Search the FAQ Archives

3 - A - B - C - D - E - F - G - H - I - J - K - L - M
N - O - P - Q - R - S - T - U - V - W - X - Y - Z - Internet FAQ Archives

Java Programmers FAQ

[ Usenet FAQs | Web FAQs | Documents | RFC Index | Houses ]
Archive-name: computer-lang/java/programmers/faq
Posting-Frequency: weekly
Last-modified: 1999/05/19

See reader questions & answers on this topic! - Help others by sharing your knowledge
Frequently Asked Questions (with answers) for Java programmers

  ________|                                                     |________
  \       |   Java Programmers FAQ   |       /
   \      |   Last modified May 19, 1999   Peter van der Linden |      /
   /      |_____________________________________________________|      \
  /___________)                                              (__________\

The Java FAQs here are intended for people who already have some programming
experience, though maybe not in Java.

Go to the FAQ home page at for other Java information and
downloads, and the most up-to-date copy of the FAQ. Report FAQ updates to
faqidea at the address


The sections of the Java FAQ are:

   * 1. Java Book Information
   * 2. Getting Started
   * 2.5 Portability
   * 3. General Information
   * 4. Compilers and Tools
   * 5. Compiler Messages
   * 6. Java Language issues
   * 7. I/O
   * 8. Core library issues
   * 9. Computer Dating
   * 10. AWT
   * 11. Swing
   * 12. Browsers
   * 13. Applets
   * 14. Multi-Media
   * 15. Networking
   * 16. Security
   * 17. For C, C++ Afficionados
   * 18. Java Idioms
   * 19. Java Gotcha's
   * 20. Further Resources
   * 21. Acknowledgements


1. Java Book Information

  1. (sect. 1) How do I choose a Java book?

     [*] There is no one right answer to "which is the right Java book for
     A lot depends on what you already know, and how you like to
     If you already know how to program in another language, and Just want
     to learn Java, consider Just Java 1.2 from the FAQ author.

     Everything in one handy volume (language basics, Swing, networking,
     I/O, database access, etc) and it comes with a CD with tons of Java
     applets, games, applications, compiler tools, complete with source.
     Includes a Java compiler for Windows, the Mac, Linux, and Solaris
     (sparc and x86). look at for details

      If you or your manager instead want a briefing on Java
      technology, thin clients, XML, CORBA, TCP/IP, Java beans,
      etc., take a look at Not Just Java from the FAQ author.
      This book doesn't teach you how to program in Java; it tells
      you why you might want to, and what kind of systems are most
      suited to Java. It describes E-Commerce and XML. look at for details

     Here are the points to check when evaluating a programming book.
        o Above all, make sure that it is a Java book. If it comes with a
          CD, check that it has a Java compiler on it, not a J++ compiler.
          VJ++ does not implement Java 2. VJ++ is also a different language
          in some important ways, and is missing the latest Java libraries
          (Swing, Collections, JFC, RMI). If your interest is Java, leave
          the downrev VJ++ book back on the shelf.
        o Does the book cover the current level of Java, which is Java 1.2
          (aka Java 2)? Look up "JApplet" in the index. If it's not there
          the book doesn't cover JDK 1.2, and you probably need a more
          up-to-date book.
        o Check that the book has a reasonable number of figures, diagrams,
          and illustrations. It is not possible to explain how to program a
          window system without pictures and diagrams. Other topics benefit
          from pictures, too.
        o Check what the book says about itself. Is it a reference work,
          intended for Java-experts to look things up in? This is the role
          of "Java in a Nutshell", and "The Java Almanac". Do you need that,
          or are you looking for a book that teaches by examples and
        o Read Peter Norvig's excellent advice on learning programming
          languages and being a programmer.
        o Appraise your own level of programming knowledge: are you
          proficient in some other language, or are you learning programming
          as well? Does the book cater to your level?
        o Read a section of the book. Does the style keep you interested, as
          it educates you? Will you get bored if you read many pages? Is the
          book too long for your initial purpose? Browse Amazon online and
          see what other readers say about the text.
        o If the book comes with a CD, how much other software is on the CD?
          You want at least a Java compiler plus all the examples from the
          book. Does the Java compiler work on your platform (Mac, Linux,
          etc)? Additional software on the CD is a big plus, as we learn the
          most from reading other people's code.

     Most people buy one book to begin with, then four or five more as they
     wish to learn more, and about more specialized topics. The FAQ author
     has purchased and read probably 60 Java books in the last three years.

  2. (sect. 1) Where can I find a some lists of Java books and book reviews?

     [*] Here are some good ones:
     (an exhaustive list -- takes a long time to load).


2. Getting Started

  1. (Sect. 2) What is the easiest way to get started with Java?

     [*] Follow these steps.
       1. Look at the books section of the FAQ to see what kind of book will
          suit you. There is no one perfect Java book. The right book
          depends on the style, pace, and detail that you are comfortable
          with. Amazon has good info and reviews on Java books.

       2. Download a free Java compiler from Service
          packs for Solaris (if any are needed) can be found at:

       3. Read the free Java tutorial, at

          (bookmark it, so you will easily find it again).

       4. Avoid Microsoft's J++ product, which is in the words of
          Microsoft's own employees "polluted Java". It is designed to
          undermine standard Java, and has many deliberate platform-specific
          incompatibilities, including new keywords in the language.

       5. To get some tips on getting started with a programming homework
          assignment, look at

       6. Search this FAQ when something in Java confuses you. Many people
          have trodden this path before you, and the FAQ contains the
          accumulated knowledge and pointers to other references.

  2. (Sect. 2) Why doesn't my "Hello World!" program compile?

     [*] There are three basic possibilities:
       1. Are you successfully running the javac compiler?

          javac -garbage

          to see if it prints out a message about correct usage. If not,
          invoke javac using the full pathname, or set your PATH variable to
          include the directory that contains javac.
       2. Is the CLASSPATH environment variable used correctly?
          In JDK 1.0.2, beginners had to set CLASSPATH, and it had to
          include both system libraries and your programs.
          In JDK 1.2, CLASSPATH is no longer needed for system libraries.
          You do want CLASSPATH to point to the directories containing any
          "user classes" you're using.
          Getting started, you will probably want at a minimum "." (the
          current directory) in your CLASSPATH.
          When CLASSPATH is wrong, javac will tell you it can't find
          definitions for classes you reference in the source file. For
          information on setting up the CLASSPATH, see Question 1.3
       3. Is the source correct?
          Here javac will emit error and warning messages. See the questions
          on compiler messages in the next section.

  3. (Sect. 2) Why doesn't my "Hello World!" program run?

     [*] There are five common mistakes that cause your VM (java or browser)
     to be unable to execute your classes
       1. First, did you write an applet or an application? If an applet,
          you must make sure that you did extend the java.applet.Applet
          class. Your starting code should be in the init routine.
          If you wrote an application, your starting code should be in a
          routine called main(). Don't mix applets with applications until
          you have a bit more experience.
       2. You must declare your main class as "public". If you don't,
          unfortunately some systems will still run the code, while others
          won't. The main class is either the one with the main() method in,
          or in the case of an Applet, the class that extends Applet.
       3. Your class name and the file name must match exactly, even letter
          case. If your class is HelloWorld, your source file must be
 and your class file will be "HelloWorld.class".
       4. If an Applet, and you used ftp to transfer the classes to the
          server, you must ftp all the classes, and you must use BINARY
          transfer not ASCII.
       5. Errors in setting the CLASSPATH (and/or codebase in an applet).
          Even seasoned programmers do this, pointing inside a package or
          mistyping a path delimiter. For information on setting up the
          CLASSPATH and codebase, see Question 1.3

     If you are running an applet, you should check the following further
       1. If your class isn't loading, recheck the HTML applet tag.
       2. If you are writing to System.out, the results are displayed in the
          browser's java console. You'll have to create a window if you want
       3. Make sure your browser is compatible with the Java language
          features you are using. Internet Explorer and older versions of
          Netscape's browsers have omitted some support for JDK 1.1. Try
          your applet in the JDK's appletviewer first.

  4. How do I set the CLASSPATH?

     [*] The CLASSPATH environment variable tells the VM's class loader
     where to find classes that are directly or indirectly invoked,
     including system classes. The CLASSPATH variable should
        o point to the directory containing the class file, for classes not
          in a package.
        o point to the package root, for classes in a package. The root is
          the parent directory of the highest directory of the package name.
        o point directly to the zip or jar file, if the classes are in an
          archive file. You may have to list the contents of the archive to
          get the correct package/path name for the class.
          Separate multiple paths and archives with a platform-specific
          separator, ";" for Windows; ":" for Solaris
     Also remember that
        o Browsers set the CLASSPATH to the directory of the HTML file, plus
          the codebase parameter.
        o in JDK 1.1 and after, java adds the system classes
          (lib/, so you don't have to.
        o JDK 1.2 versions of java add "." (current directory), so you don't
          have to. (But jre doesn't - see below.)
        o JDK 1.1 jre tool does not use the CLASSPATH variable or assume the
          current directory. (On Solaris, CLASSPATH does work.)

     From JDK 1.1.2 on, it is generally an error if the user sets the
     CLASSPATH to include But CLASSPATH will need to be set to
        o point to the roots of the programmer's own packages, and third
          party packages
        o use rmic
        o use unbundled packages like Swing in JDK 1.1
        o point to native code libraries.

     For the sake of consistency, the minimal classpath setting should be: "
     set CLASSPATH=. "
     Below you'll find examples for Windows (basic application class),
     Solaris (package class), javac (multiple packages), and browsers
     (applet codebase).

     Here's some Windows examples, assuming the application class is


             ## JDK 1.1,  no CLASSPATH set
             > cd D:\src\tries\
             > D:\jdk11\bin\java HelloWorld
               # OK: 1.1 implicitly adds and current dir

             > D:\jdk11\bin\jre HelloWorld
               # FAILS: jre does not automatically add . to CLASSPATH

             > cd D:\
             > D:\jdk11\bin\jre -cp D:\src\tries HelloWorld
               # OK: jre adds, -cp adds class directory

             ## JDK 1.1,  CLASSPATH set
             > set CLASSPATH=D:\src\tries
             > D:\jdk11\bin\java HelloWorld
               # OK: java using CLASSPATH

             > D:\jdk11\bin\jre HelloWorld
               # FAILS: jre does not use CLASSPATH (on Windows)

             ## JDK 1.0.2,  CLASSPATH set
             > set CLASSPATH=D:\jdk102\lib\;D:\src\tries
             > D:\jdk102\bin\java HelloWorld
               # OK:

             > set CLASSPATH=D:\jdk102\lib\;D:\src\tries
             > D:\jdk11\bin\java HelloWorld
               # FAILS: exception in thread NULL - wrong system classes


     Here's some Solaris examples, assuming the application class is


     and it is in package com.devjoes.killer:

             # JDK 1.1, no CLASSPATH set
             $ /usr/bin/jdk11/bin/jre  -cp /usr/src   com.devjoes.killer.App
               # OK:

             $ cd /usr/src/com/devjoes/killer/
             $ /usr/bin/jdk11/bin/java App
               # fails: class name and path are wrong

             $ CLASSPATH=/usr/src/
             $ /usr/bin/jdk11/bin/java App
               # fails: class name is com.devjoes.killer.App

             $ /usr/bin/jdk11/bin/java com.devjoes.killer.App
               # OK:


     Here's some javac examples, for both Solaris and Windows, based on the
      Source files                package           Makes the call
      /usr/src/pack/  package pack
      /usr/src/pack/sub/  package pack.sub  (nothing)

             $ CLASSPATH=""
             $ /usr/bin/jdk11/bin/javac /usr/src/pack/sub/
               # OK: works fine

             $ /usr/bin/jdk11/bin/javac /usr/src/pack/
               # FAILS: can't find pack.sub.Try

             $ cd /usr/src
             $ /usr/bin/jdk10/bin/javac pack/
               # OK: finds pack.sub.Try based on . as package root

             $ cd /usr/src/pack
             $ CLASSPATH=/usr/src
             $ /usr/bin/jdk11/bin/javac
               # OK: finds pack.sub.Try based on CLASSPATH

     Now assume the killer application class


     (in package com.devjoes.killer) uses a third-party package in a jar


     but makes no other reference to other classes. The following works

             $ CLASSPATH=/usr/jars/JShapes.jar
             $ cd /usr/src/com/devjoes
             $ /usr/bin/jdk11/bin/javac killer/

     Finally, some applet examples. Many applets only use one class, in the
     same directory as the html file:

     <applet code=ArcTest.class height=400 width=400>

     To use classes in subdirectory, use the codebase parameter:

     <applet codebase="mysubdir/" code=ArcTest.class ..

     To use classes in an archive, use the archive parameter:

     <applet archive="applets.jar" code=ArcTest.class ..

     See also: JDK 1.1 ReadMe
     Solaris JDK 1.1 tool documentation
     Win32 JDK 1.1 tool documentation

  5. (Sect. 2) How do I do keyboard (interactive) I/O in Java?

     [*] Interactive I/O in Java is very poorly supported. Programmers must
     piece together several library classes in non-obvious ways to get the
     required functionality. See the answer to Question 7.1.

  6. (Sect. 2) How do I do file I/O in an applet?

     [*] By default, an applet can read files on the server, but not write
     them, and has no access to the client. This is for reasons of security.
     It would be very unsafe to let any old applet that you downloaded from
     an unknown origin on the Internet read/write your files. It would be as
     unwise as allowing this kind of access to an ActiveX control (which is
     one reason ActiveX is dead on the Internet).

     There are several different ways to relax the default rules. See the
     answer to Question 7.8.

  7. (Sect. 2) How do I do I/O to the serial port on my computer?

     [*] Java 1.0 and 1.1 do not have a serial port API. There are several
     commercially-available libraries that supply the needed functionality.
     JDK 1.2 introduces access to the serial and parallel ports as an
     extension (optional extra) library. See also the answer to Question

  8. (Sect. 2) How do I do formatted I/O like printf and scanf in C/C++?

     [*] The java.text package introduced with Java 1.1 supports formatted
     I/O. See also the answer to questions 7.11, 7.12, and 17.7.

  9. (Sect. 2) I have spent more debugging time finding case (upper vs
     lower) typos than everything else put together and squared!

     [*] Do not forget that your remark must be phrased in the form of a
     question to win on FAQ Jeopardy. In any event, it is worth reminding
     those new to Java that letter case really matters in Java, and that the
     names of public classes should exactly match (including case) the names
     of the files they live in. See also the answer to Question 1.1.2

 10. (Sect. 2) Why do I get this compiler error: "Can't make static
     reference to method..."?

     [*] Your code probably looks something like this:

     class myclass {
                 public static void main(String args[]) {

                 public void myMethod() {
                     //some code

     The issue is this: a static method means it belongs to the class, not
     each individual object. If you leave the static keyword off (the usual
     case) as is done here with the method "myMethod()", it means that
     method can only be invoked on an object. But your call from main() has
     not told myMethod() which object it is to be invoked on. Inside a
     non-static method, you don't have to provide this information, as it
     assumes you mean the same object on which it was invoked. But when
     calling from a static method, you must provide the information, and you
     haven't - hence the error message.

     A common fix is to instantiate a member of the class, on which to
     invoke myMethod(), like this:

             public static void main(String args[]) {
                 myclass m = new myclass();

     This problem is especially common when you are writing code that you
     want to run as an applet and as an application. Naturally, you call
     init() and start() from main. What you really need to do is:

             public static void main(String[] args) {
                 Applet ma = new myApplet();

 11. (Sect. 2) Why can't I do myArray.length() ? Arrays are just objects,

     [*] Yes, the Java specification says that arrays are object references
     [JLS 10.2] just like classes are. However, arrays cannot contain
     methods. Instead you have to use myArray.length, which is a data item
     (not a method) called "length", belonging to myArray.

 12. (Sect. 2) How do I close a Java window by using the icon in the upper
     right hand corner of a window?

     [*] Create an event handler class to extend WindowAdapter. Then
     override WindowAdapter's windowClosing() to do the actions you want
     when a window's "close" action is clicked. Then add that to the
     listeners for that window.

     import java.awt.*;
     import java.awt.event.*;

     public class MyFrame extends Frame {
         public MyFrame(String s) {super(s);}

         public class WL extends WindowAdapter {
             public void windowClosing(WindowEvent e) {System.exit(0);}

         // do your other Frame stuff


     Somewhere in your initialization code, put:

             f1.addWindowListener( f1. new WL()  );

     This last syntax is not commonly known to many people yet, it's another
     wacky artifact of inner classes.

     Alternatively, combining the inner class and setting the handler in one
     go, you could do this:

             MyFrame f1 = new f("wave");

             f1.addWindowListener( new WindowAdapter() {
                 public void windowClosing(WindowEvent e) {
                     // and/or setVisible(false) and/or dispose()
                     System.exit(0); }

     See also the answer to questions 1.0.19, 1.0.30 and 15.7.

 13. (Sect. 2) Why is b+=100; OK, but b = b+100; fails to compile?

     [*] You have code like this

       byte b = 0;
       Incompatible type for =. Explicit cast needed to convert int to byte.
       b = b + 100;    // compiler error message
       b += 100;       // works OK

     The reason is "promotion". Arithmetic expressions are promoted to the
     type of the longest, floatiest operand in them, or at least to int. The
     first statement involves the assignment of an expression. The
     expression is promoted to 32 bits, and must be cast back down to 8
     bits, like this "b = (byte) (b+100);". The second is an operator
     assignment, and the cast back to byte takes place automatically. The
     Java Specification says:
          "A compound assignment expression of the form E1 op= E2 is
          equivalent to E1 = (typecast)((E1) op (E2)), where "typecast" is
          the type of E1, except that E1 is evaluated only once" [JLS

     The compile-time narrowing of constants means that code such as:

     byte theAnswer = 42;

     is allowed, with no cast necessary. [JLS 5.2]

 14. (Sect. 2) How do I add two Float objects together?

     [*] You want to write code like this

             Float One;
             Float Two;
             Float Hard = One + Two;

     but the compiler does not allow it.

     Java has two separate ways of representing a 32 bit floating point
     number, Float and float. Float is a class, that whose sole purpose is
     to "wrap" a floating point number so it can be treated as an object.
     The class does not support floating point arithmetic, because the
     performance would be too slow. float is a primitive type (like int)
     that is used for floating point arithmetic.

     You choose one or the other depending on your predominant use. If all
     you need of your floating point numbers is arithmetic, declare them to
     be "float". If you need to use them as objects, for example to place
     them in a Vector, declare them as "Float".

     If you need both, tough. You have to declare them one way and convert
     whenever you need the capabilities of the other. Your specific code can
     be written as:

             Float One = new Float(1.0);
             Float Two = new Float(2.0);
             Float Hard = new Float(One.floatValue() + Two.floatValue());

     See also questions 6.n , 5.1, and 10.1.

 15. (Sect. 2) How can I put all my classes and resources into one file and
     have java run it?

     [*] Use a JAR file. Put all the files in a JAR, then run the app like

             java -jar [-options] jarfile Mainclass  [args...]

 16. (Sect. 2) How can I see line numbers in a stack trace using JDK 1.1.6?

     [*] From JDK 1.1.5 on (i.e. JDK 1.1.6, JDK 1.2) the stack trace of an
     uncaught exception no longer has source code line numbers. It only says
     "(Compiled Code)".

     To see the line numbers where your program throws an exception, use the
     command line argument in JDK 1.2:

          java -Djava.compiler=NONE myapp

     This info is well-concealed in the release note at:


     Specify standard Java on your new PC!

             Your new PC can come with the most up-to-date standard version
             of Java, but only if you ask for it! The JavaLobby is asking PC
     vendors to support Java, and to ship new machines with the Java Plug-In

     Please help the Java Lobby to promote this initiative.


     Please support Java Portability.

     The biggest value of Java is its portability.
        o Portability makes it easier for companies to change/upgrade
          operating systems and platforms, without losing their investment
          in applications software.
        o Portability makes it easier for Java programmers to transfer their
          skills to new employers.
        o Portability makes a wider variety of software available on all
     Software portability is very much in the interest of most software
     developers and customers. Even if you only use Windows 95, portability
     matters to you. If your software was all written in Java, it would all
     just run when you moved from MS-DOS to Windows 3.1 to Windows 95 to
     Windows 98 to Windows NT, and even on Windows CE. Instead, you
     typically need to buy new applications software all over again when
     Windows changes. Portability is not in Microsoft's interest, as it
     removes a revenue stream and makes it easy for users to try other
     operating systems.

     The 1998 anti-monopoly case against Microsoft revealed a Microsoft
     internal memo. The memo revealed that Microsoft's "strategic objective"
     was to "kill cross-platform Java." by "grow[ing] the polluted Java
     market." This is Exhibit 101 (MS7 033448) in the case.

     In November 1998, a Federal judge ruled that Microsoft was probably
     breaking its written agreement with Sun by distributing incompatible
     Java, and that Microsoft had to stop doing that. If portability matters
     to you or your users, avoid Java products from Microsoft; it is
     deliberately trying to sabotage it. Microsoft's own internal documents
     make this goal unambiguous. See

     See also "The Microsoft Java Dilemma" paper at


 17. Is there a Java implementation for Windows 3.1?

     [*] Yes. See Question 1.6 It's not that great though because Windows
     3.1 has inadequate features to support great software.

 18. (Sect. 2) I'm using Win95, and my DOS window won't accept filenames
     longer than 8.3.
     "This program cannot be run in DOS mode"

     [*] Both these problems are resolved by the same process. Assuming
     you're running the Win95/98, then you've changed the MS-DOS
     Prompt options under the "Properties" menu item. In the Properties
     dialog, Program->Advanced gets you a dialog. Here, make sure the
     "Prevent MS-DOS-based programs from detecting Windows" checkbox is

     If the option is checked you get exactly the kind of behavior you're
     seeing. The option is unchecked by default, so it must have been
     selected at some time in the past. Change it back to unchecked.

 19. (Sect. 2) I'm using Notepad to edit my files, and how can I save them
     with the extension ".java"? Also, in notepad some source files have all
     the characters on one line. Why is that?

     [*] First answer: put the entire filename in quotes in the save dialog.
     Once you have created your first Java file, double click on it in
     Explorer, select "Notepad" from the "Open with" box, and Notepad will
     stop adding the spurious ".txt" to your .java files.

     Second answer: Notepad expects to see a "carriage return/line feed"
     pair at the end of each line, rather than just the "newline"
     (line-feed) commonly used on Unix. Use this program to expand all

      * Usage: jre crlf ...

     class crlf {
         public static void main(String s[]){
             byte b[]; byte p;
             FileInputStream is;
             BufferedOutputStream os;
             File f;
             for (int i=0; i < s.length;i++){
                     f=new File(s[i]);
                     b=new byte[(int)f.length()];
                     is = new FileInputStream(f);
           ; is.close();
                     os = new BufferedOutputStream(
                     new FileOutputStream(s[i]),b.length);
                     for(int j=0; j < b.length; j++){
                         if((p!='\r')&&(b[j]=='\n')) os.write('\r');
                         p=b[j]; os.write(p);
                     os.flush(); os.close();
                 }catch(IOException e){

     The source code is to show new users a way to make a simple program
     which can read a file and write it out buffered.

     Compile with "javac" and run with
     java crlf outfile.txt
     or just use Wordpad instead of Notepad. Wordpad is under

 20. (Sect. 2) How do I fix the message about "out of environment space"?

     [*] This occurs under Windows when you have long CLASSPATH names. You
     need to increase the environment space. On Windows 95,8 put this in
     your c:\windows\system.ini


     On NT you can right-click on My Computer, select System Properties then
     go to the Environment tab and then increase COMSPEC to the value you

     The previous suggestion to put this in your config.sys:

             shell=command /e:4096

     apparently causes you to create two copies of which wastes


3. General Information

  1. (Sect. 3) Is Java "Open" or "Proprietary"?

     [*] The Java specification is publicly available, and anyone is free to
     do clean-room implementations of the JVM and core Java API's. Sun
     includes a perpetual, irrevocable, free and royalty-free license in the
     front of the Addison-Wesley books containing the specification.

     Sun also provides free access to the Java source. See

     Using the Java trademark does requires licensing from Sun. It is not
     clear if the Embedded or Personal Java specifications are open, as it
     is not clear if a clean-room implementation may be done without
     licensing from Sun.

     The relative openness of Java contrasts with systems that are only
     available from one vendor, whose interfaces are developed in secret,
     without an open process for others to participate, whose owners do not
     allow competing implementations of the same API, and whose owners
     change the APIs as a strategic weapon against competitors. Typically,
     such systems also feature "private" APIs that are published late or not
     published at all, to allow the single vendor to gain a competitive
     advantage for their other products. Typically such proprietary systems
     do not make the source code available for inspection by all.

  2. (Sect. 3) What is the best way to refer someone to the FAQ when they
     ask a question I know is answered there?

     [*] The Java Programmers FAQ (at answers your
     question in section N.n. ...

     This gives them the answer, and shows them where to go for future
     questions. It also demonstrates that the FAQ can answer their
     questions, supplying an incentive to go there next time. It's regarded
     as elementary politeness to look for the FAQ of a newsgroup and read it
     before posting any questions.

     In general, FAQs for any newsgroup are available by looking at past
     postings in a group, or by searching Deja News (see Q 1.4), or via
     anonymous FTP at directories under The pathnames
     are called things like
     which may help you get to the right one directly, as it takes some time
     to get a directory listing there. Alternatively, you can look for
     newsgroup names on the same ftp site by going to the same site and
     looking under /pub/usenet-by-hierarchy/. That has subdirectories such
     as alt/, ba/, ca/, comp/, and subdirectories under them such as
     /pub/usenet-by-hierarchy/comp/lang/ and so on. This helps you explore
     the world of newsgroups with FAQs.

     If you do not have anonymous FTP access, you can access the archives by mail server as well. Send an E-mail message to with "help" in the body for more information.
     "RTFM" stands for "Read The effing Manual" - you must expect to put in
     some time and effort to master a new area of study.

     If you want to look at the definition of Internet standards like FTP,
     telnet, visit the IETF site at where all the RFC's
     (Request For Comments) can be found.

  3. (Sect. 3) What if my question is not answered in this FAQ?

     [*] Go to
     In May 1999, Dejanews changed their name to, and their web
     design to a truly horrible garish interface, covered with ads and
     harder to search. Consider using these search sites instead:
        o Under "Newsgroups" enter "" (or whatever)
        o Under "Subject" enter "Frotteur" (or other topic you find
        o Click "Create Filter"
        o It will go to a new document, and you should click the link

          nnn Documents (nnn is some number).

     The chances are that you will find several answers to your question.
     Some may not be complete or completely accurate however. That is the
     nature of Usenet, and free information. If you still don't have an
     answer, then post your question on the most appropriate of the
     newsgroups. Don't spam the newsgroups by posting to multiple groups.
     Knowledgeable posters tend to ignore questions like that.

     Also look at
     and look at for a Java newsgroup search. can search the Javaworld

  4. (Sect. 3) What Java mailing lists are there?

     [*] There are quite a few Java mailing lists. has a comprehensive list.

  5. (Sect. 3) Where can I look at the definitive Java Language

     [*] This is available online at:
 (Java 1.0)

     and the Java 1.1 inner classes document at:


     and the other Java 1.1 update at:

     and the Java API is at:

     It is also available as a book in printed form (details at website).
     Also see the "Clarifications and Amendments"

     You can also see the virtual machine (execution environment)
     specification at

  6. (Sect. 3) Where can I find information about future Java APIs?

     [*] JavaSoft has followed a policy of creating new APIs in consultation
     with leading industry participants, then posting the draft
     specification for public review and comments. Check the JavaSoft
     roadmap of new APIs and products at
     Also, some APIs that are under consideration, possibly for JDK 1.2 are

  7. (Sect. 3) I'm looking for a Java style guide on naming conventions.

     [*] Check out the section "Naming Conventions" in the language

     Also take a look at Doug Lea's draft coding standard -

     See also naming conventions for some basic rules of thumb.

  8. (Sect. 3) How do I check on known bugs in the JDK?

     [*] Look at the Java Developer Connection at

     All the Java bugs that Sun knows about are listed there, with the
     exception of security-related bugs. The legal department in Sun vetoed
     the open publication of security bugs. After you have checked that the
     bug is not already listed, you can submit a bug report through:
     You should check that the bug doesn't already exist for two reasons:
     first, you might find the bug with a workaround listed. Second, you
     don't want to waste everyone's time with a duplicate bug report.

     You can also send in an RFE (Request For Enhancement) or ease-of-use
     issue there. You can even vote on the priority you would assign to a
     particular bug fix! Join the Java Developer Connection (it's free) by
     going to Then browse

  9. (Sect. 3) What computers have Java ports? Is there a port to Win 3.1?

     [*] A partial list of JDK ports is available from
     An (impressive) list of the systems that the GPL Kaffee JVM runs on is

     A list of the software updates you should install to run Java on
     Solaris is at

     There are several Java ports to Win 3.1. IBM's ADK1.02 is available at
     the following locations:
     IBM offers a port to Linux, as do others. The IBM Jikes port is at There is a large amount of useful
     software there, including a profiling tool called jinsight.

     Netscape Navigator for Win3.1 has Java support. Java will never be
     well-supported under Win3.1 because Win3.1 lacks the basic features
     expected of a modern OS (primarily lengthy filenames and multithreading

     Also take a look at JavaSoft's JavaPC kit that can switch a PC into a
     thin client Java system (and back to Win3.1/DOS when you want). It's
     meant for software OEMs and large corporations running lots of older
     PCs, but you can use it on the latest Pentium II too. Details are at JavaPC is available now
     for $100, runs on 486's with 8Mb or more Unlike the 16-bit versions of
     Netscape Navigator and Microsoft Internet Explorer, which provide a
     Java Virtual Machine that is only compliant with the JDK 1.0.2 API, the
     JavaPC software allows IS managers to deploy JDK 1.1-compatible Java
     applications on PCs running DOS and Windows 3.x.

 10. (Sect. 3) Where can I find information on Java 3D?

     [*] The Java 3D FAQ at
     may have the answers you're looking for. It contains general
     information about Java 3D, as well as programming tips.

 11. (Sect. 3) Where can I find information about Java Certification?

     [*] Sun is sponsoring an examination which programmers worldwide can
     take. Those passing can use the designation "Sun Certified Java
     Programmer". There is also a second-level test, involving writing a
     program to spec, and taking a further test. That results in the
     qualification "Sun Certified Java Developer". You can find out all
     about the exam at:

     and then search for "sun certified java". It costs $150 to sit the Java
     Programmer exam. It is not trivial to pass the Java certification exam.
     It requires understanding the objectives of the test, and the material
     that is tested for. These are given, along with sample questions, at
     the URL mentioned above.

     There is a Java certification FAQ at:

 12. (Sect. 3) How can I find links to recent news about Java?

     [*] This site contains links to late-breaking online news stories about
     Another good Java news source is

     This site is a fine site for programmers who want to be well-informed
     about computer industry topics. It has a lot of coverage of Linux as
     well as more general news. Highly recommended.

     This site is a source of independent news and commentary on the
     computer industry, including Java.
     You have to subscribe ($10/year, 30 day free trial).

 13. (Sect. 3) What are the folks at GNU doing with Java?

     [*] First note that the URLs in this section change quickly, and soon
     become outdated. If you have an update, send it in. There is a Gnu Java
     page at
     Guava (a GPL'd Java compiler) can be found at

     Alternatively, it may be available at
     Work is progressing on the Cygnus Java frontend to gcc. See

     Kaffe (a JVM) can be found at
 This is Tim Wilkenson's company
          devoted to commercializing the Kaffe JVM for the embedded systems
          market. He also releases a version of it under the GPL. It also
          comes with a the beginnings of a class library and the Pizza

     Classpath is a free implementation of Sun's Java libraries (v1.1),
     being developed for the GNU Project ( Information
     regarding classpath is at They aim to develop
     a 100% free drop in replacement for Sun's class libraries, targeting
     first the Japhar JVM (below). They are always looking for help, so feel
     free to stop by and volunteer.

     See also This is the Hungry Programmer's JVM.
     Currently it is development grade only.

 14. (Sect. 3) What is "San Francisco"?

     [*] San Francisco is the code name for a very large Java project led by
     IBM, and involving other companies. The project is to provide a Java
     framework for data processing applications. A large number of classes
     are provided for general ledger, sales order, inventory management,
     etc., and these classes can be extended and customized for particular
     industries (vertical markets). It is a large and ambitious software

     IBM's SF project competes with products from companies like SAP and
     Baan. Of course, the SF project is multi-platform and uses Java beans
     and GUI interfaces. More information on SF is available at

 15. (Sect. 3) What large Office-style or other applications have been
     written in Java?

     [*] Well, the first one to consider is IBM's San Francisco project,
     mentioned above. There is also Lotus's e-suite - a suite of Java
     applets and beans including a spreadsheet and a word processor. See These became available in March 1998.

     Another office suite in Java is Applix Anyware at Applix became available in
     downloadable demo form in April 1998.

     Yet another is Star Division's Client/Server Office. It is an office
     suite with the client part written in Java and able to run on
     JavaStations. The server part will run on Solaris, NT, OS/390, and
     AS/400. The older (non-Java) version is bundled with all Sun
     workstations sold in Germany. The Linux version is freely downloadable

     Another is Digital Harbor's Wav word processor. It supports component
     software, and it runs in 1MB, not the 114Mb of the latest MS Word. A
     free trial is avilable. See:

     Another Java application is Formula One for Java, an Excel-compatible
     spreadsheet written in 100% pure Java, and available for all systems
     that support Java. It runs as a Java Bean, so can easily be assembled
     as one component of a larger system. It also runs as an application,
     and as an applet! Formula One is a product of Visual Components, Inc.

     Another one is Ncode Research Inc. who write Java viewers for office
     suites. They are file-format specialists. Their mission is to make all
     popular file formats available for the Java platform. They write 100%
     Pure Java viewers for Word, Excel and PowerPoint (including Office 95
     and 97 formats). See
     Another company operating in the same space is JSoft, at

     Intentia, the 8th largest ERP vendor (annual sales of $320 million),
     has moved their entire suite of applications (Movex - covering 8
     markets) from RPG to Java - 20 million lines worth. See their press

     However, the industry is seeing few new office productivity
     applications written in any language. The niche for single-user office
     productivity applications is already dominated by Microsoft products,
     and it is unrealistic to think that Java software will unseat
     shrink-wrapped software simply because it is written in Java. This is
     why Corel replanned its Java rewrite of Corel Office before taking it
     to FCS. When Corel did that, it also increased its investment in Java
     from 33% of R&D budget to 50%, at the expense of Windows.

     Most of Java development is taking place for custom applications
     internal to a company. Most programmers of any kind have never worked
     on MS Office, but work on internal applications, and so it is with
     Java. These projects don't have the high profile of major vendors'
     products, but they are the mainstay of the industry. There are many
     companies working on Java Beans, like who have
     Java graphing software (see also GraphMaker mentioned later in this
     document). EspressChart is a Java Bean that gives you the ability to
     add 2D and 3D graphs in your applications/applets. This bean is easy to
     use, 100% Java, and runs anywhere.

     There are some excellent Java games applets at Check out the pinball -- dig that crazy
     rhythm, man.
     There are more good Java games applets at
     and at
     If you want to use Java to learn math & computer graphics, visit

     ObjectDesign Inc., has an ODBMS written in 100% java. The product is
     named PSE Pro for Java. See
     Another database written in Java is available from
     Another database written in Java is available from InstantDB is available free to
     non-commercial organizations, and is very well documented and
     maintained. (Recommended!)
     Finally, note that Sun's Java compiler is written in Java. This is a
     really big application in widespread use on millions of platforms. The
     compile command "javac" is equivalent to


     javac has a script wrapper just to set the heap size as a command line
     argument, as you can do in your own programs.

 16. (Sect 3.) What Java User Groups are there?

     [*] There are scores of Java User groups around the world, mostly in
     urban areas, and centers of software technology development. A partial
     list with contact information can be found at
     If you can't find a user group in your area/school, it's easy and
     satisfying to start one.

 17. (Sect 3.) What is a Java Bean?

     [*] A Java bean is a Java class that follows some simple conventions.
     Because it follows conventions, it can easily be processed by a
     software tool that connects Beans together at runtime. Java beans are
     reusable software components.

     Think of Java beans as being the software equivalent of Lego[tm]
     bricks. Instead of plugging together plastic bricks, you can easily
     plug together classes, and have them fit and work with each other. See
     See the Java Bean FAQ at

 18. (Sect 3.) Where can I find examples of the use of the Java class

     [*] The two volumes of "Java Class Libraries" by Chan, Lee and Krama
     published by Addison Wesley, have extensive examples of how to use the
     standard libraries. One programmer comments "When I need to use an
     unfamiliar area of the class libraries one of the first things I do is
     read their examples." You can see them online at

 19. (Sect 3.) How can I find out exactly what version of Java I have on my

     [*] On a Solaris system, you can use the pkginfo command, like this:

        pkginfo -l SUNWjvrt

     It will give a reply like this:

        PKGINST:  SUNWjvrt
           NAME:  JavaVM run time environment
       CATEGORY:  system
           ARCH:  sparc
        VERSION:  1.1.6,REV=1998.
        BASEDIR:  /
         VENDOR:  Sun Microsystems, Inc.

     You may also try

        java -fullversion

     Although that's not an officially-supported command option, and has
     gone away in JDK 1.2. Try also

        java -version

 20. (Sect 3.) What Java newsgroups are there?


 (it renamed
 (it renamed
 (it renamed c.l.j.api and c.l.j.misc)
 (argue all day with anonymous shills)

     New Java newsgroups are added every so often. Try not to crosspost.


4. Compilers and Tools

  1. (Sect. 4) Is there a lex and yacc or preferably a flex and bison
     equivalent for Java?

     [*] There is a lex equivalent called JavaLex and a yacc equivalent
     called CUP.

     LALR(1) parser JavaLex and JavaCup:

     LL(k) parser JavaCC:
     LALR(1) parser SableCC from McGill U. is generously made
     available under GNU license.

  2. (Sect. 4) Where can I find a byte code obfuscator?

     [*] Java Obfuscators replace the original class, field and methods
     names in the bytecode with meaningless strings. Second generation
     obfuscators are now appearing that also obfuscate the control flow and
     encrypt String literals. People use obfuscators on their applets if
     they want to hide their code from others. Generally, you wouldn't do
     this with software that you put on your website for others to enjoy. It
     runs counter to the "open source" philosophy of learning from other's
     code and allowing them to learn from yours.

     Zelix KlassMaster is a commercially supported obfuscator. It has a free
     evaluation version at
     Another commercially supported obfuscator, with a downloadable free
     trial is at There are also some
     free works from students and others.

     Some people have reported problems using these with JDK 1.1.

     This obfuscator has been updated to be fully compatible with JDK 1.1:

     Obfuscators are intended to foil decompilers. Decompilers translate
     byte code back into Java source code. Mocha was the first and most well
     known of the decompilers; it's no longer supported. There is a
     decompiler (written in C++) at

     Because it is in C++, there are different versions for every
     architecture (hah!) There are also commercial products, such as
     SourceAgain from

     There's a very good Java Code Engineering and Reverse Engineering FAQ
     page at

  3. (Sect. 4) Which program is used to create .zip files compatible with
     the java* programs?

     [*] Use the jar-tool from JDK1.1(.1):
          jar [ options ] [manifest] destination input-file [input-files]

          jar cvf myJarFile.jar *.class

     creates a compressed archive. And watch out -- the order of the options
     ("cvf") determine the order of the arguments!
          jar cvfO *.class

     creates it fullsize (uncompressed) (note the 'O'-option used for

     On Unix you can also use:
          zip -rn ".class" *

     Info-ZIP home page:
     Latest source code:

     Netscape's command line version of its JAR packager and signing tool is
     called "zigbert". They also have a signing tool with GUI written in
     Java. More info

     If you zip your .class files for JDK 1.0.2 (for 1.1 you'll use a Jar):
       1. zip your files uncompressed (can use WinZip 6.2 up);
               Unix command:

               zip -r0 <directories>

       2. Make sure the main class has no parent directory inside the
          archive, (in other words, don't build an archive with
          foo/bar/myMain.class, unless your myMain is in a package called
 Instead start it at myMain.class). Your packages must be
          placed in the archive using their corresponding filesystem
       3. Put the archive in the same directory as the .html page.
       4. Put something like the following tag in the .html file:

          <APPLET CODEBASE="."
                  WIDTH=600 HEIGHT=250>

     From JDK 1.1 on, an example of the applet tag used with a jar file is

     <APPLET ARCHIVE=myfile.jar
                     WIDTH=600 HEIGHT=250>

     These lines will use an applet called myapplet that can be found in the
     jarfile myfile.jar. An example applet tag of a jar file used to hold
     classes in packages is

     <APPLET ARCHIVE="myclasses.jar"

     You can supply several jar filenames in a comma-separated list. Jar
     files are in compressed PKZIP format.

  4. (Sect. 4) Can I compile a Java program to a binary executable, .exe on
     a PC?

     [*] Compiling into native code destroys portability, which is one of
     the main benefits of Java. If you want to create a native executable
     because you wanted to make it easy to distribute and use programs,
     consider a Jar file instead.
     Some companies make products that do this. See the webpages for
     Symantec, Supercede,
     and Tower Technology The first two are targeted to
     Windows. Tower Technology supports several flavors of Unix.

     Also, there is a native Java compiler from IBM, known as the HPJ (High
     Performance Java) compiler. One user has reported that it created a 2Mb
     executable from a 12K java file, and did not run any faster. See

     See also Instantiations JOVE,
     the paper about the Toba project,
     Network World, "Vendors Rush To Speed Java Performance", Feb 9 1998, at

     Compiling to native code takes away the most significant benefit of
     Java: portability of executables. Further, if you want your Java DLL
     (or .exe) to interact with C++, you'll have to specify which specific
     C++ compiler and/or actually compile some sort of linkage via the
     appropriate C++ compiler. C++ does not have a standard ABI, so there is
     a big problem with interoperability. Every C++ compiler uses a
     different object model, a different way of laying out class members,
     and a different way of "mangling" names for the linker.

     C is much simpler. The only question here is how structures are
     "packed" (i.e., are integers aligned on four-byte bounds?). All the C++
     compilers can interact with C code, thanks to 'extern "C"'

     Consider carefully why you want to compile to a native executable, and
     whether there is a Java way to accomplish your goal. There may be a
     good reason for compiling to native code, but it needs to be thought

  5. (Sect. 4) How can I performance profile my Java code?

     [*]java -prof MyClass

     produces some basic output in a file called, showing the
     number of times methods were invoked. The output lines are of the form:
          # of calls     method called      called by        time spent
     On a Unix system, you can sort the file with something like

     sort -r +82 < > java.sort

     More and better Java tools are a third party opportunity. One Java
     profiler is JProbe Profiler, available from
     JProbe is said to be easy to use. Another profiler is OptimizeIt,
     available from Each of these profilers has
     performance tuning, which shows which methods took how much time, and
     also memory tuning, which shows what objects are in memory and how they
     were allocated. Both are important things to know. The latest version
     of the CodeWarrior IDE has a time-based
     profiler for Java code. Java Workshop from Sun also has a time-based

     JDK 1.2 comes with some limited profiling capability built-in.
     Depending on your needs, it may be all that you need. Execute the
     following to get a short summary of what you can do:

        java -Xrunhprof:help

     For example, you can see which methods are taking the most time to
     execute, in the context of particular stack traces.

  6. (Sect. 4) When I use javadoc and I click on any java class included in
     the JDK why do I get this message?

         Netscape is unable to find the file or directory named:

     [*] References to the JDK classes assume that all generated html files
     are in the same directory and, in fact, that all files for all classes
     referenced are generated at the same time. There is no way to generate
     files incrementally and have them all reference each other, as you
     would like.

     As long as you have source for everything involved (including the JDK
     and all third-party classes), you can list all of your packages and all
     of the others on the javadoc command line and generate the whole set at
     once, but it is burdensome. Of course, if you receive any libraries as
     .class files, even this workaround will not suffice.

     Also javadoc will not generate the image files - you need to get them
     from the images directory under the JDK API documentation files. You
     can just copy the entire directory into your own doc directory. javadoc
     is a very nice concept, with a few implementation flaws.

  7. (Sect. 4) I'm working on a project with lots of classes and I use the
     JDK. A recompile from scratch takes forever when I do it a class at a
     time. How do I recompile everything?

     [*] The first way is
          javac *.java

     Another way is
          javac -depend

     where "" is a class "at the tip of the iceberg", i.e. that
     depends on (uses) all the other classes. Typically, this may be your
     main class. However, "-depend" is known to be buggy and cannot be
     relied upon. It also doesn't issue compile commands in parallel to make
     use of multi-processor systems.

     Without the "-depend" option, the standard "javac files" doesn't look
     beyond the immediately adjacent dependencies to find classes lower down
     the hierarchy where the source has changed.

     The -depend options searches recursively for depending classes and
     recompiles it. This option doesn't help when you have dynamically
     loaded classes whose names cannot be determined by the compiler from
     the dependency graph. E.g. you use something like

     The author of the code using those classes should make sure that those
     classes are mentioned in a Makefile.

  8. (Sect. 4) Why do I get the java.lang.UnsatisfiedLinkError when I run my
     Java program containing Native Method invocations?

     [*] Your program is not able to find your shared library or DLL.

     On Windows 95/NT, make sure that the DLL exists in a path that is
     included within the PATH environment variable. (This need is true for
     both standard (untrusted) applications and trusted applets. At least,
     if you use the Java Plug-in to give yourself standard Java inside a

     On Solaris, make sure that the environment variable LD_LIBRARY_PATH
     includes the path of your shared library.

     Note that jdb looks for libraries with "_g" appended to their names.
     Thus, if you intend to use jdb on a Java application that invokes
     native methods, you must ensure that the appropriately named libraries
     are in jdb's path. The "debug" nm libraries can simply be renamed
     copies of the nondebug libraries.

     For example, if your app invokes native methods in a library named
     mynm.dll (on Windows) or (on Solaris), make a copy in the same
     directory and name it mynm_g.dll or

  9. (Sect. 4) An anonymous class can't seem to access a private outer
     method. Why is that?

     [*] It's a known bug in the JDK 1.1.4. The code is:

         public class MyDialog {

                 void Setup() {
                 addWindowListener( new WindowAdapter() {
                       public void windowClosing(WindowEvent e) {
                              myCloseWindow(); }
                       );     // anon inner class

             private void myCloseWindow() {   // private outer method

     This code sends javac into an infinite loop. The workaround is to make
     the private method non-private, or to make the inner class a named
     class. Sun put a workaround in the compiler to silently set the field
     to package access.

 10. (Sect. 4) What are the major Java releases and their contents?

     [*] There have been three Java releases from Sun so far, plus a number
     of bugfix (dot-dot) releases. The releases are:
        o JDK 1.0.2
          This was the release FCS in May 1996. It had some security fixes
          over JDK 1.0.
        o JDK 1.1
          This release (Feb 1997) introduced a new event model in the window
          system. It also made JDBC support and beans support a standard
          feature. It changed and standardized the native code interface to
          JNI. It also introduced inner classes.
        o JDK 1.2
          This release (Dec 1998) made the Swing library a standard feature.
          Swing is a set of rich platform-independent graphical components.
          It also introduced the Collections library, and Java 2D.

 11. (Sect. 4) What is the difference between jre and java?

     [*] They are functionally equivalent, with minor differences in the
     handling of default classpath and options supported. To reduce
     confusion, the jre command was removed in JDK 1.2. Instead there is a
     "java" command in both bin and jre/bin.

     jre.exe is the java launcher that comes with the Java Runtime
     Environment. It ignores the CLASSPATH environment setting in favor of
     its own internally generated default and whatever is supplied on the
     cmd line using -cp or -classpath. It's intended to be a bit simpler for
     those who are only ever running Java programs, not developing them.

     java.exe is the java launcher that comes with the JDK. It uses the
     CLASSPATH environment setting as a starting point and then tacks on its
     own internally generated entries.

     They both serve the same purpose and that's to start a Java VM, have it
     run a Java application, then terminate. The source for jre.exe is
     provided in the JDK. The source to java.exe is provided only in the JDK
     Source distribution.

 12. (Sect. 4) What IDEs (Integrated Development Environments) are there?

     [*] Some popular IDEs include: (... if you have info on the platforms
     that each of these support, send it in, and I'll add it to the FAQ).
      Apptivity (Progress)
      Blue J (free)
      Bluette (free)
      Chicory (free)
      Freebuilder (free)
      GRASP (free) 
      Java WorkShop (Sun)
      Javelin, Visual Object
      Development for Java
      JBuilder (Inprise)
      JDE for emacs
      Jirvana (free)
      Kawa (Webcetera)
      NetBeans (Swing-based)
      PARTS alpha
      PowerJ (Sybase)
      Simplicity for Java
      Super Mojo (Penumbra)
      SuperCede (Asymetrix)
      teikade (PFU Ltd)
      Together/J (Object Intl
      Visaj (Imperial SW
      VisualAge (IBM)
      Visual Cafe (Symantec)

      Visual J++ (Microsoft)
                             (not recommended)
      Xelfi 0.94   

 13. (Sect. 4) Why is Visual J++ not recommended?

     Because Microsoft's strategic objective is "Kill cross-platform Java"

     [*] It is not in Microsoft's financial interest to allow users to
     easily move software to different platforms. Microsoft is the only
     company in the computer industry that is actively trying to undermine
     Java. This is not speculation -- the Department of Justice's lawsuit
     quoted a Microsoft memo describing the strategic objective to "kill
     cross-platform Java by grow[ing] the polluted Java market". See
     VJ++ can be used in a compatible way, but the tool encourages use of
     Windows lock-ins. The J++ compiler introduced new keywords and other
     deliberate incompatibilities.

     Microsoft is being sued because of unauthorized changes it made in
     Java. A federal judge issued a preliminary injunction against Microsoft
     in March 1998, prohibiting them from labelling their incompatible J++
     product as Java. Another injunction was issued against Microsoft in Nov
     1998, requiring them to remove some deliberate incompatibilities with
     standard Java. (Recall that Microsoft did not create Java, but
     contracted with Sun to distribute it).

     Microsoft is in a real jam over Java; it contracted with Sun to
     distribute standard Java, and then made many changes to its product
     to make it deliberately non-portable. Sun sued Microsoft to enforce the
     contract, and the preliminary legal decisions have been in Sun's favor.
     Some analysts speculate that Microsoft will leave its customers in the
     lurch and cancel VJ++. See

     Speak to your management chain - how comfortable do they feel using a
     Microsoft product that is embroiled in a legal dispute, that introduces
     deliberate incompatibilities, and whose stated goal is to lock you in
     to one platform? It is a safer choice to get standard Java from any
     other source than Microsoft. You can use these facts to move your
     company to standard Java.

     As a Java programmer please join the Java Lobby, an independent
     organization dedicated to representing independent non-vendor interests
     in Java. It's free, and you can sign up by visiting for details. Other ways to encourage portable
        o Use development environments from other vendors, or convert
          Microsoft Visual J++ to use Sun's JDK, following the instructions
        o Use Netscape Communicator (not Internet Explorer)
        o If required to use Internet Explorer, use the Java Plug-In to get
          a standard Java system inside it.
        o Use a standard JVM from GNU, Kaffe, any of the IDE vendors, or Sun
          (but not Microsoft's J++ SDK)

        o Free standard Java compilers and the Java Plug-In can be
          downloaded from
        o Free standard Java Virtual Machines can be downloaded from
,, and

        o Free Java AWT software can be downloaded from
 and the files are all at
 (the linux site) too.
        o Free Java software can be downloaded from

     Just for the record, the May 1998 federal case against Microsoft has
     nothing to do with innovation or product design, as Microsoft
     frequently insists. Microsoft is actually charged with
        o taking anti-competitive action to exclude competition in browsers,
          in order to protect its monopoly in desktop operating systems.
        o using its monopoly to impose restrictive agreements that require
          PC manufacturers to accept the Microsoft browser with Windows, and
          that hinder the promotion of competing browsers.
     Many people think that contracts between "A" and "B",
     restricting/discouraging "B" from distributing "C's" products are
     sleazy. ["A" is Microsoft, "B" is AOL, many ISPs, computer vendors,
     etc. "C" is Netscape"]. Such contracts are in restraint of competition
     and illegal when used by a monopoly. This is why Microsoft is facing
     mounting legal problems in the United States, Italy, Brazil, and the
     European Union.

 14. (Sect. 4) What language is the Java compiler and JVM written in?

     [*] Sun's javac Java compiler is written in Java.
     Sun's java JVM interpreter is written in Java with some C, C++ for
     platform-specific and low level routines.
     Sun's Java Web Server is written in Java.

     Other companies have chosen other approaches. IBM's Jikes compiler
     (which is praised for its speed) is written in C++, but of course each
     version only runs on one platform. Jikes versions have been prepared
     for IBM AIX, Linux Intel (glibc), Win95/NT and Solaris Sparc at

 15. (Sect. 4) How do I turn off the JIT in the JDK?

     [*] The command that works in both JDK 1.1.6 on, and JDK 1.2/2 is

              java -Djava.compiler=NONE   ...

         One reason for turning off the JIT is to get more
         information about any exception that is thrown in your code.
         But HotSpot is able to produce line numbers in stack traces
         even for JIT'd code.  HotSpot rocks.

 16. (Sect. 4)
     How can I store the errors from the javac compiler in a DOS file?

     javac > errorfile doesn't work.
         javac writes errors to stderr,
         The problem is that DOS doesn't allow stderr to be redirected
         (as is very poor software).  So you
         have to use a special error redirection mechanism in the compiler:

         javac -J-Djavac.pipe.output=true > errors.txt

         In JDK 1.2, you can use:
         javac -Xstdout

         You typically use this when a compilation produces a lot of error messages,
         and they scroll off the DOS window before you can read them.

         you can get a scollbar to appear on a DOS window by changing the properties
         with the "Layout" tab. Change the Screen Buffer Size Height: to some
         multiple > 1 of the Window Size Height. E.g. use a buffer height of 100 and
         screen height of 25 (the default). This will give you three buffers of
         scroll "history."

          The DOS limitation is improved in NT, where you can write

         javac 2> errors.dat

 17. (Sect. 4)
     How can I pretty-print Java source?
         For tools that reformat code, try:











         For tools that print code neatly, try:



         Some Unix utilities work adequately:

                  cb (very few style choices though)

                  alias printjava 'vgrind -lC++ -t -w \!* | lp' works pretty well too.

         Perhaps the best tools are the GNU utilities. Use enscript to
         generate postscript files with Java-specific formatting. Then use
         GhostScript/GhostView to preview and print the files to a
         non-PostScript printer, if necessary. The scripts can be found at:

                  GNU Enscript - pretty
                  printer and PostScript formatter

                  Ghostscript, Ghostview, and
                  GSView for Unix

                   - GSView for NT (requires the following Ghostscript files)


         - Ghostscript configuration, initialization, and
                  example files


         - Ghostscript core binaries for Windows NT


         - Ghostscript standard fonts


5. Compiler Messages

     Most of the "questions" in this section are diagnostic messages from the
     compiler.  Each answer explains what the message means, and how to avoid it.

  1. (Sect. 5)
      Why did I get an OutOfMemory error when porting working code from
     JDK 1.0.2 to 1.1?
         The preset memory limit has changed. It went down to 16MB so as not to
         penalize low memory machines. You can adjust it with

          java -mx128m Frotz        # jdk 1.1
          java -Xmx128m Frotz       # jdk 1.2

     to get a 128MB extent.

     Also see the Runtime methods freeMemory() and totalMemory().

  2. (Sect. 5) Why do I get a "Statement not reached" error from javac for
     no apparent reason?

     [*] JDK 1.0 has a limit of 63 words of storage for local variables in
     any method. longs and doubles require two words of storage, and all
     other primitive types and all reference types require one word. If you
     assign values to more than 63 words of local variables, you will get a
     "Statement not reached" error on the statement after you assign to the
     variable that contains the 64th word. In JDK 1.1, the low limit was

  3. (Sect. 5) class MyOrdinaryClass must be declared abstract.
     It does not define void actionPerformed(java.awt.event.ActionEvent)

     [*] This is one of those error messages where the compiler tries to
     guess what you meant, and gives you a message based on a wrong guess!
     So the message is confusing.

     Your MyOrdinaryClass class implements ActionListener, which means you
     must include a definition of the methods from the ActionListener

     But you did not. You either left a method out, or (more likely) you
     misspelled its name. Perhaps you wrote "ActionListener" instead of

     So the compiler did not find the method to fulfill the interface. Since
     there was a method promised but not supplied, the compiler thinks you
     were aiming at an abstract class, and it prints an error message

  4. (Sect. 5) Variable may not have been initialized.

         URL test;
         try {
          test = new URL("");
         } catch (MalformedURLException e) {
          System.out.println("bad URL:" + e.getMessage());
         System.out.println("this is url " + test);

     [*] The compiler will warn you if you use a variable before it is
     certain to have been initialized (not just with the default value)
     since this means you probably forgot to set it.

     In the case of exceptions, you have to consider that the flow of
     control may terminate abruptly, with no operations completed. In the
     example above, if an exception is raised in the try clause, variable
     test will not be assigned a value, yet you are using it after the catch
     clause. One solution would be to declare test with an explicit initial
     value of null, but this works only because toString() works on a null
     reference. (toString() is invoked implicitly by operator + with String

     Always initialize to a value that will work notwithstanding exceptions
     being thrown.

  5. (Sect. 5) No constructor {superclass}()
     I extended the class called Frotz, and the compiler is giving me an
     error message "No constructor Frotz()" in the child class. Why?

     [*] When you define a constructor for a class, the compiler inserts a
     call to the superclass' parameterless constructor unless you explicitly
     call the superclass' constructor at the start of your constructor. If
     the superclass doesn't *have* a parameterless constructor, the compiler
     emits a message to that effect. The solution is usually to call the
     superclass' constructor at the start of your constructor.

  6. (Sect. 5) No constructor matching MyCheckbox(myApplet)
 No constructor matching MyCheckbox(myApplet)
         found in class MyCheckbox.

         bp1 = new MyCheckbox(this);

     [*] If a compiler isn't finding a constructor you thought you created,
     check whether you gave a return value to the method (remember,
     constructors have no return value). E.g.,

         public void MyCheckbox( Container parent )

     If you did, the compiler will think it is an ordinary method, not a
     constructor. This is a common mistake and hard to spot.

  7. (Sect. 5) Type expected {public method variable}

         public static void main(String[] args) {
         Statement expected.
         public static final float Conversion_Factor = 39.37;
         Type expected.

     [*] Argument and variable declarations inside methods are never public
     or static because they are local to a method. (Before JDK 1.1 they
     couldn't be final either, but there was no good reason for that
     restriction and it was dropped.) If you have public or static
     variables, move them outside the method. They are usually put at the
     beginning of the class.

  8. (Sect. 5) Can't access protected method clone in class java.lang.Object
 Can't access protected method clone in
         class java.lang.Object. OtherT is not a subclass of
         the current class.

     [*] Object.clone() is protected because subclasses might want to
     restrict access to cloning, and if Object.clone() were declared public,
     subclasses could never make it more restrictive. The subclass can make
     access to the clone() operation less restrictive.
     This means that a method can clone its own objects, but a method cannot
     clone objects of another class, unless you do something like:

         class SomeObject implements Cloneable {
             public Object clone()
                 throws CloneNotSupportedException {
                 return super.clone();

     i.e., override clone() to make it public, and call the superclass

         class Foo {
          Bar bar;
          Foo (Bar b) {
              try {bar = (Bar) b.clone();}
              catch (Exception e) {}
         class Bar implements Cloneable {
          public Object clone()
              throws java.lang.CloneNotSupportedException {
              return super.clone();

     Another refinement is to note that Object.clone() only throws a
     CloneNotSupportedException when the object doesn't implement Cloneable.
     Since you control what your classes do and don't implement, you can
     ensure that Cloneable classes implement the interface, and you don't
     need to make the overridden clone() throw the exception.

         public class X implements Cloneable {
             public Object clone() { // no throws
                 try {
                           // in case members need cloning
                     X c = (X)super.clone();
                     return c;
                    } catch (CloneNotSupportedException e) {
                    // should not happen, because of Cloneable
                        throw new InternalError();

  9. (Sect. 5) Deprecated methods
     What does "deprecated" mean? I got this in a compiler error message.

     [*] "Deprecated" means you are using an older API, that Sun has
     replaced with a newer one (usually to follow more consistent naming
     conventions). Deprecated methods are not recommended for use. They are
     supported in the short term, but your code should be updated with the
     new. To update your code, compile your old code using javac's
     "-deprecation" option to list deprecated methods, then find the
     replacement methods in the current HTML API documentation for the old
     deprecated methods.
     As an example of a deprecated API, Component.size() was replaced by

     See also
     misc/deprecation/index.html, "1.1 Deprecated Methods"
     awt/DeprecatedMethods.html, "Deprecated methods in the 1.1 AWT"

 10. (Sect. 5) double y = sin(90);
     What's wrong? That code provokes compiler messages.

     [*] You need to write it this way:

         double cvtDegToRad = Math.PI/180;
         double x = 90*cvtDegToRad;
         double y = Math.sin(x);

     sin is a static method of the Math class that takes radians. You need
     to use the "Math" classname, e.g. Math.sin instead of plain sin,
     because you have to say what class or object these methods belong to.

     A very common mistake is to assume that importing a class means that
     you don't have to qualify the names of its members. When you call a
     method you have to state the name of the class or object it belongs to,
     regardless of any imports you have done. (Except inside the class
     itself, obviously).

     The trig functions are static methods of the Math class, so you give
     the name of the class in invoking them. Further, the Math class works
     in radians, not degrees. 360 degrees = 2 pi radians, so use a
     conversion factor as shown if you are working with degrees.

 11. (Sect. 5) Can't make static reference to method...

     [*] Your code probably looks something like this:

         class myclass {
             public static void main(String args[]) {
             public void myMethod() { //some code

     Static (class) methods can only call without qualification other static
     methods, so you either have to qualify the call in (static) main() to
     (nonstatic) myMethod() with an object of type myclass, or you have to
     make myMethod() static.

     People often forget that even though main is "in" myclass, there is no
     implicit object when you are in main() because it is static. This
     happens especially when writing code to run an applet as an
     application, where you want to call init() and start() from main.

         public static void main(String[] args) {
             Applet ma = new myApplet(); // have to create object
             ma.init();  // use to qualify access to non-static methods

 12. (Sect. 5) Incompatible type for =. Explicit cast needed...

         byte b = 0;
         Incompatible type for =.
         Explicit cast needed to convert int to byte.
         b = b + 100;    // compiler error message
         b += 100;       // works OK

     [*] Arithmetic expressions are promoted to the type of the longest,
     floatiest operand in them, or at least to int. The first statement
     involves the assignment of an expression. The expression is promoted to
     32 bits, and must be cast back down to 8 bits, like this: b = (byte)
     (b+100); The second is an operator assignment, and the cast back to
     byte takes place automatically. The Java Language Specification says
     that a compound assignment expression of the form E1 op= E2 is
     equivalent to E1 = (typecast)((E1) op (E2)), where "typecast" is the
     type of E1, except that E1 is evaluated only once. (See JLS 15.25.2
     Compound Assignment Operators) The compile-time narrowing of constants
     means that code such as:

         byte theAnswer = 42;

     is allowed, with no cast necessary. (See JLS 5.2 Assignment Conversion)

     Other sites:
     JLS 5.2 Assignment Conversion
     JLS 15.25.2 Compound Assignment Operators

 13. (Sect. 5) Class {package}.{class} not found in type declaration.
     I am trying to compile file "{class2}.java" in a package, and I get
     this compiler error. {class2}.java refers to {package}.{class}, but the
     file {class}.java and {class2}.java are in the same {package}
     directory, which is the current directory and which is in the CLASSPATH
     variable. Both files have "package {package};" at the top of the file.
     What's the problem?

     [*] When the source refers to classes in packages, the CLASSPATH has to
     point to the root of the package/directory hierarchy for a reference to
     resolve correctly. This is true even for source files in the same
     package (and directory). I.e., assuming {class} and {class2} are both
     in {package}, {class} can't make a reference to {class2} unless the
     CLASSPATH is set so javac can find {package}/{class2}.java. It should
     make no difference what directory you are in when you invoke javac,
     unless you are relying on "." in the CLASSPATH to point to the package
     root or are specifying the source file with a relative path (e.g.,

     Some examples, assuming
        o - and are in /java/source/pack/
        o - Both have "package pack;" as the first statement
        o - includes "Bar b = new Bar();"

         # solaris ksh
         $ alias jc=/java/jdk11/bin/javac
         $ CLASSPATH=/java/source/
         $ jc /java/source/pack/*.java  # works fine
         $ cd /java/source/pack
         $ CLASSPATH=.
         $ jc *.java         # fails - can't find class Bar
         $ cd ..             # now . is package root, /java/source/
         $ jc pack/*.java    # works

 14. (Sect. 5) public class "Foo" must be defined in ""
     I get this message even though it is in What gives?

     [*] Javac verifies that a public class is defined in a file of the same
     name (e.g., that public class Foo is defined in Two things
     you can check:

     First, make sure the case matches exactly. public class Foo cannot be
     in; it has to be in

     Second, are you using MKS on win32? Javac on win32 assumes you are
     using the DOS path separator (\) even though MKS accepts the Unix path
     separator (/). When javac tries to parse a your Unix-style path, it
     won't produce the correct filename, the match will fail, and it will
     emit an error. You have to use the DOS path separator (\), which must
     be escaped in MKS - e.g., "javac H:\\source\\package\\".
     Alternatively, you can traverse to each source directory and avoid
     pathnames altogether.


6. Java Language Issues


  1. (Sect 6.) How do I compare two Strings?

     if (s1 == s2)

     is giving me funny results.

     [*] The comparison using "==" on objects, such as Strings, is asking,
     "Do these two objects have the same reference?" Do they have the same
     address, and hence are the same object? What you really want to do is
     ask, "Do these two Strings have the same *contents*?"
     Compare String contents with any of the following:

         if (s1.equals(s2) )
         if (s1.equalsIgnoreCase(s2) )
         if (s1.startsWith(s2) )
         if (s1.endsWith(s2) )
         if (s1.regionMatches(s1_offset, s2, s2_offset, length) )
         if (s1.compareTo(s2) < 0)

     (There are other ways, too.)
     Note that you can do this with literals:

     if ("apple".equals(s2) ) ...

     If you compare these the other way round, like this:

     if ( s2.equals("apple") ) ...

     and s2 is null, you will get a null pointer exception.

  2. (Sect. 6) How do you get the code value of a char?
     I would like to transform a char into the corresponding int value, that
     represents the code value of the char. How?

     [*] Like this.

         char c = 'A';
         int i = c;

     Going the other way is just

         c = (char) i;

     This question crops up so frequently because the BASIC language uses
     functions to map characters into ints, ASC( 'A' ) => 65 causing BASIC
     programmers to seek the corresponding Java functions. The same is true
     for Pascal, Ada, and other languages.

  3. (Sect. 6) Why does b >>>= 1 give me the same result as b >>= 1?

     [*] ">>" is a "signed" or "arithmetic" shift, namely, it replicates the
     sign bit on the left as it shifts.
     The ">>>" operator is an "unsigned" or "logical" shift; it does a shift
     right and zero fill. However, ">>>" looks like it does a signed shift
     with negative bytes and shorts, where int promotion alters the sign.

     This occurs when you have a non-canonical type, byte, or short, with a
     negative value, e.g.

         byte b = -15; // 0xf1
         b = (byte) b >>> 4; // why isn't b 0x0f ?

     The initial expectation is that an unsigned shift right of 0xf1 would
     successively be (in binary)

         0111_1000 then
         0011_1100 then
         0001_1110 then

     But that doesn't happen. The rules of arithmetic in Java say that all
     operands are converted at least to int before the operation (and
     possibly to a more capacious type). That means our byte is promoted to
     an int, so instead of shifting 0xf1, we are shifting 0xfffffff1. If you
     shift right unsigned 4 places, you get 0x0fffffff. When you cast that
     to a byte it becomes 0xff, or -1.

     The bottom line is that the final result is the same as if you had
     performed the signed shift because the unsigned shift applied to the
     intermediate int, not to the original byte. This anomaly means that
     ">>>" is useless for negative bytes and shorts. It is probably safer
     and clearer not to use it at all, but to mask and shift instead:

         // not recommended
         byte b = -15;
         b = (byte) (b>>>4);
         System.out.println("b= "+Integer.toHexString(b) );
         // recommended
         b = -15;
         b = (byte) ( (b & 0xFF) >> 4 );
         System.out.println("b= "+Integer.toHexString(b) );

  4. (Sect. 6) Why does the <unexpected> happen in floating point?

     [*] There are several unexpected things that seem to bite programmers
     with floating point. This is almost always a result of the programmer
     not being fully conversant with floating point arithmetic in general,
     rather than a problem relating to Java.

     The question of floating point accuracy comes up with Java more than
     with C++ (for example) because of Java's decision in the println()
     method to print enough digits to distinguish the number from the
     next-closest number, rather than rounding it to 6 significant digits by
     default as the C/C++ libraries do. [ISO C, Sect, line 21 of
     "the double argument"]

     If you seem to be having problems with floating point, the problem
     probably stems from the fact that floating-point arithmetic is
     inherently imprecise. You can expect up to 7 digits of precision with
     floats and 16 digits with doubles. However, that does not mean that a
     number that can be exactly represented in 7 digits decimal or can be
     exactly represented as a binary floating point number. On the contrary,
     that is usually not the case.

     Additionally, when Java converts floating point numbers to a String, as
     is done when they are output, enough digits are printed so the number
     can be read back in with no loss of precision. For this reason, you may
     see more "inaccuracies" in floating point output than you are used to.
     You are actually getting more accuracy than on systems (like C,C++)
     that suppress the less significant digits.

     For more information and detailed specifications on how Java deals with
     floating point, see the URLs listed below.

     Other sites:
     What Every Computer Scientist Should Know About Floating Point.

     JLS 4.2.3 Floating-Point Types and Values
     JLS 4.2.4 Floating-Point Operations
     JLS 3.10.2 Floating-Point Literals
     JLS 5.2.3 Narrowing Primitive Conversions
     See also "the guff from Hough" at

     If you want the rounded floating point output that most languages have,
     use the new java.text package of Java 1.1 to limit the number of digits
     that are output. If you need more precision than about 16 digits, use
     the BigInteger and BigDecimal classes of Java 1.1.

     Understanding the Java Language

  5. (Sect. 6) How can I program linked lists if Java doesn't have pointers?

     [*] Of all the misconceptions about Java, this is the most egregious.
     Far from not having pointers, in Java, object-oriented programming is
     conducted exclusively with pointers. In other words, objects are only
     ever accessed through pointers, never directly. The pointers are termed
     "references" and they are automatically dereferenced for you.

     Java does not have pointer arithmetic or untyped casting. By removing
     the ability for programmers to create and modify pointers in arbitrary
     ways, Java makes memory management more reliable, while still allowing
     dynamic data structures. Also note that Java has NullPointerException,
     not NullReferenceException.

     A linked list class in Java might start like this:

         public class LinkedList {
             public LinkedList head;
             public LinkedList next;
             public Object data;
             public LinkedList advanceToNext(LinkedList current) { ...

     Another choice for a linked list structure is to use the built-in class
     java.util.Vector which accepts and stores arbitrary amounts of Object
     data (as a linked list does), and retrieves it by index number on
     demand (as an array does). It grows automatically as needed to
     accommodate more elements. Insertion at the front of a Vector is a slow
     operation compared with insertion in a linked list, but retrieval is
     fast. Which is more important in the application you have?

  6. (Sect. 6) Are parameters in Java passed by value or by reference?

     [*] All parameters (values of primitive types and values that are
     references to objects) are passed by value. However this does not tell
     the whole story, since objects are always manipulated through reference
     variables in Java. Thus one can equally say that objects are passed by
     reference (and the reference variable is passed by value). This is a
     consequence of the fact that variables do not take on the values of
     "objects" but values of "references to objects" as described in the
     previous question on linked lists.

     Bottom line: The caller's copy of primitive type arguments (int, char,
     etc.) _do not_ change when the corresponding parameter is changed.
     However, the fields of the caller's object _do_ change when the called
     method changes the corresponding fields of the object (reference)
     passed as a parameter.

     Also in this FAQ:
     How can I program linked lists if Java doesn't have pointers?
     Other sites:
     JLS 8.4.1 Formal Parameters

  7. (Sect. 6) What are "class literals"?

     [*] A feature introduced in JDK 1.1. They are literals of type "Class"
     that hold a value representing any class. There are even values to
     represent "void" and an array, like this:

         Class myCl1 = Character.class;
         Class myCl2 = Void.class;
         Class myCl3 = Object.class;
         Class myCl4 = String[].class;
         Class myCl5 = int[][].class;

     You might use it like this:

         Class cl = thing.getClass();
         if (cl.equals(myCl1))
         System.out.println("It's a Character class");

     Note that a class literal


     is the equivalent of


     The second can throw an exception, but the first cannot. If you don't
     know the name of the class when you write the code, you cannot use the
     first form.

  8. (Sect. 6) What are the naming conventions in Java?

     [*] The naming conventions are straightforward:
       1. Package names are guaranteed uniqueness by using the Internet
          domain name in reverse order: com.javasoft.jag - the "com" or
          "edu" (etc.) part used to be in upper case, but now lower case is
          the recommendation.
       2. Class and interface names are descriptive nouns, with the first
          letter of each word capitalized: PolarCoords. Interfaces are often
          (not always) called "something-able", e.g. "Runnable", "Sortable".
          Caution: java.util.Observable is not an interface, though
          java.util.Observer is. These two are poorly designed.
       3. Object and data (field) names are nouns/noun phrases, with the
          first letter lowercase, and the first letter of subsequent words
          capitalized: currentLimit
       4. Method names are verbs/verb phrases, with the first letter
          lowercase, and the first letter of subsequent words capitalized:
       5. Constant (final) names are in caps: UPPER_LIMIT
       6. Also in the FAQ:
          Where can I find a Java style guide on naming conventions?
          Other sites:
          JLS 6.8 Naming Conventions

  9. (Sect. 6) Should I prefer importing {package}.{class} over {package}.*?

     Does it make a difference to the class file in any way, if I import a
     package versus use the full name, i.e.

         import java.rmi.server.*;
         RemoteObject ro;


         java.rmi.server.RemoteObject ro;

     [*] No, it makes no difference to the class files or runtime speed.
     Import is just a shorthand for quoting the full name package and class
     name (as in the examples in the question). Importing a class does not
     cause the class to be loaded at run time. There is no runtime penalty
     for using the * form of import. The class file will contain the name of
     the packages it uses, and the loader will look for those classes as
     needed at runtime.

     At compile time, the different forms of import may or may not make a
     difference to compile time. Such a difference is likely to be
     negligible, and should not be a factor in which form of import you use.

     However, there are style advantages. Some say that stating which
     classes you are importing can help program readability. In a program
     with many * import statements, it may take a programmer time to find
     which package an obscure class is imported from. If you explicitly list
     each class you import at the top of the program, you document which
     package each class you use comes from. These people suggest that you

     import java.rmi.server.RemoteObject;

     in preference to:

     import java.rmi.server.*;

     Other people say that it is clearer still to use the full package and
     class name, at the point where you use classes in other packages.
     These people suggest that you use:

     java.rmi.server.RemoteObject ro;

     But that gets a little lengthy when you instantiate:

             java.rmi.server.RemoteObject ro
                       = new java.rmi.server.RemoteObject();

     You always have the option of stating the full package and class name,
     whether you use import or not.

     Another good reason not to use the * form is when you are importing two
     packages that have classes of the same name and you want to use only
     one of those classes. E.g.

         import com.sun.*;

     where there is a class called Modem in both those packages. If you use
     the * form of import, you import both of the Modem classes and then
     must fully qualify the class each time you use it, to say which of the
     two you mean. In Java 1.2, the class java.util.List was introduced.
     That had the same unqualified name as java.awt.List. If your code had
     "import java.awt.*; import java.util.*;" it would no longer compile.
     You'd get a message about ambiguous classname. If you import all of a
     package indiscriminately you might get bitten when the package API

     In Java 1.0, if you import a class that has the same name as a class
     defined in that source file, you will get an error that the class names
     clash. In Java 1.1, the local class will be used when the package name
     is not given; to use the imported class, you have to use the full
     package name.

     The best advice is to write the program so that it is as readable as
     possible. Where you have a group of well-known classes, as in java.awt,
     there is no reason not to use "import java.awt.*;"

 10. (Sect. 6) How can I use Math.cos() etc. without the prefix "Math."?
     Is there some declaration that I can use to make "acos", "cos", "sin",
     etc. (from java.lang.Math) recognizable in my own class, so I don't
     have to prefix "Math." to them?

     [*] No. There is no good alternative. There are several bad
       1. Using "import" doesn't work.
          The import stament only imports packages, subpackages, and
          classes, not class members. This doesn't work:

                  import java.lang.Math.*;

       2. Minimizing class name usage is unclear and bad style.
          - You could wrap the functions in your own class.

              double sin(double x) {
                  return Math.sin(x);
              } // etc. for each function

          But you'd have to use your class name everywhere but inside your
          class, so it doesn't help.
          - You can make a null reference to the Math class and use it to
          refer to the static methods. Declare

              java.lang.Math M = null;
              angle = M.cos(i);

          Besides not being clear, this invites abuse and errors.

          - You could inherit the names
          If java.lang.Math were not final and your class did not extend
          another class, you could have your class extend Math, to bring the
          namespace in. However, it is poor OOP style to use inheritance to
          obtain a name abbreviation rather than to express a type

 11. (Sect. 6) Why is there a standard JNI?

     [*] JNI is the Java Native Interface. It defines the way that a Java
     program can call C programs. The industry has agreed on, and Sun has
     codified, JNI as the standard. Microsoft shuns the standard and uses a
     protocol of its own called Raw Native Interface, RNI.

     You might think that once a Java program uses JNI, portability is lost,
     and hence it doesn't matter if vendors diverge from the JNI standard.
     Not so. Code that accesses a native library using JNI can run on any VM
     that supports JNI, so it's portable across VMs on the same platform.
     Further, you can port a native library to all platforms Java supports
     (indeed, this is how Sun implements the Java Platform), so JNI
     _enables_ cross-platform development where it's necessary to use
     platform-specific idioms for certain functionality.

     Conversely, code that uses RNI can only run on Microsoft's VMs on the
     win32 platform. Microsoft's RNI has the effect of limiting RNI programs
     to the Microsoft VM. Further, Microsoft's failure to support JNI locks
     out JNI-based functionality on Windows. Microsoft's non-standard RNI is
     the reason that programs using the Microsoft JVM cannot use the
     standard Java jdbc-odbc library. That library has a piece written in C.
     It works for all JVMs except Microsoft's.

     The standard JNI thus has two purposes:
       1. Source code compatibility between different platforms.
       2. Binary code compatibility between different JVMs on the same
     Microsoft's use of RNI locks in programmers who use it, and Microsoft's
     failure to support JNI locks out programmers who don't use RNI. Users
     can't run standard JNI applications on Microsoft VMs, or RNI
     applications on non-Microsoft VM's. As a result, since most users will
     support only one VM, they'll be locked in to complementary software -
     in the case of Microsoft, a proprietary standard. A standard JNI means
     that you can use any standard JVM to run your code on this platform.

 12. (Sect. 6) How do I find out more about JNI? How do I find out more
     about Java Anything?

     [*] Taking the questions one at a time. Use of JNI detracts from
     program portability. So you would only do it when you need some
     critical single-platform effect. The documentation on JNI is at:

     If your interest extends to reading a book on JNI, a good one is
     "Essential JNI Java Native Interface" by Rob Gordon; ISBN
     0-13-679895-0. See

     In general, if you want to find out about topic "X" in Java, your first
     stop should be to search the website for "X". For
     example if you want to know about Internationalization in Java, a
     search at the site quickly takes you to

 13. (Sect. 6) How do I get unsigned ints in Java?

     [*] Java doesn't have unsigned ints. The reason is that this is a
     poorly designed area of C. The rules for what type you end up with when
     you mix signed and unsigned in expressions are complicated, and they
     changed between K&R and ANSI C (you might have heard this under the
     name "unsigned preserving vs. value preserving"). Worse, they depended
     on the underlying hardware, so they varied from platform to platform,
     causing bugs in all kinds of unexpected places. The book "Expert C
     Programming" goes into this in more depth (page 25). So, to avoid
     bringing over the hidden complexities, Java does not bring over
     unsigned types from C.

     Use type char if you are OK with 16-bit unsigned quantities. Otherwise,
     go to the next larger type and use masking. If you don't have to worry
     about division by numbers with the high bit set, then when you use >>>
     for right shift and remember to AND each byte with 0xFF where they are
     being expanded into a larger type, then there is no difference in the
     bit patterns generated.

     Specifically, to convert an int to its unsigned representation, use:

             ((long)i) & 0x00000000FFFFFFFFL

     This promotes the signed (and possibly negative) int to long (with sign
     extension) then chops off the sign-extension, leaving it as the same
     32-bit quantity held in a 64-bit type.
     Also worth noting is that if you're going to work with unsigned bytes,
     int is a more efficient larger type to use than short or char, since
     smaller values have to be promoted to int to do any arithmetic or
     testing on them.

 14. (Sect. 6) What happened to "private protected"?

     [*] It first appeared in JDK 1.0 FCS (it had not been in the betas).
     Then it was removed in JDK 1.0.1. It was an ugly hack syntax-wise, and
     it didn't fit consistently with the other access modifiers. It never
     worked properly: in the versions of the JDK before it was removed,
     calls to private protected methods were not dynamically bound, as they
     should have been. It added very little capability to the language. It's
     always a bad idea to reuse existing keywords with a different meaning.
     Using two of them together only compounds the sin.

     The official story is that it was a bug. That's not the full story.
     Private protected was put in because it was championed by a strong
     advocate. It was pulled out when he was overruled by popular


 15. What are the differences between an interface and an abstract class?

     [*] Some use a semantic distinction: an abstract superclass models the
     "is" relationship, while an interface models the "has" relationship.
     The rule would be, if it's a subtype, inherit; otherwise, implement.

     But, in the absence of real-world characteristics to distinguish the
     objects from their properties and parents, that becomes a circular
     argument. In this case, you have to look at the practical differences
     in Java (compared with C++).

     Most differences between interfaces and abstract classes stem from
     three characteristics:
       1. Both define method signatures that a derived class will have.
       2. An abstract class can also define a partial implementation.
       3. A class can implement many interfaces, but inherit from only one

     In greater detail, these topics are:
       1. Method signatures Both interfaces and abstract classes permit one
          to treat the derived-type class as the derived-from-type class.
          Both define a set of available methods in a way that can be
          enforced by the type-checking mechanism. This is typically used to
          permit different (derived) types to behave in the same way (as
          what they are derived from - i.e., they all support particular
          methods). For example, all java.* types can be printed as Strings
          because Object, the superclass of all java.* types, has a
          toString() method. Similarly, all types that implement the
          Observable interface can be passed an Observer to signal when an
          event has occurred. This permits an algorithm or service to
          operate on different (derived) types as if they were the same
          (derived-from) type.
          This mechanism supports not only polymorphism (one object treated
          as another), but differentiation. In either case, the (derived)
          types can implement the method in the way appropriate to that
          type. However, you're not likely to override inherited
          functionality, but you must implement interface methods, so if you
          expect significant differentiation, then an interface might be
          Finally, this mechanism supports a weak variant of access control.
          Only inherited methods are available to callers who only have a
          reference to the superclass or interface type. It's weak because
          they can attempt a narrowing cast if they know their target type.
          Nonetheless, it reduces some complexity.
       2. Inheriting implementation Inheriting an implementation is useful
          where the code should be shared. This happens where derived types
          vary the functionality only a little bit, or where a complex set
          of method interfaces can through mutual reference be implemented
          with relatively few methods that can be implemented by derived
          types. You can also reuse code by having your class use or keep an
          object of another type that implements that code, but that doesn't
          permit your callers to treat you in a particular way. To both
          "get" functionality and to be treated "as" the superclass are the
          essentials of the type/subtype relationship.
       3. Java's rule of single inheritance Java differs from C++ in
          permitting only single inheritance. This makes for some difficult
          choices, if you would like to share combinations of inherited
          functionality and polymorphism from more than one source. However,
          it does reinforce the notion of inheritance as a subtyping (is)
          relationship, and implicitly that type relationships form a tree
          rather than a network.

     Other differences to consider:
       1. Abstract class implementations may include fields
       2. Interfaces may include final data members
       3. It is slightly slower to call an implemented method via an
          interface reference. There is an even smaller penalty for calling
          a superclass method via a subclass reference (i.e., where the
          subclass does not override the method). There is almost no penalty
          for calling a subclass method via a superclass reference. (All are
          compared to a direct method call, i.e., calling the derived class
          method via a derived class reference).

 16. (Sect. 6) How do static methods interact with inheritance?

     [*] Static (per-class, rather than per-object) methods do not
     participate in overriding (choosing the right method at runtime based
     on the class of the object). Probably the best and simplest way to
     think about this (and to write your code) is to write every invocation
     of a static method using the fully qualified class name:

         class A {
         public static method1() {
         public static method2() {

         class B extends A {
             public static method3() {
             public static method2() {

     Now it is perfectly clear that the static method2() that is called is
     A.method2(), not B.method2(). A.method2() will be called regardless of
     whether you use the fully-qualified class name or not, but using "A."
     makes it obvious to all.

 17. (Sect. 6) Why is the String class final? I often want to override it.

     [*] Being final guarantees that instances of String are read-only. (The
     String class implements read-only objects, but if it were not final it
     would be possible to write a subclass of String which permitted
     instances to be changed.) Strings need to be read-only for security and

     As for efficiency, Strings are very commonly used, even behind the
     scenes by the Java compiler. Efficiency gains in the String class yield
     big dividends. If no one can change a String, then you never have to
     worry about who else has a reference to your String. It's easier to
     optimize accesses to an object that is known to be unchanging.

     Security is a more compelling reason. Before String was changed to be
     final (while Java 1.0 was still in beta) there was a race condition
     which could be used to subvert security restrictions. It had to do with
     one thread changing a pathname to a file after another thread had
     checked that the access was permitted and was about to open it.

     There are other ways to solve these problems, but the designers
     preferred making String final, particularly since the StringBuffer
     class is available as an alternative.

 18. (Sect. 6) If I extend/subclass a class, are the constructors inherited?

     [*] "Constructor declarations are not members. They are never inherited
     and therefore are not subject to hiding or overriding." The default
     constructor is not inherited, but provided. (See JLS 8.6.7 Default

     If you don't give your child class any constructors, a default no-arg
     constructor that invokes the superclass' constructor is provided for
     you. If the superclass doesn't have a no-arg constructor, you should
     create a constructor and call the appropriate superclass constructor.

     Also in the FAQ:
     Compiler message No constructor {superclass}()

     Other sites:
     JLS 8.6.7 Default Constructors

 19. (Sect. 6) How can I safely store particular types in general
     I often want to store particular types of objects but don't want to
     subclass my basic storage classes to enforce the particular type; that
     would make for too many subclasses (e.g., IntegerLinkedList,
     StringLinkedList, etc.).

     [*] Generic programming in java (the rough equivalent of C++'s
     templates) works reasonably well since all java classes are subclasses
     of Object. There is, however one potential problem - there is always a
     possibility that a generic container may contain different classes of

     This naturally leads to the question of how to do this in a type-safe
     way. If you've created a generic LinkedList class, how can you be type
     safe without having to create a multitude of subclasses
     (IntegerLinkedList, StringLinkedList, etc.)?

     One way to handle this would be to offer up an additional constructor
     in your generic class that takes a parameter of type "Class" and uses
     that parameter along with Class's "isInstance" method to guarantee that
     Objects added to the container are the expected type.

         public class LinkedList {
             Protected Class type = Object.class;

             public LinkedList(Class type) { this.type = type; }

             public void addElement(Object element) throws Exception
             if(!type.isInstance( element ))
                 throw new Exception(
                      "Expected element of type (" + type    + ")" +
                      " got element of type ("     + element + ")"   );

     Note that the comments in the source for isInstance() refer to a
     "specified Class parameter", suggesting that you are supposed to write
     something like:

             public void addElement(Object element) throws Exception
             Class c = element.getClass();

     This works, but the documentation for isInstance is clear that the
     parameter should be an Object rather than a Class. Also, note that
     "Collections" are coming in JDK 1.2, and they provide a much safer and
     more extensible mechanism. More information about this is available at
     the Java Developer Connection at the Java website:

     Method interfaces

 20. How do I send a variable number of arguments to a method?

       1. (Easy) Use method overloading to support different parameters.
          This makes things easy on the caller but can get out of hand if
          you want to support a wide number and variety of parameter types.
          You should ask yourself if your code design is well-organized if
          you need to do this.
       2. (More complicated) Use arrays. It's even possible to declare
          arrays inline as shown below:

              foo("A param",
                  new Object[] {"param3", "param4", new Integer(5)} );
           // ...

           void foo(String param1, Object param2[]) {
               for (int i = 0; i < param2.length; i++) {

          You can even pass arrays of arrays using this method. Of course,
          inside the method you need to be able to decode what the arguments
          are and how you use them.
       3. Alternatively you can invent a class that just contains all the
          possible fields you might want to pass into a method (plus
          booleans to say if each field is set or not), and make an object
          of that class be a parameter to the method. You can return
          multiple values from a method the same ways; either have the
          method return an array or a wrapper object.

     However, remember the wise words of Professor Alan Perlis, "if your
     procedure has more than about half a dozen parameters, you probably
     forgot a few." Passing large numbers of arguments into a function
     suggests your function is badly organized.

 21. (Sect. 6) How can I return a different object in a method parameter?
     How can I pass an object to a method, and have the method change the
     reference so it points to a different object back in the calling code?

     indirection". Wrap the object in another class, whose purpose is simply
     to be passed as a parameter, allowing the nested object reference to be
     The second alternative is a clearer variant of this. Pass in a single
     element array. Since arrays are objects, this works.

             void jfoo(Object ref[]){
             ref[0] = new Object();
         Object kludge[] = new Object[1];
         kludge[0]= myObj;
         if (kludge[0] == myObj) ...
         else ...

     Note that changing a global variable/object inside a method is an
     egregious programming practice; it usually violates basic OOP

 22. (Sect. 6) How do I get multiple return values back from a method?

     [*] You can just have the function return a Vector. This is
     particularly convenient when you're not sure how much you are going to
     be returning, based on what occurs in the method. A Vector is
     essentially a dynamically-growable array. Regular arrays can't grow
     after you declare them - you have to declare a bigger array and move
     the old stuff into it.


 23. (Sect. 6) How do I allocate a multidimensional array?

     [*] There are several ways. If you want a rectangular array, you can
     allocate the space for the array all at once. The following creates a
     4x5 array:

         int arr[][] = new int[4][5];

     If you want each row to have a different number of columns, you can use
     the fact that a two-dimensional array is actually an array of arrays.
     The following code allocates a triangular array:

         int arr[][] = new int[4][];    // allocate the four row arrays
         for (int i = 0; i < 4; i++) // initialize each of the four rows
         arr[i] = new int[i + 1];       // row i has i + 1 columns

     Note that if you allocate an array of any kind of object (as opposed to
     primitive type), all the references will be null by default. These null
     references can result in NullPointerExceptions if you try to
     dereference them.
     In other words, after doing:

         int arr[] = new int[4];

     you can say

             if (arr[2] == 0)

     But after doing

             Integer Iarr[] = new Integer[4];

     you must fill in the object reference before using it. E.g.,

             Iarr[2] = myInt;


             arr[2] = new Int(27);

     before you can say

             if (Iarr[2].equals(myInt))

 24. (Sect. 6) How do I copy an array?

     [*] If the array only contains primitive types or if you want to copy
     only the object references, not duplicate the objects, then use the

         java.lang.System.arraycopy(Object src, int src_position,
             Object dst, int dst_position, int length);

     Otherwise, if you want to duplicate the objects, you have to initialize
     your new array and write a loop that duplicates each object in the old
     array into the new.

     Note that the documentation for arraycopy() says that if src and dst
     refer to the same object, then arraycopy behaves as if the source array
     elements are copied into a temporary array (i.e., they are preserved).
     Some interpret this as meaning a temporary array will be so allocated,
     but that's not Sun's implementation.

     Other sites:
     JLS 20.18.16 {java.lang.System.arraycopy()}

 25. (Sect. 6) How do I clear an array?

     [*] There is no method to clear an array to 0.0, 0, null, false,
     '\u0000' etc. When you allocate an array, the elements are set to their
     default values, but that doesn't help when you want to reuse an array.

     If you want to set the same array to the same set of values many times,
     create a template array. Fill it with the reset value, then use
     System.arraycopy() to copy it into the work array each time you need to
     set the work array.

 26. (Sect. 6) What is a fast way to set all elements of an array?
     I don't want to use a template array. I would like to set all array
     elements to a given value without duplicating the (possibly large)

     [*] Using a loop that does it one by one is probably 20 to 40 times
     slower than good old memset() in C.

     A fast way on many VM's is to set the first byte of the array, then use
     System.arraycopy() repeatedly to fill the next byte, the next two
     bytes, the next four bytes, the next eight bytes, etc., and when you
     get past halfway, fill in the rest.

         public static void bytefill(byte[] array, byte value) {
         int len = array.length;
         if (len > 0)
         array[0] = value;
         for (int i = 1; i < len; i += i)
             System.arraycopy( array, 0, array, i,
                 ((len - i) < i) ? (len - i) : i);

     This is faster on Sun's VM than a simple loop, and probably even faster
     under JITs because it only performs at most log2(array.length) bounds
     checks. This is a clever code idiom applying the binary chop algorithm
     to arrays even when their size is not a power of 2.


7. I/O

  1. (Sect. 7) How do I read a file containing ASCII numbers?

     [*] There are several ways to do this. Here is one way. Let's assume
     your file is called "C:\work\mydata.txt" and it contains lines like:

         135   7512   3659814  328   1 54829
         68522 19982810  38

     i.e. lines contain several ASCII strings that are numbers separated by
     The code fragment is as follows:

     //  Open the file with
     RandomAccessFile f = new RandomAccessFile("c:\\work\\datafile.txt", "r");

     // read an entire line from it
     String s= f.readLine();

     // get some methods to break up a line into tokens
     StringTokenizer st = new StringTokenizer(s);

     // extract the next int from the line
     i = Integer.parseInt(st.nextToken());

     We use a RandomAccessFile because that supports the readLine() method
     directly. An alternative would be to instantiate a FileReader, and wrap
     a BufferedReader around it. Putting it all together, including the
     exception handling in the event the file is missing, the code looks

     import java.util.*;
     public class c  {
         public static void main(String args[]) {
           try {
             RandomAccessFile f = new RandomAccessFile
                                             ("datafile.txt", "r");
             String s;
             while ( (s=f.readLine()) != null )  {
                 System.out.println("read: "+s);

                 StringTokenizer st = new StringTokenizer(s);
                 int i=0;
                 while (st.hasMoreTokens()) {
                    i = Integer.parseInt(st.nextToken());
                    // i now holds the next int on the line
                    // could also use Double.parseDouble(), etc.

                    System.out.print(" "+ i);

           } catch (Exception e) {System.out.println("Excpn: "+e); }
           // file I/O, from book "Just Java" by Peter van der Linden

     See also the next question on how to read data interactively.

  2. (Sect. 7) How do I read a String/int/boolean/etc from the keyboard?

     [*] The easiest way is to pick up the source for the 100% pure Java
     class EasyIn from (same place as this FAQ). Compile
     it with your code and use it like this:

     EasyIn easy = new EasyIn();

     int i = easy.readInt(); // gets an int from
     boolean b = easy.readBoolean(); // gets a boolean from
     double d = easy.readDouble(); // gets a double from

     ... etc.

     EasyIn is free, comes with source, and you can do what you like with
     it, including improve it, and send me back the results.

     If, instead, you want to "roll your own" code (why?!), in JDK 1.0.2 in = new;
     String s = in.readLine();

     One way in JDK 1.1: in =
      new new InputStreamReader(;

     String s = in.readLine();

     Once you have the token in a String, it is easy to parse it into one of
     the other types, as shown earlier in the FAQ. Yes, it is bone-headed,
     as it makes the simplest case of keyboard I/O unnecessarily
     complicated. A bug was filed with Javasoft to record this problem, but
     don't count on this being fixed any time soon.

  3. (Sect. 7) Why do I have trouble with System.out.println()?

     [*] Check the spelling. The last two characters are the letters "ell
     enn" not "one enn".

     The name of the method stands for "print line", since it prints a
     String and goes to the next line, rather than staying on the same line
     as System.out.print() does. Yes, the name is yet another Java naming
     inconsistency, since the input equivalent is readLine(), not readln().

  4. (Sect. 7) How do I write to the serial port on my PC using Java?

     [*] There is a platform-independent serial port API introduced in JDK
     1.2. You can download the documentation by registering with the Java
     Developer Connection (it's free, and browsing

     For systems prior to JDK 1.2, read on. At least two companies have
     written a library to drive the port. See
        o has a library for Windows 95, WindowsNT,
          OS/2, Macintosh PPC, Solaris Sparc, Linux x86, FreeBSD x86, HP/UX
          PA-RISC, and possibly others too.
        o In addition, there is a Unix serial port utility available with
          source at It's free under the GPL,
          and works on Linux, Irix, Solaris, Windows 95, and NT.

     While not helpful to typical home users, there is an alternative
     portable COM port solution for Java 1.1 and even 1.0. Buy your COM
     ports in the form of "terminal servers". Using a COM port is now as
     easy as connecting to it with a Socket. Port parameters can be changed
     programatically using SNMP for most terminal servers (but this is never
     necessary when a modern modem or other fixed-rate equipment is
     attached). Any networked box can serve as a terminal server - even
     Win95 - with a simple native server application for that box, but
     buying an actual firmware based hardware box is much easier.

     Furthermore, your Win95 native applications can now share the COM ports
     (and any attached modems) via a Win95 product called "Dial-out IP" at

     If the port exists as a pathname in the filesystem, you can open it as
     a file and read/write. You can also print text this way by writing to
     "prn" or "lpt1" on a pc, and "/dev/something" on Unix. Writing a
     formfeed at the end of the file is essential on Windows 95. Here is
     some sample code:

     // class that opens the printer as a file
     // and writes "Hello World" to it

     public class lpt {
         public static void main (String[] argv) {
             try {
                 FileOutputStream os = new FileOutputStream("LPT1");
                 //wrap stream in "friendly" PrintStream
                 PrintStream ps = new PrintStream(os);

                 //print text here
                 ps.println("Hello world!");

                 //form feed -- this is important
                 //Without the form feed, the text will simply sit
                 // in print buffer until something else gets printed.
                 //flush buffer and close
             } catch (Exception e) {
                 System.out.println("Exception occurred: " + e);

     If you wish to change the characteristics of the port (e.g. baud rate,
     parity, etc.), not just read/write data, Java currently offers no
     portable way to do this. You will need to use one of the packages
     mentioned above or some native code or a system command.

  5. (Sect. 7) How do I append to a file?

     [*] There are two ways. JDK 1.1 introduced new constructors for two of
     the output classes that allowed you to set a boolean flag:

     public FileWriter(String fileName, boolean append) throws IOException
     public FileOutputStream(String name, boolean append) throws IOException

     Another way is to do this:

     RandomAccessFile fd = new RandomAccessFile(file,"rw");;

     Then write using fd. Note that the latter method does not take
     advantage of the "append" mode present in many operating systems (such
     as all Unixes). Such a difference may make a difference with multiple
     processes or threads appending to the same output file. This can happen
     frequently, even if not intended by the programmer, e.g. with logfiles
     in multitasking environments.

  6. (Sect. 7) Is it possible to lock a file using Java ?

     [*] JDK 1.2 introduces the ability to lock a file (indirectly) using
     the File class. Use createTempFile() with delete on exit. Prior
     releases of Java do not feature an API to lock a file or regions within
     a file. Code that needs to do this must take one of four approaches:
       1. Implement an advisory locking scheme using features that Java does
          have (synchronized methods). This allows you to lock files against
          use by other Java code running in the same JVM.

       2. Use the atomic operation File.createNewFile(), and mark it as
          deleteOnExit(). Have all processes (Java and non-Java) follow the
          same protocol: if the create operation succeeded, you have the
          lock. To give up the lock, you either delete the file or exit the

          Note that this may fail if the file is remotely mounted using NFS
          version 2. (There's a window of opportunity bewteen the LOOKUP to
          see if it's already there, and the CREATE if it's not). However,
          NFS version 3 does guarantee exclusive create of remotely mounted
          files, and is not subject to this race condition failure.

       3. Make calls to native code to issue the locking ioctls. This
          approach is not portable, but gives you a shot at having your
          locks respected by legacy code in non-Java programs using standard
          locking ioctls.

       4. Push the work to a central server. Since socket connection
          requests arrive in a single queue on the server, this can be used
          to serialize lock requests. There might be some merit in copying
          the NFS lockd protocol for a general approach. Rolling your own
          simple version for a specific application is pretty easy. A
          database would be better off locking records or fields, not byte
          offsets. In theory, the server socket approach would make it
          easier to perform automatic cleanup of a lock on abrupt VM process
          failure, e.g. by asking "are you still alive?" to the lock holder

  7. (Sect. 7) How do I make the keyboard beep in Java?

     [*] In JDK 1.1, java.awt.Toolkit has the method beep(). The pre-1.1
     alternative of


     (the ASCII BEL character) doesn't work on Macs, but does on some other
     platforms. Java doesn't support the C abstraction of '\a' for an alert

  8. (Sect. 7) How do you do file I/O from an applet?

     [*] For security reasons, untrusted applets accessed across the network
     are restricted from doing certain operations, including I/O. This
     prevents rogue applets from sending out your private data, or deleting
     it. A trusted (signed) applet can perform these operations (JDK 1.1

     The simplest approach for server-side I/O is to use the Linlyn class
     available from This is free software under the GNU
     license, and uses FTP to move files between an applet and the server.
     It is suitable for low-volume non-critical use like keeping a
     high-score file. The Linlyn class has a very simple application
     programmer interface.

        o The following suggestion is for server-side input.

          You can read a file on the server if you can create a URL
          referencing the file. Then open a stream, then use any of the
          stream-based methods to read.

          This allows reading but not writing. It requires an http daemon
          running on the server, which will usually be the case.

             URL url = new URL("");
           // or URL url = new URL( getDocumentBase(), filename);
             BufferedReader in = new BufferedReader(
                                   new InputStreamReader(
                                      url.openStream() ) );
             String s = in.readLine(); //read till you get a null line.
             } catch(MalformedURLException e){
             } catch(IOException e){

          You cannot write a file on the server this way.
        o The following suggestions are for server-side output.

          It absolutely requires the cooperation of the server to allow an
          applet to write a file to the server. This cooperation may take
          any of several forms:
             + FTP server
             + File server (webnfs or custom written)
             + Listening on a socket for data from applets
             + CGI script
             + Java RMI (remote method invocation)
             + JDBC process

          In particular:
             + FTP code. Use the Linlyn class mentioned above.
             + WebNFS. This is an evolution of the NFS (Network File System)
               to make file resources visible in browsers. More information
             + Open a socket back to the server and read/write the data.
               Have a process on the server that listens for socket
               connections from applets and does the requisite I/O. This
               does I/O on the server.
             + Or use a CGI script or servlet on the server to write when

        o The following suggestions are for client-side I/O. Use a trusted
          applet (see section on security). This will permit local I/O
          without any of the restraints mentioned above. In this regard, the
          appletviewer and many other browsers regard applets loaded from a
          local filesystem (rather than across the net) as being more
          trustworthy, and perhaps even allowed to do I/O.
        o The simplest form of output is probably for the applet to connect
          to the mailserver port on the server, and prompt the user to enter
          his email address. Then email him the data to save locally if he
          wishes. If a small amount of data he can later enter it by
          cut-and-paste into the applet when he next invokes it.
        o Or use a browser that has a security policy that is configured to
          allow file I/O (such as Sun's appletviewer).
        o Read this article

          for an introduction to the basics.

  9. (Sect. 7) What are Resources files and how do they work?

     [*] In Java 1.1 and above, a resource file is a file that your code
     should be able to access no matter where it was loaded from: the local
     file system, via http, or some other means. This is a different concept
     than system resources. See question (TBD) for more information about
     system resources. (Volunteer sought to write answer on system resource

     There are two ways to specify resources.
        o using an absolute path name, such as
          With an absolute path name, the resource file is actually relative
          to the classpath.
        o using a relative path such as "dir/myResource.txt".
          If you specify a relative path, then the file is found relative to
          where the class loader found the package of the Class that is
          loading the resource. The consequence of this is that relative
          resource files can only be in the same directory as your class
          file or in a directory below.
     If you want to access resources from an unsigned applet, use relative

     There are a couple of methods to get at resources, including methods in
     the java.lang.Class and java.lang.Classloader. It uses the class loader
     because that code knew where to find the class on the filesystem. If
     the resource file is nearby, it can be found the same way. One simple
     method to get a stream for a resource for a particular class, say
     Mypackage.MyClass, is as follows:

     String relativePath = "resourceDir/somefile.txt";
     String absolutePath = "/somePackage/somefile.txt";
     InputStream in=Mypackage.MyClass.class.getResourceAsStream(relativePath);

     If the class cannot be loaded, "in" would be assigned a null value.
     Otherwise, you can use the inputStream just as any other.

     A brief note about the syntax used above. In java 1.1 and above, a
     java.lang.Class object for a particular class can be acquired by
     appending .class to the class's name. Though it looks like every object
     has a static member variable, this is not actually the case.

     When using resources with Netscape, be aware of the Netscape
     restrictions that:
        o Netscape does not implement the getResource() methods in
          java.lang.Class and java.lang.ClassLoader, but only
          getResourceAsStream() methods.
        o All resources must be in a Jar/archive file
        o Resource files must have a Netscape-approved extension, or you
          must call certain functions before hand. See the following for

     Internet Explorer does not seem to have these restrictions.

     One final "gotcha" is that jpgStream.available() (wrongly) returns 1
     when .gif files in the .jar file are compressed! Some people now create
     a .jar file in two steps, essentially:

             jar cvf x.jar *.class
             jar uf0 x.jar *.gif

     This ensures .gifs are not compressed, and hence that available()
     doesn't lie to you. David Alex Lamb stumbled onto this after wondering
     why his .gif files looked bad, then realized that compression of a .gif
     might throw away detail. The conjecture is that available() returns 1
     because it has to block to do uncompression.

 10. (Sect. 7) How do I execute a command from Java? How do I do I/O
     redirection in Java using exec()?

     [*] See the answer to Question 18.7.

 11. (Sect. 7) I used a C program to write a binary file. When I instantiate
     a DataInputStream on the file in Java, and try to readInt, I do not get
     the correct numbers. Why is this?

     [*] Java does everything in network byte order (big-endian order), as
     do many computers including Motorola, and SPARC. The Intel x86 uses
     little-endian order in which the 4 bytes of an int are stored least
     significant first. Rearranging the bytes on the way in will get you the
     results you need. This is only necessary when the file was written by a
     non-Java program on a little-endian machine such as a PC.

     The following code will byte-swap little-endian integers into network
     standard order:

     public int swap(int i) {
         int byte0 = i & 0xff;
         int byte1 = (i>>8) & 0xff;
         int byte2 = (i>>16) & 0xff;
         int byte3 = (i>>24) & 0xff;
         // swap the byte order
         return (byte0<<24) | (byte1<<16) | (byte2<<8) | byte3;

     Alternatively, the following code assembles bytes from a byte array
     that is in big-endian order (as used by Java) into an int:

     byte[] bytes = ... // whatever
     int start_index = ... // wherever

     int value = 0;
     for ( int i = start_index; i < start_index+4; ++i ) {
         value = ( value << 8 ) | ( bytes[i] & 0xFF );

     If the bytes are in little-endian order, just change the "for"

     for ( int i = start_index+3; i >= start_index; --i )

     And this code will assemble a double that has been written in reverse
     byte order:

       byte[] gnol = new byte[8];;

       long l = (
                  ( (gnol[7] & 0xff) << 56) |
                  ( (gnol[6] & 0xff) << 48) |
                  ( (gnol[5] & 0xff) << 40) |
                  ( (gnol[4] & 0xff) << 32) |
                  ( (gnol[3] & 0xff) << 24) |
                  ( (gnol[2] & 0xff) << 16) |
                  ( (gnol[1] & 0xff) <<  8) |
                    (gnol[0] & 0xff)

       double d = Double.longBitsToDouble(l);

 12. (Sect. 7) How do I make I/O faster? My file copy program is slow.

     [*] This is the purpose of BufferedInputStream. It is a flaw in Java
     that buffered I/O is not the default, with a flag or different
     constructor to turn it off. I/O is the second worst designed package in
     Java, after the Date class. ,

 13. (Sect. 7) How do I do formatted I/O of floating point numbers?

     [*] Use the class java.text.NumberFormat.

     Or use Or use Cay Horstmann's
     Although many utilities claim to handle all varieties of C's printf, as
     far as has been found, this is the only one to correctly handle the
     equivalent of %e in printf.

     See also the standard packages java.text.DecimalFormat and

 14. (Sect. 7) How do I read numbers in exponential format in Java?

     [*] The program below (written by Steve Chapel) uses StreamTokenizer to
     read data from the standard input and recognizes doubles in exponential
     format (e.g. -1.23e-45).


     public class ReadExponential {
         public static void main(String argv[]) {
             DataInputStream in = new DataInputStream(;
             StreamTokenizer st = new StreamTokenizer(in);
             try {
                 while (st.nextToken() != StreamTokenizer.TT_EOF) {
                     switch (st.ttype) {

        case StreamTokenizer.TT_NUMBER:
                  double num = st.nval;
                  int exp = 0;
                  st.ordinaryChars('\0', ' ');
                  st.whitespaceChars('\0', ' ');
                  if (st.ttype == StreamTokenizer.TT_WORD &&
                      Character.toUpperCase(st.sval.charAt(0)) == 'E') {
                      try {
                         exp = Integer.parseInt(st.sval.substring(1));
                      } catch (NumberFormatException e) {
                  } else if (st.ttype < 0 || st.ttype > ' ')
                  System.out.println("Num " + num * Math.pow(10, exp));
         case StreamTokenizer.TT_WORD:
                  System.out.println("Word " + st.sval);
                  System.out.println("Char '" + (char) st.ttype + "'");
              } // end switch
          }  // end while
        } catch (IOException e) {
          System.out.println("IOException: " + e);
      } // end main

 15. (Sect. 7) I'm trying to read in a character from a text file using the
     DataInputStream's readChar() method. However, when I print it out, I
     get ?'s.

     [*] Remember that Java characters are 16-bit Unicode characters, while
     many hosts systems store characters as 8-bit ASCII characters.
     Therefore, to read individual chacters from a text file, you need to
     ensure the proper conversion. The proper way to do this is to use an
     InputStreamReader, which converts from 8 to 16 bit streams:

     FileInputStream fis = new FileInputStream("myfile.txt");
     InputStreamReader isr = new InputStreamReader(fis);
     char c3 = (char);

     The less-favored way (because it is not so portable, as the encodings
     translation is not done) is just to read a byte and cast it into a

     FileInputStream fis = new FileInputStream("myfile.txt");
     DataInputStream dis = new DataInputStream(fis);
     char c1 = (char) dis.readByte();

 16. (Sect. 7) How do I delete a directory in Java?

     [*] JDK 1.0 did not support directory removal. JDK 1.1 supports
     directory removal with the method:
          public boolean delete() in class

     Make sure you don't have any open streams in the directory you're
     trying to remove. Do a close() on all streams, even if the underlying
     file is gone.

 17. (Sect. 7) How do I tell how much disk space is free in Java?

     [*] There currently aren't any good Java APIs for system introspection.
     There is no Java way to control processes, or look at system resources.
     You can use Runtime.getRuntime().exec() to do "df" on unix or "dir" on
     Windows right now.

     Alternatively, check out JConfig:
     JConfig is a cross-platform library that fills in many of the gaps in
     the core Java API, and makes it possible to work with files, processes,
     file types, video monitors, etc. in a much more Windows- and
     Mac-friendly manner.

 18. (Sect. 7) How do I get a directory listing of the root directory C:\ on
     a PC?

     [*] The obvious approach of calling File.list("C:\"); does not work.
     There are two reasons why this fails. First, slash is an escape
     character in Java, so if you want a literal slash, you have to repeat
     it. Second, you need to give the name of the directory, i.e. dot.
     Putting this together, either of the following calls will work




     Note: a file separator of "/" works just as well as "\" in most Windows
     programs and library calls. It is an artifact of DOS's origin's as a
     ripped-off port of CP/M. When Microsoft bought the rights to DOS from
     Seattle Computer Products (what, you didn't know Microsoft didn't
     develop DOS? They didn't even own the rights to DOS at the time the
     contracted with IBM to supply DOS for the PC) they were buying software
     which was an unauthorized port of the CP/M operating system.

     CP/M didn't have directories, so it didn't use pathname separators. The
     forward slash "/" was already used for giving options to CP/M commands,
     so "\" was pressed into service as the DOS pathname separator, but the
     DOS shell was taught to understood "/" for compatibility with other
     OS's. See also JConfig in Q6.15.

 19. (Sect. 7) What is the difference between the various ZIP formats: ZIP,
     GZIP, and PKZIP?

     [*] Zip is an archive file format, popularized on PCs, that contains
     multiple compressed files.
     GZIP comes from Gnu. It is essentially a one file subset of the Zip
     format. You can't put a whole directory into a GZIP archive, just a
     single file. It's intended for compressing a tarball of many files.
     PKZIP is a set of commercially available programs that create Zip
     files. All three use the deflate compression format, which is based on
     the LZ77 algorithm. This compression is also used by the ZLIB library
     and hence the PNG graphics file format (which uses ZLIB). PNG -
     Portable Network Graphics - provides a patent-free replacement for GIF
     and TIFF.

     An alternative compression technology, LZW compression, is encumbered
     by Unisys's patent. LZW is used in GIF files and by the Unix compress
     command. Luckily, as well as being free from patent restrictions, LZ77
     also gives better compression than LZW. LZW is the initial letters of
     the last names of the three computer scientists who developed the
     algorithm (Lempel, Ziv, Welch).

     The basic classes (all in that read LZ77 Zip format are
     Deflater and Inflater. These are used by the stream classes
     DeflaterOutputStream and InflaterInputStream. The classes
     GZIPInputStream and ZipInputStream inherit from InflaterInputStream.

     PKZIP is a commercial program for DOS, Windows, and OS/2, sold by
     PKWARE Their FAQ, at, specifically
          "Because PKWARE has dedicated the .ZIP file format to the public
          domain, it is possible for other people to write programs which

     The "other people" PKZIP's FAQ refers to is the InfoZIP project, a
     group of public-minded programmers spread over the world producing free
     software that works on most ANSI C compilers and platforms. See

     Jar files are in ZIP format, but are not as complete as a full
     filesystem archive format since file permissions are not saved. Some
     versions of WinZip are known to be inadequate for processing the full
     PKZIP format. Use InfoZIP instead.

 20. (Sect. 7) How can I use characters other than ASCII in Java?

     [*] Search for the article titled "Adding Fonts to the Java Runtime" or

     The article explains how to add fonts to Sun's JDK 1.2 using the file. [If anyone has summarised the information, please
     send it in].

 21. (Sect. 7) I did a read from a Buffered stream, and I got fewer bytes
     than I specified.

     [*] This is the way that BufferedInputStream works up to and including
     the current release. The behavior is so unintuitive that it really
     represents a bug. Javasoft has "resolved" the bug by writing comments
     in the program so that the broken behavior is in the range of legal
     outcomes. Ugh.

     When you instantiate a buffered input stream, you can specify the size
     of buffer it should use. Let's call this the internal buffer. When you
     call read() you can say how many bytes to read. Let's call this the
     request. If the request is smaller than the internal buffer and not a
     multiple of the internal buffer, then the last read returns only the
     odd bytes left in the internal buffer! The more reasonable and
     intuitive behavior would be for the internal buffer to be refilled, so
     that the whole request can be granted.

     For example, if you create a BufferedInputStream with an internal
     buffer of 1000 bytes, and try to read 512 byte chunks, your first read
     will return 512 bytes, but your second read will only return
     (1000-512), or 488, bytes. (Assuming that the file has at least that
     many bytes in it). The following code illustrates the problem.

     // troubleshooting by Tov Are Jacobsen
     class filebug {
         public static void main(String args[])
                       throws FileNotFoundException, IOException  {
         BufferedInputStream bis =
            new BufferedInputStream(
            new FileInputStream("test.txt"), 1000 );
         byte[] buf = new byte[2000];
         int numread;
         System.out.println( "Available: "+bis.available() );
         while (true) {
             numread =,0,512);
             if (numread<0) break;
             System.out.println( "got "+numread
                                +", avail:"+ bis.available());

     Of course, a valid reason for getting less than you asked for is that
     you asked for more data than is actually available in the Stream, e.g.
     you requested 512 bytes from a file that only contains 40 bytes. In
     general, there are no guarantees about how much data is returned for a
     given buffered input stream read request. To avoid this problem, push a
     DataInputStream on top of your buffered stream. Then you can call
     readFully(), which will do what you want.

     A similar "got less than I asked for" occurs when reading a socket.
     Network protocols frequently packetize data and send it across in
     bursts. Nothing is lost of course, and you are always told how many
     bytes you actually got. You will get the remaining bytes on a
     subsequent read. This happens regardless of the language used. Be sure
     to check the "amount of data returned" when using the read(byte[], int,
     int) method of BufferedInputStream, or when reading from a socket.

     Another problem with[], int, int) is that
     it catches and ignores IOExceptions. Instead, these exceptions should
     be passed on to the caller. Ace programmer Jef Poskanzer,,
     has a version to do this at See Jef's read() and
     readFully() routines.

 22. (Sect. 7) How do I redirect the System.err stream to a file?

     [*] You cannot assign a new FileOutputStream to System.err, as it is
     final. Instead use the System.setErr() library call, like this:

     FileOutputStream err = new FileOutputStream("stderr.log");
     PrintStream errPrintStream = new PrintStream(err);

     This was introduced with JDK 1.1. There is also a corresponding setIn()
     for redirecting standard in, and a setOut() for standard out.

     Note that you will get a compiler warning about a deprecated construct
     when you do this. PrintStreams are deprecated in favor of PrintWriters,
     which handle Unicode properly. The PrintStream is marked as deprecated
     by marking all its constructors as deprecated. There is no way to
     create the PrintStream needed to redirect System.err or System.out
     without triggering the deprecated warning.

 23. (Sect. 7) What are the values for the Unicode encoding schemes?

     [*] If you review the String constructor with this signature

         String(byte[] bytes, String encoding)

     you can see that one argument is a value for the encoding scheme that
     the conversion of 8-bit bytes to 16-bit Unicode chars is to use.

     There are three: "Unicode", "UnicodeBig" and "UnicodeLittle". The first
     one expects the first two bytes of your data to be a Byte Order Mark,
     FEFF or FFFE, which specifies whether the data is in little-endian or
     big-endian order. If there isn't a BOM but you already know the
     endianness, you can use "UnicodeBig" or "UnicodeLittle" directly.

     There is also a Sun document at
     with some related information (not much).

     There is another Sun document at which
     shows the table of encodings. There is a new system property called
     "file.encoding" which translates between codes known to Java like
     "Cp1252", and locale encoding names like "Windows Western Europe /

 24. (Sect. 7) Is there a way to read a char from the keyboard without
     having to type carriage-return?

     [*] You can do this in a GUI (e.g. in a text component). There is no
     pure Java way to do character-by-character I/O without using a GUI.


8. Core Libraries

  1. (Sect. 8) I can't seem to change the value of an Integer object once

     [*] Correct. Integer (Float, Double, etc) are intended as an object
     wrapper for a specific value of a number, not as a general-purpose way
     of shipping a primitive variable around as an Object. If you need that
     it's easy enough to create: class General { public int i; }

  2. (Sect. 8) How do I print from a Java program?

     [*] Use the Toolkit.getPrintJob() method

     Component c = this.getParent();
     while (c!=null && !(c instanceof Frame))
     // With a JComponent use   c=getTopLevelAncestor();

     PrintJob pj = getToolkit().getPrintJob((Frame) c, "test", null);
     Graphics pg = pj.getGraphics();

     This feature was introduced with JDK 1.1. A common place to put this is
     in the code that handles a button press. Printing from an untrusted
     applet is subject to a check from the SecurityManager.

     The JDK 1.1 printing API is more a screen hardcopy facility than a full
     blown publishing and illustration hardcopy API. JDK 1.2 offers a more
     full-featured printing API.

     If you simply want to print text, then write it to a file and print the
     file. Or open a filename that corresponds to the printer. On Windows,
     that is "LPT1" and the code looks like:

     try {
         FileOutputStream fos = new FileOutputStream("LPT1");
         PrintStream ps = new PrintStream(fos);
                 ps.print("Your string goes here");
     } catch (Exception e) {
         System.out.println("Exception occurred: " + e);

     The final formfeed is needed by windows to start the printjob.

  3. (Sect. 8) What are the properties that can be used in a PrintJob?

     [*] The properties are
        o awt.print.destination - can be "printer" or "file"
        o awt.print.printer - printer name
        o awt.print.fileName - name of the file to print
        o awt.print.numCopies - obvious
        o awt.print.options - options to pass to the print command
        o awt.print.orientation - can be "portrait" or "landscape"
        o awt.print.paperSize - can be "letter","legal","executive" or "a4"
     paperSize=letter, and numCopies=1.

     You can search for info like this by joining the Java Developer
     Connection (it's free) at

     and doing a search for "PrintJob".

  4. (Sect. 8) Is there any package in Java to handle HTML?

     [*] See the answer to Question 13.14.

  5. (Sect. 8) Why don't Dialogs work the way I want them to?

     [*] Modal dialogs (dialog windows that stay up until you click on them)
     are buggy in many browsers and in the 1.0.2 JDK. One bug is that the
     dialog is not necessarily put on top when it is displayed. Most of the
     modal dialog bugs are fixed in JDK 1.1.

  6. (Sect. 8) Where can I find information about the sun.* classes in the

     [*] You're not supposed to. Those classes are only to support functions
     in the java.* hierarchy. They are not part of the API, and won't be
     present in Java systems from non-Sun vendors. Some people have
     reverse-engineered the code and published an API for these classes but
     you use it at your own risk, and it may change without warning.

     Worst of all, those programs will not have the portability of true Java
     but will only run on Sun JDKs. For the same reason you shouldn't use
     classes outside the java.* packages when using JDKs from other vendors.

     If you still insist on going ahead, check these URLs:

  7. (Sect. 8) How do you read environment variables from with a Java

     [*] Environment variables are not used in Java, as they are not
     platform-portable. The Mac doesn't have environment variables, for
     example. A Windows 95 application not started from a DOS window does
     not have environment variables. Use properties instead. It was a design
     error in JDK 1.0 that programmers had to set the CLASSPATH environment
     variable. This should have been set in a property file

     Create your own properties file (see java.util.Properties) or specify
     them with the -D option when you invoke the interpreter or JRE.
     Additionally, on some systems you can set a property from the command
     invocation line like this:

     java -Dfoo=$foo MyClass (Unix)


     java -Dfoo=%foo% MyClass (Win95/NT)

     This sets the "foo" property to the value of the environment variable
     foo, and makes it available in the System properties. Make sure you do
     not leave any spaces after the -D or around the = sign. Inside the
     program you get the value with:

     String env = System.getProperty("foo");

     More simply, just put the environment variable on the command line and
     read it as arg[0].

     java MyClass %FOO% ; Win32
         java MyClass $FOO ; Unix

     Finally, you could execute a Runtime process to get the environment
     variables if you are on a platform that has them.

     import java.util.Properties;

     public class Main {
         public static void main(String[] argv) {
             Properties envVars = new Properties();

             try {
               envVars.load(   // use "set" on Windows
             } catch (Throwable t) {t.printStackTrace();}

             System.out.println("\n\n" + argv[0]
                              + " = <" + envVars.get(argv[0]) + ">");

     This is not a Pure Java approach as it builds platform-specific
     knowledge into the program. See Question 10.6 for more details. On
     Unix, the command that prints environment variables is "/usr/bin/env".
     On Windows95, it is "set"

  8. (Sect. 8) How do I get Java talking to a Microsoft Access database?

     [*] Use the JDBC-ODBC bridge. It is not especially challenging to set
     up, but it does require painstaking attention to detail. There is a
     step-by-step example in the van der Linden text "Just Java" mentioned
     in the sponsorship section of this document.

     Note that the Microsoft version of the Java kit does not support
     JDBC-ODBC access because it uses a non-standard native code interface.
     The JDBC FAQ can be found at

  9. (Sect. 8) I can't seem to change the current working directory.

     [*] Correct. This missing functionality is an oversight that we hope
     will be corrected in the future. The bug id is 4156278, please feel
     free to join the JDC, and vote to have this (or any other) fixed.
     Changing the user.dir property merely changes the text property, not
     the underlying reality that it is supposed to reflect.

     There are several workarounds.
        o Run your java app from a .bat or .sh file and do the "cd" in that
          (before you run your java app), assuming that all the external
          processes you need to exec can be run from the same directory.
        o Do: exec("cd /home/wherever; externalApp.exe") on unix, (there
          doesn't seem to be an equivalent on NT).
        o Instead of running the .exe directly, run (or write on the fly) a
          .bat or .sh file that does the cd and then runs the .exe for you
          (this could well create trouble with getting back the correct
          return status)

 10. (Sect. 8) How do I create a Vector of ints?

     [*] ints are primitive types and hence can't be stored by the Vector
     class, which stores objects. You'll need to wrap the ints. Try this:

     int i =7;
     Vector holdsInts = new Vector(5,1);

     holdsInts.addElement(new Integer(i));
     int j = ((Integer)holdsInts.elementAt(0)).intValue();

 11. (Sect. 8) I have several worker threads. I want my main thread to wait
     for any of them to complete, and take action as soon as any of them
     completes. I don't know which will complete soonest, so I can't just
     call Thread.join on that one. How do I do it?

     [*] You need to use the wait/notify mechanism to allow any of the
     worker threads to wake up your main thread when the worker has

 12. (Sect. 8) How do I get random numbers?

     [*] If you just need a quick random double between 0.0 and just less
     than 1.0

     double myrandom = Math.random(); // [0,1)

     The notation "[0,1)" is common math notation for "zero to .9999999 etc"
     The Sun documents say this returns 0.0 to 1.0, but inspection of the
     source shows they are wrong. However, due to the inherent inaccuracies
     of floating point arithmetic, multiplying N by 0.999999 etc can result
     in an answer of N, not N * .999999. So watch out when N is big.

     JDK 1.2 adds another version of nextInt that accepts a parameter for
     returning ranged random numbers.

     Where things get trickier is when you use JDK 1.1 and want an int in a
     certain range, say 1 to 6 to simulate the throw of a die or 1 to 52 to
     represent a playing card. Class Random has a nextInt method that will
     return any integer:

     import java.util.Random;
     Random r = new Random();
     int i = r.nextInt();

     However, that has an (almost) 50% chance of being negative, and it
     doesn't come from the right range. So you just take the abs() value and
     then mod it into the right range:

     int dice_throw = 1 + Math.abs(i) % 6;

     Except, the abs() method fails gracelessly in the presence of the
     Integer.MIN_VALUE (it returns the same, negative, result!). So it is
     better to AND to get the non-negative value: In general, to get a
     random int between high and low limits (inclusive):

     java.util.Random r = new java.util.Random();
     int j = (r.nextInt() & Integer.MAX_VALUE) % (high-low+1) + low;

     The sentence above states "(almost) 50% chance" because there is one
     more value in the negative integers than in the positive integers in
     two's complement arithmetic as used by Java. For most purposes, the
     bias introduced will be insignificant, but we "and" the nextInt() to
     convert it to zero. Sure, it's unlikely to occur, but you don't want
     the reactor going critical just because you missed this case while

     A worse problem is that with the algorithm used, the low order bits are
     significantly less random than the higher order bits. And the low order
     bits are precisely the ones you get when you do a (mod 2^n) operation.
     You may consider using which provides much
     better randomness by using a Cryptographic hash, although it is much
     more expensive to compute.

 13. (Sect. 8) What does "deprecated" mean? I got this in a compiler error

     [*] The compiler will flag a now-obsolete API as "deprecated". The word
     means "officially disapproved of". Compile again with the
     "-deprecation" option to see what is deprecated. In almost all cases,
     the old API has been replaced by a new one. Update your code to use the
     new one.

     An example of using a deprecated API is calling component.size(). That
     has been replaced by component.getSize().

 14. (Sect. 8) Where/why should I use the Enumeration interface?

     [*] It's a very convenient way to step through some of the library data
     structures, such as HashTable, Vector, and ZipFile. It is thread safe.
     If you're looking at an element in one thread while another thread is
     trying to delete it, it won't half vanish.

     Here's how you might look at every file in a ZIP file:

     ZipFile z = new ZipFile("");
     Enumeration e = null;
     for (e=z.entries(); e.hasMoreElements(); ) {
        ZipEntry ze = (ZipEntry)e.nextElement();
        System.out.println("got " + ze.getName() );

     And here's how you might look at all the tokens in a String:

     StringTokenizer tokens =
         new StringTokenizer (SomeArbitraryString, "\n", false);

     Enumeration enum = tokens.elements();
     while enum.hasMoreElements()

     You should look for opportunities in your own data structures to
     implement Enumeration anywhere where the structure has repeated

 15. (Sect. 8) Which version of WinZip is compatible with

     [*] You need WinZip version 6.2 or later. Version 6.1 or earlier is not
     good enough. WinZip can be downloaded from The pkzip software works fine.
     Infozip is better than WinZip because it lacks the winzip "feature" of
     failing to recreate directories unless given a special option. Use

 16. (Sect. 8) How can Java access other fonts on my system?

     [*] You do it by editing the fontnames in the file in
     the lib directory of your JDK release. Watch out for program
     portability when you do this.

     For more details, check the website and the JavaSoft site
     They have lots of information on this.

 17. (Sect. 8) How can I trap Control-C in Java?

     [*] Control-C is used on some OS's to break into a running program
     interactively and terminate it. On Unix Control-C is sent to the
     process as a signal. If a C program declares a handler for that signal,
     the program will be able to continue even if Control-C is sent to it.

     Control-C is not a Java concept, and there is no way to do this
     On Unix you can write the signal handler in C, and impose the handler
     by calling the C routine through the Java Native Interface.
     On Win32 in a GUI a KeyPressedListener will see a control-C as
     On Win32 in a command line app you can't read a control-C in Java
     because it is sent to the process as a signal (as with Unix).
     Instead of interposing a signal handler thru JNI, you could launch java
     from perl or a shell script which is capable of doing this.
     Alternatively, use javaw to launch the JVM so that it has no keyboard
     which can ctrl-C it.

 18. (Sect. 8) How do you parse commandline arguments to a Java program?

     [*] Perhaps surprisingly, commandline arguments are not encouraged, as
     they make a program not 100% pure Java. The reason is that some systems
     like MacOS don't normally have a command line or command-line
     arguments. Consider using properties instead so that your programs fit
     more naturally into the environment.

     If you must use command-line arguments, have them comply with the POSIX
     conventions, i.e. they should
        o use a "-" instead of a "/"
        o be case sensitive, and
        o be concatenatable (-pst == -p-s-t).

     If such arguments are used, it should NOT be up to the invocating
     application (ie java.exe) to propperly parse them. According to the
     getopts routine from gnu, getopts is required to know exactly what the
     usable parameters are before it can properly parse them and create

     Because of that, it becomes unduly hard for calling programs such as
     java.exe to properly parse class arguments by themselves. Instead, they
     should create an argc/argv pair (actually a java specific array
     dependant on the way in which the method is called) where the number of
     elements equals the number of text words in the options. Text word
     means a group of ascii characters outside of quotation marks, not
     separated by white space. Once these text words are passed to the
     class, it is up to the class to actually parse them correctly.

     The text words passed to the class can be parsed however the class
     author wishes. It is up to the author to correctly parse the options.
     There are several implementations of getopts which perform such
     parsing, in C, Perl, and Java. You will find one at

 19. (Sect. 8) How many threads can I create?

     [*] By default, on a Win32 machine, each ("native") thread gets one MB
     (1024*1024 bytes) of stack space reserved for it. The memory is not
     actually allocated. If it were, you'd run out of memory with dozens of
     threads instead of hundreds. The space needs to be reserved as a
     contiguous block of memory so the stack can grow as needed.

     Under Win32, the max amount of memory is 2GB, which corresponds to
     2,000 threads each reserving 1MB of memory. That's an upper bound; in
     practice you'd run out of other OS resources before running out of
     memory. Each thread has some system resources that will be used to
     manage the thread -- register storage area, thread-local storage, state
     information, exception condition info, etc.

     You should design your programs so that you use threads only where
     necessary. It's almost always a mistake to have one thread per object.
     Successful programs are much more likely to have 5 threads (or fewer)
     than 500.


9. Dates and Times

     Credit to Paul Hill who completely rewrote this section, and to
     the programmers at IBM who implemented much of the Java Date code,
     and reviewed this section of the FAQ for accuracy.


  1. (Sect. 9) Is Java "Year 2000"-compliant?

     [*] Java is Y2K compliant in release JDK 1.1.6 and later. See Prior to this release there were
     certain corner case bugs that had to be fixed.

     The Date class, as you can see from the discussion, contains more than
     enough resolution to represent dates in this century and the next and
     the last. The SimpleDateFormat when parsing a 2 digit year could cause
     problems; see discussion below.

  2. (Sect. 9) What happen to java.util.Date between JDK 1.0 and JDK 1.1?

     [*] In JDK 1.1 the java.util.Date class was split to provide better
     support for timezones, and internationalization.

     The classes specifially related to dates are summarized below:

            1. The class Date represents a specific instant in time,
               with millisecond precision.
            2. The class TimeZone is an abstract class that represents
               a time zone offset, and also figures out daylight
               savings time adjustment.
            3. The class SimpleTimeZone is the only concrete subclass
               of TimeZone in the JDK.  It is what defines an ordinary
               timezone with a simple daylight savings and daylight
               savings time period.
            4. The class Calendar is an abstract class for converting
               between a Date object and a set of integer fields such
               as year, month, day, and hour.
            5. The class GregorianCalendar is the only concrete
               subclass of Calendar in the JDK. It does the
               Date-to-fields conversions for the calendar system in
               common use.
            6. The class DateFormat is an abstract class that lets you
               convert a Date to a printable string with fields in the
               way you want (e.g. dd/mm/yy or dd.MMM.yyyy).
            7. The class SimpleDateFormat is the only concrete subclass
               of DateFormat in the JDK. It takes a format string and
               either parses a string to produce a date or takes  a
               date and produces a string.

     At least one critic has used the term "baroque" when describing the
     complexities of the Java date related classes, but other critics would
     spell that "broke". The good news is that as of JDK 1.2 all of the
     common problems have been corrected and many of the bugs were corrected
     in 1.1.4 and 1.1.6. Even in 1.1.1, you can avoid most of the most
     annoying bugs by always keeping in mind which timezone each class is

  3. (Sect. 9) Exactly what is a java.util.Date?

     [*] A java.util.Date stores a moment in time as a long integer
     representing the number of milliseconds since 00:00:00 Jan 1, 1970 UTC
     (Coordinated Universal Time). This zero moment is known as the "Epoch".
     This is the same Epoch as is used on Unix systems. Dates earlier than
     the Epoch are represented as negative numbers, counting away from

     The scheme is sufficient to represent dates from 292,269,053 B.C. to
     292,272,993 A.D. (64 bits covers -9,223,372,036,854,775,808 to
     +9,223,372,036,854,775,807 milliseconds. But note that prior to JDK
     1.2, a GregorianCalendar will not accept values earlier than 4713 B.C.

     A java.util.Date is the light-weight object intended to just hold a
     millisecond value. It is used to hold, communicate and store a moment
     in time. Other tasks like creating a formated string, or calculating
     dates are best done using other classes.

  4. (Sect. 9) Does a java.util.Date really represent the true UTC?

     [*] No, but it is close enough for most human time-keeping purposes. On
     most computers, it only represents the time since the epoch as
     converted from the value on the underlying hardware. If you have
     hardware that is synchronized with an atomic clock your time is UTC;
     most hardware assumes a day is 24 hours long, but there have been more
     than 20 leap seconds added to UTC, since the first one was added in

  5. (Sect. 9) How do I create a Date object that represents the current

     [*] The default value of a date object is the current time, so the
     following code creates a date object that contains the current time.

     Date now = new Date();

  6. (Sect. 9) I want to create a string that represents a date in a format
     other than what is returned by java.util.Date.toString() do I have to
     use a Calendar?

     [*] No. Instead of creating a Calendar and pulling out all of the
     appropriate fields and making a string, you could use
     SimpleDateFormat.format() to create a string.

  7. (Sect. 9) Why are all the methods in java.util.Date deprecated?

     [*] Mostly because the original java.util.Date was not completely aware
     of the timezone and "not amenable to internationalization". To make it
     timezone aware and internationalizable would have required adding some
     of the functionality which can now be seen in java.util.Calendar and
     some of the functionality in java.util.DateFormat. If you find the
     combination all of the date related classes complex, just be glad they
     were separated into different classes.

  8. (Sect. 9) I really don't need a internationalizable, timezone aware,
     extra flexible formating set of date classes. Is there anything else I
     can use that stores only a date, and allows me to do some date

     [*]You could consider using the BigDate class written by Roedy Green,
     and available in his very informative glossary (search for BigDate). If
     you want to store the result in a database as a Date or TimeStamp, you
     should read the section below on java.sql.Date.

  9. (Sect. 9) Since the Date( String ) constructor is deprecated what do I
     use instead?

     [*] The best choice is to use SimpleDateFormat.parse() to create a
     java.util.Date object.

     The Date constructor that accepts a string calls Date.parse( String ).
     The Date.parse() function had its own rules for converting 2 digit
     years (it used a 1980 pivot date) and other limitiations which makes it
     of limited value. Other "features" of Date.parse() that are not
     supported in SimpleDate have not been missed by many developers.

 10. (Sect. 9) Since Date(int year, int month, int date) and related
     constructors are deprecated what do I use instead?

     [*] The constructor GregorianCalendar(int year, int month, int date) is
     the newer replacement. Other choices are the Calendar.set( year, month,
     day ) method. Note that the year in the GregorianCalendar starts at 1
     A.D., not at 1901 like the old Date constructor.


 11. (Sect. 9) How can I see if my JVM is using the right timezone?

     [*] The following codes displays the ID of the current default

       System.out.println( TimeZone.getDefault().getID() );

 12. (Sect. 9) The value of TimeZone.getDefault is not what I expected. What
     is the problem?

     [*] The value of the default timezone is based on the value of the
     system property "user.timezone". The JVM  is supposed to set this
     value. In releases such as JDK 1.1 the value of user.timezone was often
     not set to anything, so TimeZone.getDefault() used its own built in
     "fallback" value (the default when there is no default value). In later
     JDK 1.1 releases and in JDK 1.2 the setting of the value of
     user.timezone is much better and the "fallback" value is now GMT
     (Greenwich Mean Time). Up until JDK 1.1.3, the fallback value was "PST"
     (North American Pacific Timezone).

 13. (Sect. 9) Do all the standard objects use the same default timezone?

     [*] Not until JDK 1.2. In JDK 1.1, Date.toString() and Calendar used
     the value of TimeZone.getDefault() which could often be undefined (see
     the previous question). In JDK 1.1, The Calendar in a SimpleDateFormat
     was set to the 1st timezone resource for the locale (for the US this is

     System.out.println( "Date format TZ = " + TimeZone.getDefault().getID() );
     sdf = DateFormat.getDateTimeInstance( DateFormat.LONG, DateFormat.LONG );
     System.out.println( "Date format TZ = " + sdf.getTimeZone().getID() );
     Calendar cal = Calendar.getInstance();
     System.out.println( "Calendar TZ = " + cal.getTimeZone().getID() );

     When run on a system running JDK 1.1.6, NOT in the North American
     Pacific Time nor in the GMT timezone results in:

     Timezone default = GMT
     Date format TZ = PST
     Calendar TZ = GMT

     This example shows two bugs, the value of user.timezone is undefined,
     so its defaulting to GMT (see discussion of TimeZone.getDefault()) and
     it shows that the DateFormat depends on the 1st locale entry which in
     this case is PST.

     If you don't want the DateFormat to use the Locale timezone, see the
     code provided below.

 14. (Sect. 9) If I explicitly set the default timezone, don't I need code
     to choose between (a) the daylight savings version or (b) the standard
     version of a timezone?

     [*] No. The ID that you use to select a timezone with
     TimeZone.getTimeZone refers to a predefined timezone that contains
     daylight savings information (when applicable). For example, the
     following code selects the timezone used in New York, USA.

     // Get the North American Eastern Time definition.
     TimeZone theTz = TimeZone.getTimeZone( "EST" );
     // get a really detailed date format and set it to the right timezone
     DateFormat df = DateFormat.getDateTimeInstance( DateFormat.LONG,
     DateFormat.LONG );
     df.setTimeZone( theTz );
     // create a date in the locale's calendar, set its timezone and hour.
     Calendar day = Calendar.getInstance();
     day.setTimeZone( theTz );
     day.set( 1998, Calendar.FEBRUARY, 1 );
     day.set( Calendar.HOUR, 12 );

     // print that date/time and
     // that date/time 150 full days of milliseconds later.
     System.out.println( df.format( day.getTime() ) );
     System.out.println( df.format(
     new Date( day.getTime().getTime() +    // get the millis
             150L*24*60*60*1000L ) ) );     // add exactly 150 days of millis

     Results in:

     February 1, 1998 12:00:00 PM EST
     July 1, 1998 1:00:00 PM EDT

     Notice that this example selected something refered to as "EST", but
     that this TimeZone was aware of the daylight savings time change and
     either printed as "EST" or "EDT".

     The confusion is reduced in JDK 1.2, you can use longer TimeZone IDs
     each maps to its own set of text resources. For example the following
     IDs are 5 hour West of GMT and have various DST rules:
     "America/Nassau", "America/Montreal", "America/Havana",
     "America/Port-au-Prince", "America/Grand_Turk", "America/New_York" and

     You can look at a list of other timezone names and offsets in the file

 15. (Sect. 9) How do I create my own Time Zone to apply to dates?

     [*] You can create a TimeZone object with the GMT offset of your
     choice. The following code creates British Time, a timezone that was
     not defined in 1.1.

     britTime = new SimpleTimeZone(0*ONE_HOUR, "Europe/London" /*GMT/BST*/,
         Calendar.MARCH,  -1, Calendar.SUNDAY /*DOW_IN_DOM*/, 1*ONE_HOUR,
         Calendar.OCTOBER,-1, Calendar.SUNDAY /*DOW_IN_DOM*/, 1*ONE_HOUR,

     TimeZone.setDefault( britTime );

     Or you can then apply that TimeZone to a particular Calendar object
     like so:

     Calendar myCal = Calendar.getInstance();
     myCal.setTimeZone( britTime );

     If you are running 1.2, You can choose this existing timezone as the
     default with the code:

     TimeZone.setDefault( TimeZone.getTimeZone( "Europe/London" ) );

     Note that BST is defined from JDK 1.1 and later, but it is Bangladesh
     Standard Time. For a longer example of creating and testing the British
     timezone, Tony Dahlman provides a nice example in his

 16. (Sect. 9) How do I create the BST timezone specifically?

     [*] You can create an arbitrary TimeZone object with the code below. In
     most or all other timezones, daylight savings time is handled
     automatically and internally. But GMT is the reference for all other
     timezones, and so does not have the summertime update applied
     automatically. The rules for BST can be found at

     Here is the code to create a British Summer Time timezone that is
     offset one hour from GMT between two dates:

     import java.util.*;
     import java.text.*;
     // create a BST timezone (code courtesy of Tony Dahlman).

     public static GregorianCalendar setBritSummTime(String zoneName){
        // Set up the default GMT0BST time zone
        SimpleTimeZone bst_tz =
           new SimpleTimeZone( 0,   // no offset from GMT
                        zoneName,   // individualized tz id
         // last Sun Mar 1AM
         // last Sun Oct 2AM

        // Apply TimeZone to create a Calendar object for UK locale
        return (new GregorianCalendar(bst_tz,Locale.UK) );

     and here is how you would print out values using BST:

     // create a template for printing the date
     DateFormat df = DateFormat.getTimeInstance(

     // tell the template to use BST tz.
     System.out.println("Using British Summer Time "
                       +"the time is: "
                       + df.format( BritishSummerTime.getTime() ) );

     // Now get and compare with current time in GMT
     df.setTimeZone(TimeZone.getTimeZone("GMT") );
     System.out.println("\nCurrent time in GMT is: "
                       + df.format(BritishSummerTime.getTime() ) );

     In the winter, this BST zone is aligned with GMT; in the summer it is
     one hour later (4 a.m. GMT is 5 a.m. BST).

     You can look at a list of timezone names and offsets in the file

                java.util.Calendar and java.util.GregorianCalendar

 17. (Sect. 9)How do I a create a specific date in the Gregorian Calendar?

     [*]If you have a Date use:

     myCal.setTime( myDate );

     If you have a set of integers representing the year, month and day of
     month use:

     Calendar myCal = Calendar.getInstance();
     myCal.set( 1998, Calendar.MARCH, 15 );

     Note: Months start with January = 0!

 18. (Sect. 9) How do I use a GregorianCalendar to extract a few fields from
     a Date?

     [*]The following code shows how to get some fields from a Date.

     Calendar g = Calendar.getInstance();
     g.setTime( aDate );
     int year = g.get( Calendar.YEAR );
     int mon = g.get( Calendar.MONTH );
     int date = g.get( Calendar.DATE );
     mon++;             // in class Calendar & GregCal, months run 0-11 ;-(
     System.out.println( mon + "/" + date + "/" + year);

     If you want to build a string that has a formated date consider using

 19. (Sect. 9) Some people use Calendar.getInstance() while others use new
     GregorianCalendar(). Which one is the correct way to get a Calendar?

     [*] Either way is correct, it depends on what you want to be able to
     do. You should use Calendar.getInstance(), if you want your code to be
     ready when the loading of other Calendars are added to the JDK and some
     other calendar is the default for the locale. A particular locale might
     have configured a Hebrew or Islamic Calendar as the default calendar
     and you might want a user to enter a date in his own Calendar, i.e.
     1-Jan-2000 (Gregorian) = 23-Tevet-576 (Hebrew) = 24-Ramadan-1420
     (Islamic). If you really are trying to place a particular Gregorian
     date, i.e. 4-July-1776, into a Date object, you might as well create a
     GregorianCalendar directly.

 20. (Sect. 9) I changed a field using Calendar.set() and then I checked
     another field. Its value is inconsistent with the value I just set.
     What is going on?

     [*]In JDK 1.1.0 the Calendar class did not update all of its fields
     until you called getTime to retrieve the Date that corresponds to the
     fields in the Calendar. To get the earlier version of the Calendar to
     "turn the crank" and calculate all fields you can use the trick:

     myCal.setTime( myCal.getTime() )     // pull the date out and put it back

 21. (Sect. 9) When I create the date July 4th, 1776 using new
     GregorianCalendar( 1776, 7, 4 ) the month is off by one. What is the

     [*]You need to be be aware that months start with January equal to 0. A
     better way to create that date would be:

     independanceDayUSA = new GregorianCalendar( 1776, Calendar.JULY, 4 );

 22. (Sect. 9)Why aren't there constants for milliseconds per day, week or

     [*]The short answer is: these values are not constants. While some date
     calculations would find these useful, it is important to remember that
     in areas with daylight savings time rules, there are two days per year
     that are not 24 hours long, therefore not all weeks are the same length
     (2 out of 52). Also, because of leap years, not all years are the same

     If you adding values to a calendar consider using either add or roll;
     for example:

     myCal.add(Calendar.YEAR, 1 );  // get a value 1 year later.

 23. (Sect. 9) By my count the week of the Year is off by one. What is the

     [*]The GregorianCalendar class uses the value set by
     setMinimalDaysInFirstWeek() to determine if the fractional week at the
     beginning of the year should be week 1 or week 0. If you don't change
     it, any fractional week could be week 1, depending on the value defined
     for the locale.
 24. (Sect. 9) What timezone does a calendar use when I don't explicitly set

     [*]The Calendar uses the TimeZone.getDefault() (see discussion under

 25. (Sect. 9) Should I stop using Date all together and just use Calendar?

     [*] Probably not. The Calendar class is a much larger than a Date
     object. Many other interfaces in standard APIs are defined using a Date
     object. Use Date objects to hold, store or communicate a date-time
     value. Use a Calendar object to manipulate a date-time value.

 26. (Sect. 9) The GregorianCalendar will not accept a date prior to 4713
     B.C. Why?

     [*]January 1, 4713 B.C. is the "epoch" date for the Julian Day calendar
     which was invented in the 16th century by the Joseph Justus Scaliger.
     "[T]he Julian day calendar, ... does not use individual years at all,
     but a cycle of 7980 astronomical years that counts a day at a time,
     with no fractional days, no mean year, and no leap years. He came up
     with his number by mulitplying three chronological cycles: an 18-year
     solar cycle, a 19-year lunar cycle, and the 15-year indication period
     used by Romans. All three cycles began together at the same moment at
     the start of the "Julian cycle. ... [This] Calendar lives on among
     -- David Ewing Duncan, "Calendar", Avon Books, 1998; p 207

     Note that the Julian Day calendar is not the same as the Julian
     calendar. The Julian Calendar is named for Julius Caesar. The Julian
     Calendar was used in the Europe from what we now call January 1, 45
     B.C. until at least October 4, 1582 and is still used today by the
     Eastern Orthodox Church to date holidays.

     The limitation on dates prior to 4713 BC has been dropped in JDK 1.2.

 27. (Sect. 9) The Calendar class is said not to handle certain historical
     changes. Can you explain some of the limitations?

     [*]The date of change from the Julian to the Gregorian calendar depends
     on where you lived at the time. The date can vary from 1582 (most
     Catholic countries, which of course followed the Papal edict) to 1949
     (China). The date of the cutover from using the Julian Calendar (leap
     years ever 4 years) to using the Gregorian Calendar (every 4 years,
     except every 100 unless divisable by 400) is controlled by the method

     It is also the case that January 1 was not always the beginning of the
     year. January 1 was standardized by Julius Caesar in 45 B.C. and Pope
     Gregory XIII in 1582, but others who used the Julian Calendar between
     those dates used other dates for New Years Day. (Anyone who has ever
     been involved in a standardization effort may find it interesting that
     neither an emperor nor a pope could actually complete the
     standardization effort).

     The Calendar class uses a TimeZone which does not handle historical
     changes, i.e. the SimpleTimeZone contains only two dates the "spring
     forward" and "fall back" dates and a date that the DST starts (see
     SimpleTimeZone.setStartYear() ). If the local definitions have changed
     then a date/time may not accurately reflect the historical local time.

     As noted above, the Date object does not usually include leap seconds,
     unless your hardware includes leap seconds.

     While the Calendar class is more than useful for international
     business, it may not be what you want for doing UTC timebased
     calculations or historical dates and times without a more careful
     analysis of its design limits.

               java.text.DateFormat and java.text.SimpleDateFormat

 28. (Sect. 9) How do I convert a date in one format to another?

     [*] The following code illustrates the technique:

     import java.text.*;
     public class DateTest {
       public static void main( String[] args ) {
         SimpleDateFormat df1 =
           new SimpleDateFormat("yyyy-MM-dd hh:mm:ss.S");
         SimpleDateFormat df2 =
           new SimpleDateFormat("dd-MMM-yy");
         String startdatetime = "1998-09-09 06:51:27.0";

         try {
           System.out.println("Date is " +
             df2.format( df1.parse(startdatetime) ));
         } catch (ParseException pe) {
           System.out.println("ParseException " + pe );

     When run, the program outputs "Date is 09-Sep-98"
 29. (Sect. 9) How do I use DateFormat to parse a string containing a date?

     [*] The easiest way to parse a date that is in a known format is to use

     DateFormat df = new SimpleDateFormat( "HH:mm" );
     df.setTimeZone( TimeZone.getDefault() ); // if using JDK 1.1 libraries.
     df.setLenient( false );                  // to not allow 26:65 etc.
     Date lateLunchOnDayZero = df.parse( "12:30" );
     System.out.println( lateLunchOnDayZero );

     The above code would result in (when in the MST timezone).

     Thu Jan 01 12:30:00 MST 1970

     To parse other date and time fields, refer to the SimpleDateFormat

 30. (Sect. 9) How do I use a DateFormat to create a text string from a

     [*] The easiest way to create a string from a date is to use a
     SimpleDateFormat.format(). The following code illustrates how this can
     be done.

     DateFormat df = new SimpleDateFormat( "yyyy.MMM.dd HH:mm:ss.SSS z" );
     df.setTimeZone( TimeZone.getDefault() ); // JDK 1.1
     System.out.println( df.format( d ) );    // where d is a Date

     For other possible fields from the calendar, see the document for

 31. (Sect. 9) What timezone does a SimpleDateFormat use when I don't
     specify one?

     [*]In JDK 1.1, the SimpleDateFormat uses the first timezone defined for
     the locale. In JDK 1.2, it uses the default timezone. See the
     discussion above on how this differs from the Calendar class).

 32. (Sect. 9) I'm not yet using JDK 1.2 and I don't want the DateFormat to
     use the 1st timezone for the locale. How do I change the timezone in a
     SimpleDateFormat to use a different timezone?

     [*] The following code sets the timezone of a DateFormat to the current

     DateFormat df = DateFormat.getDateInstance();

     or to set it to a timezone of your chioce.

     df.setTimeZone(TimeZone.getTimeZone( "MST" ) ) // Mtn Time, Denver USA

 33. (Sect. 9) What century is assumed when I use a two digit year in a
     SimpleDateFormat string?

     [*]In JDK 1.1, the default start for the century used by
     SimpleDateFormat for 2 digit years is 80 years before the current date.

     This means that in 1998: 1 = 2001, 2 = 2002, ... 17 = 2017, 18 = 2018,
     19 = 1919, 20 = 1920, ... 98 = 1998, 99 = 1999,

     In JDK 1.2 you can change this "default century start date" with the
     method set2DigitYearStart( Date) and get its current value
     with the method get2DigitYearStart(). One thing to note is that since
     set2DigitYearStart takes a date not a year, you can have your default
     century begin at any day or hour.

     When running under JDK 1.1, it is probably best to avoid two-digit year
     fields, when the dates entered could possibly fall outside of the range
     -- now less 80 years and now plus 20 years. If you want to allow
     two-digit year fields in JDK 1.2 and beyond, consider setting the
     2DigitYearStart property to something appropriate,  For example, set it
     to today, when all dates to be entered are in the future (i.e. an
     expiration date), or set it to today less 100 years, when the value is
     always in the past (i.e. birthdate, death date).

 34. (Sect. 9) Does the above mentioned limitation of 2 digit years in JDK
     1.1 mean that java.text.SimpleDateFormat is not Y2K compliant?

     [*] No. It means that any code you write that (1) allows the entry of 2
     digit years and (2) does not make sure they are in an appropriate
     century, would not pass a careful Y2K analysis. This code was put here
     so you could sensibly read old files with non-Y2K compliant dates, not
     so you could create new ones. Once you are using JDK 1.2 it is better
     to set the 2DigitYearStart property to something appropriate for any
     two-digit year field which you are parsing.

                       java.sql.Date and java.sql.TimeStamp

 35. (Sect. 9) What timezone does a use when converting to an
     SQL DATE?

     [*]This is another hidden use of the default java.util.TimeZone. If you
     have carefully set every timezone in every Calendar and DateFormat you
     are using, but you don't set the default in java.util.TimeZone when a
     java.util.Date is converted to a java.sql.Date you may not end up with
     the value you expected in your database.

 36. (Sect. 9) When I print a jave.sql.Timestamp it doesn't include any
     milliseconds. What is the problem?

     [*] If you print the java.sql.Timestamp directly you will see this
     problem. The following code demonstrates this surprising behavior.

     // incorrect use of java.sql.Timestamp
     DateFormat df = new SimpleDateFormat( "MM/dd/yy hh:mm:ss.SSS a" );
     df.setTimeZone( TimeZone.getDefault() );    // needed in JDK 1.1

     java.sql.Timestamp t = new java.sql.Timestamp( 94, Calendar.JANUARY, 1,
             13, 45, 59, 987654321 );
     System.out.println( df.format( t ) ) ; // Wrong! no fractions of a second.

     The results of the above code are:

     01/01/94 01:45:59.000 PM

     The above code is using whatever is in the super class (java.util.Date)
     and assumes all of those parts are filled in. java.sql.Timestamp could
     have stored the whole milliseconds in the millisecond part of a
     java.util.Date, and stored the nanoseconds that are not whole
     milliseconds in an additional field. They chose to ignore the fractions
     of a second in the java.util.Date and put all fractional parts in an
     additional nanosecond field.

     The following code shows how to convert a java.sql.timestamp to a

     Date d = new Date(t.getTime() + (t.getNanos() / 1000000 ));
     // 1 Milli = 1x10^6 Nanos
     System.out.println( df.format( d ) ) ; // Right! At least we have the millis

     The result of the above code is a better approximation of the timestamp

     01/01/94 01:45:59.987 PM

 37. (Sect. 9) How do I calculate the number of days between two dates?

     [*] There is no API for this (there should be), but you can calculate

         static final long ONE_HOUR = 60 * 60 * 1000L;

         Calendar earlierDate = new GregorianCalendar();
         Calendar laterDate = new GregorianCalendar();

         earlierDate.set(1997, 1, 5, 0, 0, 0);    // FEB!! 05, 1997
         laterDate.set(1998, 1, 5, 0, 0, 0);      // Feb 05, 1998

         // the first getTime() returns a Date, the second takes
         // that Date object and returns millisecs since 1/1/70.
         // The API has misleading and horrible naming here, sorry.
         long duration = laterDate.getTime().getTime() -

         // Add one hour in case the duration includes a
         // 23 hour Daylight Savings spring forward day.
         long nDays = ( duration + ONE_HOUR ) / (24 * ONE_HOUR);
         System.out.println("difference in days: " + nDays);

     Note: this method works only if the two times are both created to be
     exactly midnight. Otherwise you might end up finding that for example,
     11 PM tonight is 0 days away from 6 AM tomorrow, instead of 1 day as
     many applications expect. This is because 24 hours haven't elapsed
     between 11pm on one day and 6am on the next.

     Or, use int julian = myCalendar.get(Calendar.DAY_OF_YEAR); and
     subtract, making sure to subtract the years too. Alternatively, use
     BigDate at

     For the ultimate in generality, you could get ACM's Collected Algorithm
     199, recode it in Java (takes about 30 minutes), compute the Julian
     date for each end point, and subtract the two numbers. Here is the code
     in Java to get the GMT Julian day number and a "local Julian Day" that
     leverages the existing algorithms in the GregorianCalendar.

     import java.util.*;
     import java.text.*;

      * This Calendar provides Julian Day and "Calendar Day" methods,
      * The Calendar Day starts at _local_ midnight.
      * @author Paul A. Hill
      * @version 1.1
      * @since JDK 1.1.7 3/99
      * @see java.util.Calendar
     public class FAQCalendar extends GregorianCalendar implements Cloneable {
          ** this constant and the ones that follow are actually private
          ** in the GregorianCalendar, so we'll redefine them here.
         public static final long
             ONE_SECOND  = 1000,
             ONE_MINUTE  = ONE_SECOND * 60,
             ONE_HOUR    = ONE_MINUTE * 60;
          * this next constant and the others with it have their uses,
          * but watch out for DST days and the weeks that contain them.
          * Note also that 1/1/1970 doesn't start on the first day of the week.
         protected static final long
             ONE_DAY     = ONE_HOUR * 24,
             ONE_WEEK    = ONE_DAY  * 7;

         /** The number of days from Jan 1, 4713 BC (Proleptic Julian)
          ** to 1/1/1970 AD (Gregorian).  1/1/1970 is time 0 for a java.util.Date.
         public static final long JULIAN_DAY_OFFSET = 2440588L;

          * Same as GregorianCalendar(). Creates a calendar with the time set to
          * the moment it was created.
     Note: for Brevety I have not provided all of the other
          * constructors that you can find in GregorianCalendar.
          * @see java.util.GregorianCalendar#GregorianCalendar
         public FAQCalendar() {

          * A calendar day as defined here, is like the Julian Day but it starts
          * and ends at _local_ 12 midnight, as defined by the timezone which is
          * attached to this calendar.  This value is useful for comparing
          * any date in the calendar to any other date to determine how many days
          * between them, i.e. tomorrow - today = 1
          * @return the calendar day (see above).
          * @see #getCalendarDay
         public long getCalendarDay() {
             TimeZone tz = getTimeZone();
             // Figure the exact offset for the exact time of day.
             int offset = tz.getOffset( get( ERA ), get( YEAR ),
                     get( MONTH ), get( DAY_OF_MONTH ), get( DAY_OF_WEEK ),
                         (int)((long)get( HOUR_OF_DAY ) * ONE_HOUR +
                         get( MINUTE ) * ONE_MINUTE +
                         get( SECOND ) * ONE_SECOND ) );
             return round( ONE_DAY, getTime().getTime() + offset ) + JULIAN_DAY_OFFSET;

          * Sets the date in the calendar to 00:00 (midnight) on the local calendar day.
          * See getCalendarDay for the definition of calendar day as used in this class.
          * @param calendarDay the day to set the calendar to.
          * @see #setCalendarDay
          * @see java.util.TimeZone#getRawOffset
         public void setCalendarDay( long calendarDay ) {
             // Set to the beginning of the Julian day.
             // Then add in the difference to make it 00:00 local time.
             setJulianDay( calendarDay );
             setTimeInMillis( getTime().getTime() - getTimeZone().getRawOffset() );
             // we may have gone slightly too far, because we used the
             // raw offset (diff between Standard time to GMT/UT, instead of the
             // actual value for this day, so during DLS we may be at 1 AM or whatever
             // the local DLS offset is), so we'll just drop back to midnight.
             set( HOUR_OF_DAY, 0 );

         * Finds the number of days after 12/31/4312 BC 24:00 GMT on a proleptic
         * Julian Calendar (i.e. extending the Julian Calendar into pre-history)
         * to the current time.
     The Astronomers Julian Day begins at noon.  The Julian Day used here
         * sometimes called the Chronologists or Historians Julian Day
         * starts at midnight.  For more information see
     Note: This routine does NOT take into consideration
         * leap seconds.
         * @return the day number of the current time from 1/
         * @see #getCalendarDay
         public long getJulianDay() {
             return round( ONE_DAY, getTime().getTime() ) + JULIAN_DAY_OFFSET;

         * Sets the current date contained in this calendar to exactly
         * 00:00 GMT on the date defined by the Julian Day provided.
         * @param julianDay the Julian Day to set the calendar to
         * @see #setCalendarDay

         public void setJulianDay( long julianDay ) {
             setTimeInMillis( ( julianDay - JULIAN_DAY_OFFSET ) * ONE_DAY );

          * This is a utility routine for rounding (toward negative) to the nearest
          * multiple of the conversion factor.
     BUG? Why is this different than the formula given in
          * java.util.GregorianCalendar private millisToJulianDay?
          * @param conversion typically one of the constants defined
          * above ONE_MINUTE, ONE_DAY etc.
          * @param fractions the value to convert expressed in the same units
          * as the conversion factor (i.e milliseconds).
          * @return the value divided by the conversion factor, rounded to the negative.
          * @see java.util.Calendar
         protected static final long round( long conversion, long fractions ) {
             long wholeUnits;

             // round toward negative:
             // For secs rounded to minutes -61/60=-1, -60/60=-1, -59/60=0,
             //      but we want -2, -1, -1 not -1,-1,0
             // or month 0..11 => year 1; -12..-1 => 0; -24..-13 => -1

             if ( fractions >= 0 ) {
                 wholeUnits = fractions / conversion;
             else {
                 wholeUnits = ( fractions + 1 )/ conversion - 1;
             return wholeUnits;
     } // FAQCalendar


10. AWT

                          Text, Textfield, and TextArea

  1. (Sect. 10) How can I write text at an angle?

     [*] Check out Jim has some code
     to do exactly this. A good way to do it is to draw the text to an
     offscreen image and write an ImageFilter to rotate the image.

     Also, from JDK 1.2 on, the Java 2D API handles arbitrary shapes, text,
     and images and allows all of these to be rotated, scaled, skewed, and
     otherwise transformed in a uniform manner. A code example would be:

     import java.awt.*;
     import java.awt.geom.*;
     public class r extends Frame {

         public static void main(String args[]) { new r(); }

         r() { setSize(200,200); setVisible(true); }

         public void paint(Graphics g) {
             Graphics2D g2D = (Graphics2D) g;
             AffineTransform aft = new AffineTransform();
             aft.setToTranslation(100.0, 100.0);
             aft.setToRotation(Math.PI / 8.0);
             String s = "Rotated Hello World";

             for (int i = 0; i < 16; i++) {
                 g2D.drawString(s, 0.0f, 0.0f);

     There is more info about the 2D API at and

  2. (Sect. 10) How do you change the font type and size of text in a

     [*] Like this.

     myTextArea.setFont(new Font("NAME", <STYLE>, <SIZE>));

        o NAMEis the name of the font (e.g. Dialog or TimesRoman).
        o <STYLE> is Font.PLAIN, Font.ITALIC, Font.BOLD or any additive
          combination (e.g. Font.ITALIC+Font.BOLD).
        o <SIZE> is the size of the font, e.g. 12.

     Example: new Font("TimesRoman", Font.PLAIN, 18);

  3. (Sect. 10) Can you have different fonts for individual words in a

     [*] No. If you're trying to write a word processor, use the Canvas
     class to render on. Note that this can be done using the Swing JText

  4. (Sect. 10) How much text can be put in a TextArea?

     [*] TextArea just uses the corresponding widget of the underlying
     window system. It will be bounded by the limit imposed in the native
     window system. In Windows 95 TextAreas can hold about 28Kb. The native
     widget allows 32Kb, but there is some overhead which reduces the amount
     available to the programmer. The limit is removed in JTextComponent in
     Swing (JDK 1.2) which dispenses with peer controls.

  5. (Sect. 10) How do I clear the contents of a TextArea?

     [*] Set it to an empty String with this:


  6. How do I get back to a normal echo after I have used

     [*] TextField.setEchoChar('\0') works on most Windows-based
     browsers...but for most other platforms (i.e. Netscape under UNIX), it
     just locks up the textfield.

     There is only one good solution, and that is to make two TextFields on
     top of each other, one normal, and one with .setEchoChar('*'), and
     switch between them.

  7. (Sect. 10) How do I get word wrap in a TextArea?

     [*] It's a little obscure. Creating a TextArea with no horizontal
     scrollbar causes wrapping to occur automatically. The idea is that if
     you ask for a scroll to scroll viewing over to the right, there is no
     reason for the widget to do word wrap. So take away the scrollbar, and
     word wrap will be done instead.

     the TextArea constructor to get word wrap. By default, a TextArea is
     created with both horizontal and vertical scrollbars.

  8. (Sect. 10) How can I limit a TextField to no more than N characters, or
     to only allow numeric input?

     [*] The approach is to look at keystrokes as they happen, and disallow
     input that does not meet your criteria.

     A neat variation is to extend the basic AWT component, and in your
     subclass also include the handler that will look at the keystrokes.
     This bundles everything neatly in one place. The code may look like:

     import java.awt.*;
     import java.awt.event.*;

     public class XCTextField extends java.awt.TextField implements
     java.awt.event.TextListener {

         public XCTextField(int columns) {

         // other constructors may be useful, too

         public void textValueChanged(java.awt.event.TextEvent event) {
             int col = this.getColumns();
             int len = getText().length();
          // int caret = getCaretPosition();

             if (col > 0 && len  > col) {
             // or if the char just entered is not numeric etc.
                     String s = this.getText();
                     this.setCaretPosition(col-1);  // caret at end


         public void processFocusEvent(java.awt.event.FocusEvent e) {
         // this routine highlights according to focus gain/loss.
             int id = e.getID();
             if (id==java.awt.event.FocusEvent.FOCUS_GAINED)
             else if (id==java.awt.event.FocusEvent.FOCUS_LOST)

     Here is a much briefer example, which very cleverly does the work in
     the Listener. Oracle really dislikes the "apostrophe" character in a
     data text fields, as it is interpreted as part of an SQL statement.
     Here is the code that James Cloughley wrote to suppress apostrophes
     ("ticks") in a TextField.

     import java.awt.*;
     import java.awt.event.*;
     public class NoTick extends KeyAdapter {
         final char tick = '\'';

         public void keyPressed( KeyEvent event ) {
             TextComponent tc = ( TextComponent )event.getSource();
             char c = event.getKeyChar();
             if ( c == tick ) { event.consume(); }

     Use it like this:

     TextField sometextfield = new TextField();
     sometextfield.addKeyListener( new NoTick() );

     Brief and clever - make the event handler consume unwanted characters.
     However, it doesn't filter out text that arrives in the component via
     cut & paste! If you use ctrl-v to paste, you get key events for the
     ctrl and v, but not for the characters that are pasted.

     Finally, check out iDate, iTime, and iNumeric from IBM's alphaworks
     javabeans, available free at
     These beans do the kind of validation you want.

                                Size and Position

  9. (Sect. 10) I use add(Component) to add Components to the Container. Is
     there any way to explicitly set the z-order of these Components?

     [*] JDK 1.0 has no way to explicitly set the z-order of components. You
     can try it heuristically, based on the browser you're using, or you can
     use CardLayoutManager to ensure the panel you want is at the front.

     In JDK 1.1, the z-order of components ("z-order" means "front-to-back"
     order, i.e. which window is in front of which) can be controlled by
     using the the method add(Component comp, int index). By default,
     components are added 0 to N. The method paint of class Container paints
     its visible components from N to 0.

 10. (Sect. 10) How can I get the dimensions and resolution of the screen?

     [*] Use




     Screen resolution is in dots-per-inch.

     Take a look in the Toolkit class for other useful methods.


     gets you the color model in terms of bits per pixel.

     Math.pow(2, Toolkit.getDefaultToolkit().

     gets you the color model in terms of number of colors. Or use this:

     1 << Toolkit.getDefaultToolkit().

     That does a shift left to calculate the power of two.

 11. (Sect. 10) How do I allow for the size of the title bar and border when
     I draw a Frame?

     [*] Use MyFrame.getInsets(). This returns a java.awt.Insets object
     which has four ints: top, left, bottom, right, giving the number of
     pixels each of those margins are inset from the top. You can use these
     value to adjust the Dimension object returned by component.getSize().

     If you are doing this in the constructor you need to ensure that the
     Frame's peer object is created first. Otherwise the Insets object
     returned by getInsets() will have all zero values. Make a call to
     Frame.addNotify() to have the peer created.

 12. (Sect. 10) How do I resize a List? I had a List defined as

     List tlist = new List(10);

     but the Strings in the list were 80 characters long and only the first
     15 were being shown. I was not able to resize the List to display the
     contents without using the scroll bar.

     [*] A List cannot be resized in a constructor, so add the following to
     the Applet (or wherever):

     public void paint (Graphics g) {

     Then before showing panel/frame with the List:


 13. (Sect. 10) How can my program tell when a window is resized?

     [*] Override the setBounds(int,int,int,int) method of Component to do
     what you want. Of course, have it call super.setBounds() as well. Note
     that setBounds() replaces reshape() which is deprecated.

     Note the new APIs call the deprecated APIs instead of the other way
     round. For example, Component.setBounds calls Component.reshape,
     instead of reshape calling setBounds. This is because the AWT sometimes
     needs to call these for its own purposes. If it called the old one
     which then called the new one, and you overrode the new one, the AWT
     would (wrongly) not call your routine. By having the AWT call the new
     one (and then the new one call the old one), any overrides of the new
     one will correctly be called by the AWT as needed. If that didn't make
     sense, forget I mentioned it.

 14. (Sect. 10) How do I center a dialog box?

     [*] You cannot currently get the applet's absolute screen coordinates.
     Its location (0,0) is relative to the browser, not the screen itself.
     But you can center something that it pops up or displays centered on
     the screen with code like this:

     Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();
      ( screen.width - my_window.size().width ) / 2,
      ( screen.height - my_window.size().height ) / 2 );

     In a related fashion, you can center something on its parent like this.
     Note the intelligent use of APIs like translate() to do the work for

     void center(Component parent) {

         Point p = parent.getLocation();
         Dimension d = parent.getSize();
         Dimension s = getSize();

         p.translate((d.width - s.width) / 2,
                     (d.height - s.height) / 2);

 15. (Sect. 10) How can I get the absolute mouse coordinates?

     [*] You mean, if the browser is about 640x480, you want a y-coord
     between 0 and 480. If the browser window is about 800x600 you want a
     y-coord between 0 and 600. This might be needed for a pop-up menu,
     where you want to pop up at the absolute mouse position.

     The approach is to sum up the event's (x,y) and the locations of the
     target and its parents until there is no parent. Though on some
     browsers, it seems this is not reliable. [Better suggestions are

 16. (Sect. 10) How do I detect a resize of a Frame or other Component?

     [*] If you are using JDK 1.0.2, you can override the reshape(int, int,
     int, int) method of Component to do what you want; of course, have it
     call super.reshape() as well.
     In JDK 1.1.x, setBounds() replaces reshape(), which is deprecated -
     however, there is a better way of detecting the resize using the new
     event model, than overriding setBounds(). Note the new APIs call the
     depecated one.

     The proper way to detect the resize in 1.1.x is to register a
     ComponentListener on the Frame, like this:

     import java.awt.*;
     import java.awt.event.*;

     class MyFrame extends Frame {
         public MyFrame() {
             addComponentListener(new CmpAdapter());

         class CmpAdapter extends ComponentAdapter {
             public void componentResized(ComponentEvent evt) {

     Alternatively, the same effect can be achieved like this:

     class MyFrame extends Frame implements ComponentListener {
         public MyFrame() {

         public componentHidden(ComponentEvent evt) { }
         public componentMoved(ComponentEvent evt) { }
         public componentShown(ComponentEvent evt) { }
         public componentResized(ComponentEvent evt) {

     Or even with an anonymous inner class

       public MyFrame() {
               addComponentListener(new ComponentAdapter() {
                 public void componentResized(ComponentEvent evt) {
                   // doSomething;
           } );

 17. (Sect. 10) What are those preferredSize() and minimumSize() methods in

     [*] Those methods allow a LayoutManager to calculate the preferred and
     minimum sizes of the Components it is arranging. You can control the
     values that the LayoutManager gets by creating subclasses of the
     Components you are using and overriding these methods. You don't call
     them; you override them and they are called on your behalf.

 18. (Sect. 10) Why isn't my component showing up at all? Or in some cases
     not until I resize the main window?

     [*] The initial sizes of components are not always exactly what the
     programmer would expect. When a component doesn't show up, often it is
     getting added to its parent, but with a size of 0x0. Even when
     getPreferredSize gives a non-zero value. If this seems to be what's
     happening, try calling setSize(getPreferredSize()). If that doesn't
     seem to be the problem, look into your layout manager.

                                Drawing and Pixels

 19. (Sect. 10) How do I plot a single pixel to the screen?

     [*] Use g.drawLine(x1,y1,x1,y1) to draw a line one pixel in length. If
     you are drawing a very large number of individual pixels, consider
     using a java.awt.MemoryImageSource object and measure whether this
     offers better performance.

 20. (Sect. 10) Is it possible to draw a polygon or a line more than 1 pixel

     [*] JDK 1.1 doesn't have support for this. The standard workaround for
     drawing a thick line is to draw a filled polygon. The standard
     workaround for drawing a thick polygon is to draw several polygons.

     There is a useful class at which extends the
     drawxxx and fillxxx methods of java.awt.Graphics. It adds a Line Width
     argument to most of the drawxxx methods, a Color argument to most of
     the drawxxx and fillxxx methods, and a Font argument to drawString and

     JDK 1.2 introduces the java.awt.BasicStroke class. You set the stroke
     on a Graphics object, and line rendering is done using that info.

             BasicStroke bs = new BasicStroke(width, BasicStroke.CAP_BUTT,
                                      BasicStroke.JOIN_MITER, 1.0f, null, 0.0f);
             g.drawLine( ... );

 21. (Sect. 10) How can I make an offscreen image with transparent pixels?
     How can I grab the pixel values from an offscreen image?
     How can I use AWT drawing primitives (e.g. drawString() or drawOval())
     on an image I created from an ImageProducer?

     [*] None of these things can be done.

     Despite the fact that there is only one class called Image in the AWT
     libraries, it suffers from a (currently undocumented) severe case of
     schizophrenia: The code behaves as though there are two unrelated types
     of Image. The first type are those created by the
     Component.createImage(int, int) call, known as "offscreen" images, and
     the second are those created by the
     Component.createImage(ImageProducer) call, or by the
     Toolkit/Applet.getImage() calls, which I will call "produced" images.

     The only common ground between these kinds of Image is the following:
        o You may find their width and height by the methods of the Image
        o You may use them as the argument to the various
          Graphics.drawImage() calls.
     The differences between these objects are the following:
        o You may not put transparent pixels into an offscreen - note that
          all Java primitives accept Color objects, which all represent
          completely opaque colours as if Produced from an int with the
          upper 8 bits equal to 0xff. (See also Question 8.3.)
        o You may not call Image.getGraphics() on a produced image, and
          hence may not use any AWT primitives.
        o You may not grab pixels from an offscreen image using
        o Anyone know of any other limitations?
     In these cases, "you may not" generally means "you may not
     successfully". Symptoms on attempting these range from Exceptions to
     garblings of the Image. Any or all of these restrictions may be removed
     in Java 1.2, which features a new 2D API. Wait and see.
     Workaround: cause a peer to be created for the Image, and then do the
     operation. It will work. You can add it to a Frame, for example. You do
     not have to show() the Frame. Causing the peer to be created is enough.

     There are some relevant bugs shown in the Java Developer Connection:
     Bug ID 4098505. Apparently, from the report from the Sun engineer,
     PixelGrabber is specified to work with offscreen images, just it is
     currently buggy, and invariably gets the wrong color model. No fix has
     been scheduled yet.

     Bug ID 4077718 reports that setting transparent Colors in offscreen
     images has been available since Java 1.2b1. I am personally unable to
     verify this.

     There is an incorrect answer from Sun to the third matter, of
     getGraphics() on produced images, in article 1501 in Questions&Answers.

 22. (Sect. 10) How can I grab a pixel from an Image object?

     [*] This is the purpose of the java.awt.image.PixelGrabber class. A
     fragment of code showing its use is:

     import java.awt.image.PixelGrabber;
     import java.awt.Image;
     public static int pixelValue(Image image, int x, int y) {
     // precondition: buffer must not be created from ImageProducer!
     // x,y should be inside the image,
     // Returns an integer representing color value of the x,y pixel.
         int[] pixel=new int[1];

     // pixel grabber fills the array with zeros if image you are
     // trying to grab from is non existent (or throws an exception)
         PixelGrabber grabber = new PixelGrabber(image,
                                          x, y, 1, 1, pixel, 0, 0);
         try {
         } catch (Exception e) {System.err.println(e.getMessage());}
         return pixel[0];

     By the way, one issue on working with images is that the Java VM will
     consume virtual memory pretty fast if you are loading lots of images
     without calling the Image.flush() method when done. The getImage()
     method probably caches old images so they aren't garbage collected.

                                  Other AWT FAQs

 23. (Sect. 10) How do I change the icon on my Frame or JFrame from the Java
     coffee cup to my own icon?

     [*] Just use

     f.setIconImage( Toolkit.getDefaultToolkit().getImage(iconfilename) );

     On Windows, the icon must be 16-by-16 pixels.

 24. (Sect. 10) What's all this about subclassing Canvas and overriding
     paint() ? Can't I just do a getGraphics() for a component, and draw
     directly on that?

     [*] You can do that, and it might work up to a point (or it might not).
     A problem arises when the window system wants to refresh that component
     e.g. because it has been partially obscured and is now revealed. It
     calls paint(), and paint() has no knowledge of the other g.drawing()
     you have just done.

 25. (Sect. 10) But couldn't the AWT just remember what has been drawn to a
     Graphics context, and replicate that instead of calling paint()?

     [*] Possibly it could, but how do you unremember something that has
     been drawn? How do you start drawing over again with different
     contents? You could solve these by creating extra methods, but that is
     not how it works. In practice it is a lot simpler to be able to look at
     the paint method, and see explicitly all the things that will be done
     to draw that component. Bottom line: Use paint(), not g=getGraphics();
     g.drawString( ...

 26. (Sect. 10) When I call repaint() repeatedly, half my requests get lost
     and don't appear on the screen. Why is this?

     [*] repaint() just tells the AWT that you'd like a paint to happen. AWT
     will fold several adjacent repaint requests into one, so that only the
     most current paint is done. One possible workaround might be to use a
     clip rectangle and only paint the different areas that have changed.

 27. (Sect. 10) Why do I get this when using JDK 1.1 under X Windows?

     at sun.awt.motif.MFramePeer.<init>(
     at sun.awt.motif.MToolkit.createFrame(
     at java.awt.Frame.addNotify(
     at java.awt.Window.pack(

     [*] There's a missing font on your system. Move from
     the "lib" subdirectory aside to Then it won't look
     for the font and fail to find it.

     The problem occurs because the Motif AWT libraries use the Font "plain
     Dialog 12 point" as a fall-back default font. Unfortunately, when using
     a remote X server sometimes this font isn't available.

     On an X terminal, the diagnostic may be slightly different, a segv

     % appletviewer HelloWorldApplet.html
     SIGSEGV 11* segmentation violation
     si_signo [11]: SIGSEGV 11* segmentation violation
     si_errno [0]: Error 0
     si_code [1]: SEGV_ACCERR [addr: 0x14]

     To determine which fonts you have, issue a command such as

     xlsfonts > ~/fonts.txt

     Then pick through the long list of fonts to determine which ones you
     want to use. The xfd program will let you look at a font:

     xfd -fn "your font name here" &

 28. (Sect. 10) Why is GridBagLayout so hard to use?

     [*] There are two reasons. First, while simple layouts are easy.
     detailed GUI layout is difficult. Second, GridBagLayout wasn't designed
     with human factors and ease of use in mind. If that bothers you (it
     bothers me) then don't use it. Create your GUI on several panels and
     use the other layout managers as appropriate to get the exact effect
     you want. The official story from the project leader of the AWT
     project, as explained to the Mountain View Java Users Group on December
     4 1996, is:
          "The case has been made and is now accepted that GridBagLayout is
          too hard to use for what it offers. GBL will continue to be
          supported, and something better and simpler will eventually be
          provided as well. This 'better GBL' can be used instead of GBL."
     Bottom line: nobody has to spend any effort on GBL, there are simpler
     alternatives available now. In addition GBL is the source of a memory
     leak. GBL puts "added" components into a hashtable, but
     removeLayoutComponent() never removes them. See bug 4195295.

     GBL documentation is hard to come by. Based on the obvious similarity,
     it could have been derived from the Tk (of Tcl/Tk) grid layout manager.
     If you're not a fan of nesting panels, and none of the other layout
     managers do what you want (or you're working with legacy code that
     already uses it), you might find some Tk documentation worth a look.

 29. (Sect. 10) MyClass works fine except when I try to set a particular
     font. I just can't seem to get it to work in Win95, but I can get it to
     work on a MacOS and Unix.

     [*] You probably specified a font name that isn't available under your
     Win95 installation; this is one of those cross-platform differences
     that can bite you if you over-specify for one platform, like specifying
     "Arial" as a font and expecting it to work on something other than

     On both Windows 95 and Solaris 2.6, these fonts
        o Dialog
        o SansSerif
        o Serif
        o Monospaced
        o Helvetica
        o TimesRoman
        o Courier
        o DialogInput
        o ZapfDingbats
     are revealed by this code:

     import java.awt.*;

     class foonly {
         static public void main(String s[])
             String n[]= new Frame().getToolkit().getFontList();
             for (int i=0;i<n.length; i++)


     In other words, You can get a String array of the names of the fonts by

     String[] fonts = Toolkit.getDefaultToolkit().getFontList()

     The names of actual fonts like Helvetica, TimesRoman, and Courier are
     deprecated in JDK 1.1 in favor of font styles like SansSerif, Serif,
     and Monospaced (respectively). The font style will be mapped into the
     nearest actual font on a platform.

     The font styles are now mapped into a system font name using the
     entries in one of the files in $JAVAHOME/lib. There are
     multiple files corresponding to different locales. If
     you wanted a quick hack for testing, you could modify the file or add
     to it so a different mapping is done to a new font you want to try.

 30. (Sect. 10) I've made a Lightweight Component (a Component directly
     extending Component), and it keeps flickering/doesn't repaint itself
     properly. Why is this?

     [*] Lightweight Components, since they are notionally meant to be
     "transparent", aren't painted directly in response to repaint(), but in
     fact, Component.repaint() goes up the stack of Components until it
     finds an "Opaque" Heavyweight Component (necessarily a Container), and
     then calls repaint() on *that*.

     At this point, a call is eventually scheduled to Container.update().
     His first action is to call super.update, plunging us into
     Component.update(), which clears the component to the background color,
     since it has been called on a heavyweight, and returns. Then
     Container.update() proceeds merrily to call update on all contained
     Lightweight Components, recursively.

     The bottom line: "transparency" of lightweight components will only
     work correctly (without flickering) if the first upwardly accessible
     heavyweight component in the containment hierarchy is
        o a double-buffered heavyweight Component (necessarily a Container),
        o a heavyweight that never updates, but always paints (i.e. one that
          has overriden the default update() mechanism to not clear the
     If this is not done, the default Component update() will always clear
     the background before any repainting is done, leading to annoying

     Another important point is that if your Container has its own paint()
     method, that paint method of the container must call
     super.update/paint(), otherwise the contained lightweight components
     will never be painted. Putting this all together, the minimal
     alteration to code to cause it to work in this case is to place the

     public void update(Graphics g) {

     in the most closely containing heavyweight Container, in a Component
     hierarchy where you want to smoothly render lightweights that do not
     paint areas extending past that painted by their parents, i.e. ones
     that are not "transparent". This is dirty, but quick.

     If you want smooth transparency, the call above should read

       public void update(Graphics g) {
         // standard offscreen generation here.
         offg.fillRect(required background colour, full size);
         g.drawImage(myimage, 0, 0, null);

       public void paint(Graphics g) {
         // can generally expect resizes will hit update() first.
         g.drawImage(myimage, 0, 0, null);

     It's possible to intertwine these, by having this.update() calling
     this.paint(), with various replacings of the argument, but it is
     clearest to override them separately, as in the example.

 31. (Sect. 10) What is the difference between Component's
     setForeground(Color c) and Graphics's setColor(Color c) ?

     [*] First of all, these methods do the same thing: Set the foreground
     color to the value of the parameter. The difference lies in where you
     use them. There is also a Component.setBackground that will set the
     background color.

     If you are in a constructor or an event handler (e.g. "click here to
     turn the canvas blue") you have a Component and should use the
     setForeground() method. If you are in a paint() method, that takes a
     Graphics context as its argument so you will typically use

     Unlike a Component, a Graphics object doesn't have a background color
     and a foreground color that you can change independently. A Graphics
     object arrives in the color(s) inherited from the drawing surface. From
     then on, any rendering (drawLine(), drawRect(), fillOval(), etc.) will
     be done in the setColor() color. Because they do different things, the
     Component and Graphics methods have different names.

 32. (Sect. 10) When I start a mouse drag inside a Component, and go outside
     the Component, still dragging, the mouse events still get sent to the
     Component, even though I am outside it. Is this a bug?

     [*] No, it is the specified behavior. The Java API documentation says:
          "... Mouse drag events continue to get sent to this component even
          when the mouse has left the bounds of the component. The drag
          events continue until a mouse up event occurs...."
     It is done for the convenience and ease of the application programmer.
     It allows you to handle all drags from the place of origin. If you
     don't want this, simply look at the coordinates of the mouseDrag Event,
     and if they are outside the Component, ignore them.

 33. (Sect. 10) Why doesn't my window close when I click on the X in the
     title bar?

     [*] Here's how to make your program do that.
        o JDK 1.0.2: Handle Event.WINDOW_DESTROY to do a hide() and
          dispose() on the Frame.
        o JDK 1.1:
             + Listen for WindowEvent and do hide(); dispose(); in
               windowClosing() - this really ought to be the "default"
               behaviour, so was made so for a Swing JFrame.
             + Enable AWTEvent.WINDOW_CLOSING and do the hide() and
               dispose() in processWindowEvent().
        o JDK 1.2: The Component JFrame does a close by default (see section

 34. (Sect. 10) How can I force a synchronization of the graphics state,
     e.g. of a cursor change, or an animation frame to be rendered?

     [*] This is done by the sync() method in Toolkit. So just use:


 35. (Sect. 10) How can I tab between components?

     [*] In JDK 1.0, you have to read the key press, and program it
     explicitly. JDK 1.1 supports tab and shift-tab (previous field)
     automatically. The tab order is the order that the components were
     added to the container.

 36. (Sect. 10) What is the difference between "low level" and "semantic"

     [*] Low-level events are tied to a specific component (resizing a
     window, dragging the mouse, striking a key, adding a Component to a
     Container, etc.). Semantic events are those generated when you frob a
     control (move a scrollbar, click on a button, select from a menu,
     etc.), and the same kind of event can be generated by several different
     components. A Button and a List both fire an Action event when they are
     clicked on.

     To the programmer, the important difference is that you can change a
     low-level event such as the key value in a keypress, and it will
     display the new value. You can also consume low level events so that
     they do not appear in the widget. You can't do these things with
     semantic events - they have already "occurred" to the widget.

     Semantic events: Use the method addXListener() to add a listener object
     which implements the XListener interface to get XEvent objects
     delivered (usually via the AWTEventMulticaster). Low level events: Use
     the method enableEvents() and override performX() to grab those events
     in the object itself.

 37. (Sect. 10) Is it possible to have a Java window float above all other
     windows. For example, a tool palette floats in a super-layer always
     above all the regular document windows on which you use the palette's
     [*] On MS Windows, a Window object floats above all other windows,
     unlike a Frame, which is layered in with ordinary windows. This
     behavior yields a "floating" effect. Whether a Window object is really
     supposed to float is another question entirely.

     On Mac, a Window object is either layered in with other windows, just
     like a Frame is, or else it is entirely modal - depending on which VM
     you use. In Java - there appears to be no easy way to get floating
     behavior. If anyone knows otherwise, please send in your comments.

 38. (Sect. 10) How can iconify/deiconify a window in Java?

     [*] JDK 1.1 had no way to write code to force a window to iconify or
     deiconify. Support was added in JDK 1.2.

          MyFrame.setState( Frame.ICONIFIED );
          MyFrame.setState( Frame.NORMAL );

     will do the trick. There is a corresponding getState();

 39. (Sect. 10) How do I know which mouse button was pressed, and how often?

     [*] To handle mouse events you have to implement the MouseListener
     interface, or derive from the MouseAdapter class in order to use one of
     the mouse-handling methods. The MouseEvent argument passed to the
     handling methods contains fields that say which button was pressed, and
     the click count. Use code like this.

     public void mouseClicked(MouseEvent m) {
      boolean leftButtonPush   =
         (m.getModifiers() & java.awt.event.InputEvent.BUTTON1_MASK) != 0;
      boolean centerButtonPush =
         (m.getModifiers() & java.awt.event.InputEvent.BUTTON2_MASK) != 0;
      boolean rightButtonPush  =
         (m.getModifiers() & java.awt.event.InputEvent.BUTTON3_MASK) != 0;

      int click = m.getClickCount();    // might be 1,2,3 clicks or more

     You can also call at m.isPopupTrigger(). If it returns a true value,
     the user has asked for a pop-up menu. On a lot of window systems, the
     right mouse button is the trigger for pop-up menus.

     You can overload processMouseEvent for your component.

     public void processMouseEvent(MouseEvent e) {
         if (e.isPopupTrigger())  {
            // do what you want

     The code above applies to JDK 1.1. You can also call
     java.awt.swing.SwingUtilities.isRightMouseButton(MouseEvent me).

     See also question 15.10.


11. Swing

  1. (Sect. 11) What is Swing?

     [*] Swing is a new GUI toolkit bundled with JDK 1.2, and available as
     an add-on extension library for JDK 1.1. Swing is part of the Java
     Foundation Classes and supports a GUI toolkit that lets developers
     create components that have a pluggable look-and-feel. From an
     architectural standpoint, the Swing component set extends - but does
     not completely replace - the Abstract Windowing Toolkit (AWT).

     Swing has many components that can be used in place of components in
     the AWT (e.g. JFrame instead of Frame, JButton instead of Button,
     JApplet instead of Applet, JPanel instead of Panel). It also has many
     components that don't exist in the AWT (e.g. tool tips, toolbars, and
     progress bars). However Swing relies on the underlying AWT being there.

     The Swing toolkit allows the creation of GUI's that are every bit as
     sophisticated as native code toolkits like MFC -- with the Java
     advantage that they run on every platform. The pluggable look and feel
     means that they can have the same appearance on every platform, or you
     can choose to have it look like Windows on a PC, like Motif on a Unix
     box, etc, just as the user chooses.

     Swing also supports the Accessiblity API. That API allows the adaptive
     software used by disabled computer users to directly query the Java VM
     and extract information about the running program, the usual purpose
     for this being to determine the UI components. Then the existing
     adaptive software can interpret it to the user (e.g. read the contents
     of a window, etc). Swing doesn't use native components, and the
     adaptive software taps into native components, so without the
     accessibility API has a real role in bringing the two together. The
     beauty of the Accessibility API is that developers need to do little
     work to support it, but they do need to be made aware of it if they add
     new components.

     With Swing, native window behavior is confined to external window
     frames (and their borders) and a few other things such as fonts and the
     buffers used to hold window contents. The composition, layout, and
     drawing of controls is now all handled by Java code. So identical code
     is executed to create and manage your user interface on every platform.
     Swing provides a much greater consistency of behavior across different

     Swing works with JDK 1.1 if you download the swing.jar file and add it
     to your path. Swing is built in to JDK 1.2, and Javasoft has just
     changed its 1.2 Swing package-naming strategy. It is now called

  2. (Sect. 11) Should I use Swing or AWT to build my GUIs?

     [*] Use Swing to build your apps now instead of AWT components,
     wherever you have a choice. Swing is a GUI toolkit that is at least as
     good as other commercial GUI toolkits, and better in several respects.

     With Swing, it is easier to build an application that is portable
     between Mac, Solaris, Windows 95 and NT, than it is to use Win32 and
     build an application that just runs on Windows 95 and NT.

  3. (Sect. 11) Where can I find a Swing tutorial?

     [*] There is a Swing tutorial at which is
     part of this tutorial:
     There is also a Swing FAQ at
     See also
     There is a Swing Developer Connection website with white papers and
     examples at
     Please let this FAQ maintainer know about other good Swing tutorials
     and online resources.

  4. (Sect. 11) What is the Model/View/Controller paradigm?

     [*] Model/View/Controller is a design pattern or framework originally
     developed by Prof. Trygve Reenskaug at Xerox PARC in 1978/9. MVC was
     developed to allow Smalltalk to conveniently support GUIs.

     Model/View/Controller is a design pattern used extensively in Swing.
     Basically, the "model" contains your data, the "view" is the graphical
     representation, and the "controller" is responsible for the interaction
     between the other two. As an example, think of visually editing the
     Tree widget that represents a directory. The display is the view.
     Selecting a file, and dragging it to the trash can will delete the
     file. In order for the delete to happen, the controller must tell the
     model what just happened in the view.

     In practice, inter-communication between the view and the controller is
     complex, so the two are bundled together in one category in Swing. The
     model (data) is separate though.

     There's a reasonable white paper on MVC in Swing at .
     There is information on other OO design patterns at

  5. (Sect. 11) When I run the Swing demo on Windows 95 I get an error "Out
     of environment space."

     [*] That's because you don't have enough space for your DOS
     environment. You can fix this with:
        o Right click your MS-DOS Prompt icon or window and choose
        o Choose "Memory" and on "Initial Environment", choose 4096 instead
          of "auto".
        o Run Swing again, you'll be OK.

  6. (Sect. 11) How can I run Swing code in a browser?

     [*] Most current browsers have to be specifically set up to run Swing
     applets. Read the article at for
     information about this. The article also contains a simple Swing
     example applet, so you can confirm that that's your problem.
     Another approach is to use the Java plug-in, which automatically gives
     a Swing-compatible Java in the browser. See

  7. (Sect. 11) Why is my menu showing up behind other components when I use

     [*] The answer relates to lightweight and heavyweight (peer-based)
     components. There is a good article about it at

     For those who want the quick fix, and will read the article later,
     adding the line:


     before you create any menus will probably fix it (even if you're using
     menus other than JPopupMenu).

     The summary answer is that a Lightweight component will not appear over
     a heavyweight component by default.

  8. (Sect. 11) Why is there no JCanvas? How do I get a lightweight Canvas?

     [*] Use a JPanel as a Swing replacement for Canvas. All Swing
     components have a paint(Graphics) routine that you can override, just
     as you would have with Canvas, (but you probably want to override
     paintComponent(Graphics) instead, see next question).

  9. (Sect. 11) Why don't the borders of my Swing components look right when
     I override paint(Graphics)?

     [*] Swing splits painting into several different routines:
        o paintComponent(Graphics),
        o paintBorder(Graphics),
        o paintChildren(Graphics)
     all of which are called from paint(Graphics). If you override paint(),
     unless you remember to do it, the paintBorder() and paintChildren()
     won't get done.

     In most cases, what you really want to do is override paintComponent()
     instead of paint().

 10. (Sect. 11) Why does my JFrame go away after I choose system close on
     the window?

     [*] Assume that you have a Swing JFrame component, and you handle the
     windowClosing event, but do nothing in the handler. You will see that
     the JFrame disappears anyway.
     The reason is that JFrame's have default handling of the system close
     operation, separate from the windowClosing event. You have to override
     that by calling:


     on your JFrame.

 11. (Sect. 11) Why can I run the Mac Look and Feel only on Mac OS?

     [*] (This answer comes from the Swing Connection, see
     Sun has not determined that it has the right to deliver the Mac look
     and feel on platforms other than MacOS. If Apple were to confirm Sun's
     right to deliver this look and feel on other operating systems, Sun
     would be delighted to remove the lock. To date, Apple has declined to
     do this.

     Although you cannot use the new Mac L&F on non-Macintosh platforms,
     there is a way to examine the source code so developers can use it as
     an example of how to create a custom L&F. The Mac L&F is distributed in
     "stuffed-binhexed" format, which is standard for the Macintosh. If you
     develop on a MS-Windows platform and would like to examine the source
     code for the Mac L&F then you can do that by downloading and using a
     program called Aladdin Expander for Windows. You can download Aladdin
     Expander from this URL:
     When you have downloaded Aladdin Expander, you can use it to decode the
     Mac L&F file posted on the JDC.

     A recent posting on suggested the following user

     class MyOwnWindowsLookAndFeel extends WindowsLookAndFeel {
         public isSupportedLookAndFeel() { return true; }

     The desire on Sun's part to avoid infringing the Windows Look and Feel
     is also the reason why the JTree uses colored circles (and soon, little
     circles with a short line coming out of them) for the nodes to indicate
     whether they are open or not. The Swing team could have used the '+'
     and '-' as Windows does, or even the triangles that MacOS uses, but
     decided against it.

 12. (Sect. 11) When I set the cursor to WAIT_CURSOR why does it only change
     when my cursor is over specific components?
     How can I change the cursor to a WAIT_CURSOR over my entire window and
     all of its components, preventing any user action, while some other
     process is happening. (i.e. database access, opening another window,
     downloading an image, sorting some data, etc.)

     [*] In JDK 1.0.2 only the awt Frame could change its Cursor. With the
     advent of JDK 1.1 the Cursor manipulation has been move to the
     Component clss. Now all Components have access to the Cursor class.

     You could change your Cursor to a WAIT_CURSOR for each Component. This
     could be time-consuming. You could have a potentially large number of
     Components. With the advent of the JFC Swing, there is a mechanism to
     change the Cursor over the entire Window regardless of the number of
     components. The Swing component JFrame has a method:

                  public void setGlassPane(Component glassPane)

     which sets an awt Component as the 'glassPane' for the JFrame.

     This Component will cover the entire JFrame's visible user accessible
     (when visible), area excluding the border set by the underlying
     windowing system. With the 'glassPane' Component you can set the
     'WAIT_CURSOR' over an entire JFrame, prohibiting user input (the
     'glassPane' Component gets all user input) and blocking the user until
     your 'other' processing is complete.

     NOTE: You must spawn a Thread to accomplish your 'other' work if you
     want to see the WAIT_CURSOR while the 'other' processing is happening.
     When the 'other' work is being accomplished, the 'glassPane' is visible
     with a WAIT_CURSOR and gobbling up all user input. When the 'other'
     work is finished, the Thread uses your waitCursor() method to hide your
     'glassPane' until its needed again.

 13. (Sect. 11) Why does the compiler complain that the
     javax.swing.ProgressMonitor method "isCancelled()" isn't found?

     [*] In American English there are two acceptable spellings: "canceled"
     and "cancelled". Note that the first one has one "el" and the second
     two "el's". Sun spells this "canceled" for ProgressMonitors but many of
     the secondary sources of documentation spell it "cancelled". To make
     matters worse, Sun spells it "cancelled" at other times, such as the
     method "isCancelled()" for PrinterJob.

 14. (Sect. 11) Why doesn't pressing the Enter key activate the default
     button on a Swing dialog?

     [*] The default keymap for Swing text components (derived from
     JTextComponent) binds the Enter key (VK_ENTER) to the ActionEvent for
     text fields. This was done to be compatible with the behavior of
     java.awt.Textfield. To enable use of the Enter key to activate the
     default button, remove the Enter key binding from the default text
     component keymap as follows:

         static {
           KeyStroke enter = KeyStroke.getKeyStroke(KeyEvent.VK_ENTER, 0);
           Keymap map =

 15. (Sect. 11) How do I create non-rectangular Components?

     [*] Take a look at the following example that comes with the JDK:


12. Browsers

  1. (Sect. 12) When will my favorite browser support Java 1.1?

     [*] All popular browsers now have JDK 1.1 support. Netscape
     Communicator 4.04 plus patch "J" fully supports the features of Java
     1.1. It was released in December 1997, and is only missing the JavaSoft
     support for applet signing (Netscape has gone its own way on this). See
     If you have Netscape 4.05, and the console says anything other than
     Java 1.1.5 then you do not have a fully 1.1 compliant Netscape. There
     is a special preview version available here:

     Netscape badly fumbled its Java support in 1997 as its market was
     seized by Microsoft. Microsoft is using IE as a strategic tool to
     deploy what Microsoft employees call "polluted Java". For both of these
     browsers, the Java Plug-in is currently a good approach.

     Sun's HotJava browser fully supports the JDK 1.1 features. People who
     are obliged to use a browser without standard Java support should use
     the Java Plug-In. The Java Plug-In substitutes a standard Java virtual
     machine for the one that shipped with the browser. It allows you to use
     RMI, JavaBeans components, and Java Foundation Classes in Internet
     Explorer 3.02, 4.0, and 4.01. The Java Plug-In also works flawlessly
     with Netscape browsers. You can download the Java Plug-In from
     Note that you need to change the HTML a little, to ensure that the
     plug-in JVM is invoked, not the browser JVM. A tool is included to do
     the changes automatically.

  2. (Sect. 12) What applet routines get called in various browsers and the
     plug-in on different browsing actions (back, forward, load, etc)?

     [*] Java supporter Dave Postill did the work to get this answer.
     The life cycle of an applet is illustrated by logging calls to init(),
     start(), stop() and destroy(). Use caution when your applets have
     threads since in most sample applet code, the stop() method calls stop
     on any separate threads within the applet, and then sets them to null.

     This reckless threadicide is because most people think of the stop()
     method as something called only when the user leaves the page and wants
     to forget about it. But since Netscape calls stop() when you resize the
     window, your users would lose the applet's state when they thought they
     were only making a minor adjustment.

     See "Java Tip 8: Threads, Netscape, and the resize problem - How to
     deal with applet resizing in Netscape Navigator", JavaWorld Sadly the JavaWorld
     workaround does not completely fix the problem, since it relies on
     start() being called soon after stop() to identify a resize. However if
     you minimise the browser it may send a stop() to the Applet and then
     may not send a start() until the Browser is either restored or
     maximised. In this case, using the workaround results in the Applet
     being destroyed following minimising of the Browser - unless the
     Browser gets un-minimised within the killThreads timeout.

               Netscape    Netscape    Applet-    Internet    Internet
               [4.04/JDK   with        Viewer     Explorer 4  Explorer
               1.1.4]      Plug-In     [JDK       SP1         with Plug-In
               [4.05/JDK   [4.05/ JDK  1.1.5]     4.72.3110.8 [5.00.0518.10
               1.1.5]      1.1.5/      [JDK       on NT 4.0   / Plugin 1.1]
                           Plugin 1.1] 1.1.6]     SP3         on NT 4.0 SP3
      1. Clear
      browser  nothing     nothing     nothing    nothing     nothing
      2.       start() or
      Initial  init(),     init(),     init(),    init(),     init(),
      load of  start(),    start()     start()    start()     start()
      .html    stop(),
               start() [1]

      3. Back  stop()      stop(),     [4]        stop(),     stop(),
                           destroy()              destroy()   destroy()
      4.                   init(),                init(),     init(),
      Forward  start()     start()     [4]        start()     start()
               stop(),     stop(),                stop(),     stop(),
      5. Reloaddestroy(),  destroy(),  [4]        destroy(),  destroy(),
               init(),     init(),                init(),     init(),
               start()     start()                start()     start()
      reload   stop(),     stop(),                stop(),     stop(),
      [NS],    destroy(),  destroy(),  [4]        destroy(),  destroy(),
      <ctrl>   init(),     init(),                init(),     init(),
      reload   start()     start()                start()     start()

      7. Resizestop(),     [3]         [3]        [3]         [3]
      Minimize [2]         [3]         stop()     [2]         [3]
      Restore  [2]         [3]         start()    [2]         [3]

      10. Exit stop(),     stop(),     stop(),    stop(),     stop(),
               destroy()   destroy()   destroy()  destroy()   destroy()

     [1] Results not repeatable.
     [2] Not tested.
     [3] Tested, and found that no logged methods are called.
     [4] Test not applicable.

  3. (Sect. 12) Is it possible to set and retrieve cookies from Java, in a
     manner that is compatible with all browsers supporting cookies?

     [*] Short answer: no.
     Longer answer: probably no.
     Ultimate answer:
     A cookie is a morsel of tasty data that a server sends back to the
     client, and can retrieve on demand. It allows the server to retain some
     state information for each of its clients. The information is typically
     something like "what pages has the user seen?" or "is this a privileged
     The DevEdge site on Netscape's home page has a Javascript-Java example
     on getting cookies. Also has info on
     connecting an applet with JavaScript functions. It's quite involved.
     Stick to just Java if you can.

  4. (Sect. 12) I am developing an applet and testing it in Netscape
     Navigator. I find that after I recompile, I press reload, clear the
     caches, retype the URL of the HTML wrapper, and I still have the old
     version. Why is this?

     [*] Note: a reader reports that as of Netscape Communicator 4.05 it is
     possible to force the browser to reload the applet by holding down
     "control"+"shift" while clicking "Reload"
     In the past Netscape has completely failed to improve the defective
     code that does this monstrously wrong thing. It has been like this for
     many successive releases.
     Flushing the network cache will make no difference; that isn't where
     the caching is taking place. Although applets are sometimes "pruned"
     and their ClassLoaders garbage-collected, this doesn't happen
     predictably, so restarting Netscape is the only reliable work-around at
     the moment.
     A related question is "how do I make the browser reload from a
     URLConnection instead of just getting the content from the local
     cache?" The answer is to use

     Browsers seem to vary in their conformance to this programmatic
     request. Netscape caching varies depending on whether a proxy server is
     in use, and which thread in the applet made the get request.

     Another approach is adding "?" to the URL, e.g.


     and increasing this number each time the Applet fetches the image.

  5. (Sect. 12) Why didn't Netscape reload the applet when you pressed the
     Reload button?

     [*] For the applet to be reloaded, the new version would have to be
     loaded in a different ClassLoader. Navigator/Communicator's policy for
     assigning ClassLoaders to applets doesn't take into account whether a
     reload has been done (although there is no technical reason why it
     Some versions of Netscape reload the Applet if you use
     Edit/Preferences/Advanced/Cache to Clear Memory Cache and Clear Disk
     Cache, then <Shift> while you click on reload.
     In Explorer, use View/Options/General/Delete Files, then <Control>
     'Reload' button to reload the page containing the applet.

     Until they fix it, use the appletviewer to test applets. And send them
     mail - developers can only fix the bugs they know about.

  6. (Sect. 12) Should I use Microsoft CAB files or Java JAR files?

     [*] The question contains its own answer.
     CAB format is a Microsoft-only format. So do not use it as it destroys
     software portability.
     JAR format is the Java standard format, based on PKZIP format including
     data compression. JARs were introduced with JDK 1.1.
     See for more
     You should use the Java standard format JAR (Java Archive) files, not a
     vendor-specific format. JAR files are not just a Java standard, they
     are in industry-standard PKZIP format. One reader comments that both
     formats can be used with this tag:
          <APPLET NAME=myapplet
          <PARAM NAME="cabbase" VALUE="">
     IE3 does not support JAR
     IE4 supports compressed and uncompressed JAR, but not signed JAR

  7. (Sect. 12) How can I tell the version of Java that my browser supports?

     [*] See
     This page tells you whether your browser supports JDK 1.1.

     See This page tells
     you which classes you may expect to be present in the browser's

  8. (Sect. 12) How can I tell the options/commands that Netscape's JVM

     [*] You can open the Java Console and type a "?" to get a list of all
     the commands/options available to you in Netscape's built-in JVM.


13. Applets

  1. (Sect. 13) What is the difference between an application, an applet,
     and a servlet?

     [*] An applicationis a standalone program.
     An applet is a downloadable program that runs in a web browser.
     Typically an applet has restricted access to the client system for
     reasons of security. Other than that it is virtually no different from
     a regular Java program.
     A servlet is a Java program whose input comes from a server and whose
     output goes to a server. Other than that it is virtually no different
     from a regular Java program. Think of a servlet as an application, but
     one that (like an applet) requires a context in which to run, namely
     web server software. Servlets are used like CGI scripts, but take much
     less processor resources, and they allow the server end to be written
     in Java as well as the client. There is a page with much servlet
     information at:

     There is a servlet tutorial at
     and another servlet tutorial at
     There is a servlet FAQ at

     The web server starts up a servlet when the URL is referenced, and now
     your applets have something that they can talk to (via sockets) on the
     server that can write files, open connections to other servers, or
     There is also a software technology from IBM called an "Aglet".
     An aglet is a mobile agent that can go from machine to machine,
     performing tasks, serializing data collected, and "shipping itself"
     (code and data) to the next machine. It's too early to say if aglets
     are a flash in the pan or a dawning technology. Read about aglets at
     Finally, there is the ticklet (Tcl/Tk) plugin for your browser
     (Netscape or Explorer) available at
     Don't confuse Sun's JWS "Java Web Server" with JWS "Java Workshop".
     Java Web Server supports servlets, as does the lightweight and free
     server at

  2. (Sect. 13) My applet works on my machine, but fails when I put it on
     our web server. Why?

     [*] It could be one of several reasons, and unfortunately the messages
     that you get in this situation aren't much help. In general, you can
     assume that either your applet's class files are corrupted somehow, or
     the web server can't find one or more of them when the browser needs
     Be careful of the following things:
        o Make sure you transfer the class files in binary mode, rather than
          text or ASCII mode. An error from the browser saying "cannot start
          applet ... bad magic number" usually means that one of the class
          files on the server is corrupted. Replace your class binary files
          on the web server, clean up the cache of your browser, and reload
          your applet.
        o Make sure you transfer all of the class files that are a part of
          your applet. Sometimes people are surprised by how many there are.
          There will be a class file for every class and interface you
          define, even if you define more than one in a single source file.
          If you use the Java 1.1 "inner classes" feature, there will be
          class files for each inner class as well.
        o Make sure you maintain the appropriate case distinctions in your
          filenames. If a class is called StUdLy, it must be found in a file
          called StUdLy.class.
        o Make sure you maintain the directory structure that matches your
          package structure. If you declare a class in package,
          the class either needs to be in a Jar file, or the class file
          needs to be in directory com/foo/util under the applet's codebase
          directory. Again, case distinctions are important for
          package/directory names, just as they are for class/file names.
        o Make sure that the web server process will have read access to the
          class files, and search access to the directories that the files
          are in. For example, if the web server runs on a Unix machine, use
          the command "chmod o+r filename" for the files, and "chmod o+x
          dirname" for the directories.

  3. (Sect. 13) How do I load a webpage using an applet?

     [*] Use code like this,

                    new URL("") );

     Or, to show the page in another window or frame,

             new URL(""), "windowname" );

  4. (Sect. 13) How do I use an image as the background to my applet? How do
     I set the background color of my applet the same as the browser?

     [*] You can simply do a g.drawImage(yourImage, x, y, this) in the
     paint() routine of your applet. If the image isn't big enough to fill
     the entire background, tile it or scale it. Here is some code to tile

         // The background image is named "bg".
         int w = 0, h = 0;
         while (w < size().width) {
             g.drawImage(bg, w, h, this);
             while ((h + bg.getHeight(this)) < size().height) {
                 h += bg.getHeight(this);
                 g.drawImage(bg, w, h, this);
             h = 0;
             w += bg.getWidth(this);

     Alternatively, the AWT can scale your background image to the size of
     the applet. The result quality will depend on the kind of image. Inside
     an applet class, you can use:

        drawImage(img, 0, 0, size().width, size().height, this);

     You can set the background color to match the background color of the
     browser by passing the value in as a parameter, like this:
          In the HTML applet tag:
               <param name=BrowserColor value=F1F1F1>
               (value should be the same hex as the HTML COLOR value).
          In the Applet init() method:

              String colparam = getParameter("BrowserColor");
              int col = Integer.valueOf(colparam,16).intValue();
              setBackground( new Color(col) );

     An applet cannot override the size imposed by the HTML. If you make the
     applet larger, the browser will still clip to the original size. If you
     need more room, open up a new Frame, Window or Dialog to show your

  5. (Sect. 13) How do you make the applet's background transparent?

     [*] There is no way to give an applet a transparent background that
     lets the web browser background show through. You can simulate it by
     giving the applet a background that matches the underlying browser
     background. (For a straight color, it will be the value of <BODY
     BGCOLOR=nnnnnn> in the HTML file). It doesn't produce satisfactory
     results with a patterned background because of problems aligning the
     Lightweight components (new in JDK 1.1) have a transparent background,
     but that merely allows other components to show through. A lightweight
     component is always ultimately positioned in a heavyweight component.

  6. (Sect. 13) How do you do file I/O from an applet?

     [*] Unsigned applets are simply not allowed to read or write files on
     the local file system (see answer to question 7.8).

     Unsigned applets can, however, read (but not write) non-class files
     bundled with your applet on the server, called resource files.

     See answer to question 7.8 and question 7.9.

  7. (Sect. 13) How do I pull a non-class file, such as a .gif, out of a jar

     [*] [Question has been retired, in favor of question Q7.9]

  8. (Sect. 13) How do I read a text file stored in a Jar?

     [*] [Question has been retired in favor of Q7.9]

  9. (Sect. 13) How do you get a Menubar/Menu in an applet?

     [*] In your applet's init() method, create a Frame instance and then
     attach the Menus, Menubar etc to that frame. You cannot attach the Menu
     or a Menubar to an applet directly.
     Or get the parent Frame like this (doesn't work in all execution

         Container parent = getParent();
         while (! (parent instanceof Frame) )
             parent = parent.getParent();
         Frame theFrame = (Frame) parent;

     This second suggestion definitely doesn't work in the appletviewer, and
     probably won't work on Macs (where would the Menubar go?) or in some
     browsers. In JDK 1.1, just use a popup menu, which isn't attached to a

 10. (Sect. 13) Can I get rid of the message "Warning:Applet Window" along
     the bottom of my popup windows in my Applet?

     [*] This is a security feature that prevents the applet programmer from
     popping up a window that looks like it came from the native OS and
     asking for passwords or credit card info (etc.). Users must always be
     aware of when they are talking to an unsigned applet. You can get rid
     of it by signing the applet, if the user accepts signed applets from
     you. In Netscape (only), using the Capabilities API to make the call


     before creating the Frame eliminates the message, if the security
     manager passes it.

 11. (Sect. 13) When I subclass Applet, why should I put setup code in the
     init() method? Why not just a constructor for my class?

     [*] The browser invokes your constructor, then setStub, then init().
     Hence when your constructor is invoked, the AppletStub (and through it
     the AppletContext) is not yet available. Although in principle you can
     do things in the constructor that don't rely (even indirectly) on the
     AppletStub or AppletContext, it is less error-prone to simply defer all
     setup to the init() method. That way you know that anything that needs
     the stub/context will have it available.

 12. (Sect. 13) I want to know about {applets,applications} but the lousy
     book I got just talks about {applications,applets}. What can I do?

     [*] The truth is that 95% of the material is the same, whichever your
     book chooses to focus on. Some people write their apps to work
     completely in a Panel, then depending on whether they're running
     stand-alone or in a browser the Panel is either added to a Frame or an
     Applet. The trick is that you need to add a listener to the
     application's Frame to handle the WINDOW_CLOSING (previously
     WINDOW_DESTROY) event yourself.
     If you fail to do this, when running as an application, the window
     won't close. See Question 15.7 for a sample of the right handler.
     In this scenario the following code will tell you which environment
     you're running in:

        public boolean isRunningInBrowser() {
             Component p = getParent();
             while(p != null && !(p instanceof Frame)) {
                 p = p.getParent();
             return (p == null);

 13. (Sect. 13) How do I print a page with an applet?

     [*] Browsers are starting to introduce support for this. Until they all
     have it, your best bet is to print a screendump. Using the browser to
     print the page may leave a blank where the applet is. Putting print
     support in the applet will print the applet only, not the rest of the
     browser page.
     Also in the FAQ: Q5.2.

 14. (Sect. 13) How can I position my dialogs centered (not top left)?

     [*] Use some code like this:

         void center(Component parent) {
             Point p = parent.getLocation();
             Dimension d = parent.getSize();
             Dimension s = getSize();
             p.translate((d.width - s.width) / 2, (d.height - s.height) / 2);

 15. (Sect. 13) How can I get two applets on the same page to communicate
     with each other?

     [*] This is the purpose of the InfoBus protocol. See

     The older way to do it was as follows. In your HTML page, give a NAME
     in the APPLET tag for the applet receiving the message, say <APPLET ...
     NAME=someName ...>. In the Java code of the other applet do

         Applet anotherApplet = getAppletContext.getApplet("someName");

     Cast anotherApplet to the correct applet subclass, and you can call any
     methods in the applet subclass. Don't forget to use appropriate
     synchronization when two threads tweak variables. This only works when
     the applets are truly on the same page. If they are in different
     frames, it doesn't work.
     You can walk through all the applets on an HTML page using code like
     that below. However this appears to be broken in Communicator 4.04 on

     Applet otherApplet;
     AppletContext ac =getAppletContext;
     Enumeration applets = null;
     for (applets=ac.getApplets(); applets.hasMoreElements(); ) {
         if (otherApplet!=this) break;
         // do something with otherApplet, e.g.
         // if (otherApplet instanceof FooApplet) ...

     Some people suggest using the static members of a common class to
     communicate information between the applets. This is not recommended as
     it relies on class-loading behavior that may change in future. Netscape
     changed it in one Beta so it didn't work, then changed it back again so
     it did. It doesn't work if you use the "mayscript" tag though.
     Inter-applet communication sometimes arises when you have a
     multi-screen type program and you don't want to force the user into
     downloading everything at once. One alternative is to make them into
     one applet with two GUIs. Try to avoid the need for applets to talk to
     each other. Also check the URL
     which explains how it can be done in HotJava 1.1. Recommendation: avoid
     code which is browser-specific.

 16. (Sect. 13) How can I resize an applet?

     [*] If you want resizing behavior from an applet, you should launch an
     external Frame that can be resized independently.
     One programmer suggests using percentages for the height/width
     parameters in an applet tag, like this:

         <APPLET CODE="lewinsky.class" WIDTH="100%" HEIGHT="100%">

     You can't resize the applet directly, but it does get resized when you
     resize the browser window (tested with Netscape 3.04 and 4.04, but does
     not work with appletviewer). If you have nothing else on your HTML page
     and use 100% for your width and height, the browser window looks almost
     like a real application.
     For the extremely tricky: have the browser reload the page with the
     applet when the browser resizes using new values for width and height
     (probably not what you want most of the time). You would need
     Javascript to generate a page dynamically using document.write("...")
     when the browser resizes. Not recommended. Another possibility is to
     use the new SplitPane class in JFC.

 17. (Sect. 13) How do I sign an applet?

     [*] The browser vendors have produced independent and different
     solutions for applet signature (alas). Here are some URLs on this

     See the Java Signing FAQ at

     Read the basics of signing at:
     Be aware that the mechanism of signing and administering signed code
     changed significantly between Java 1.1 and Java 2.

     Netscape signing:

     Microsoft signing:


14. Multi-Media

  1. (Sect. 14) Are there any good Java Image libraries?

     [*] Yes. Try the Java Image Management Interface (JIMI), which offers a
     free trial period. JIMI is a toolkit that lets your Java programs read
     and write many graphics file formats (PNG, JPG, BMP, GIF etc). JIMI is
     written in 100% Java, and best of all, it's a breeze to get started
     with. See

  2. (Sect. 14) Why won't my audio file play?

     [*] Java 1.1 and earlier releases use one audio format exclusively. The
     audio file must be in .au format, recorded at 8 KHz, mono, in mu-law
     encoding. If your audio clip is in a different format (e.g., .wav) or a
     different frequency it must be converted to the exact specifications
     above before Java can play it. Support for .wav and other formats is
     part of the Java Media Framework coming in JDK 1.2.
     Search at for GoldWave for Win 95, sox for Unix
     and similar conversion utilities for other systems.
     Other sites:
        o One conversion utility in Java is at

        o The source of a Java class to play linear PCM .WAV files is at:
          can be used in any Java application or applet.

  3. (Sect. 14) How can I do video streaming using Java?

     [*] That is the purpose of StreamBean. See

  4. (Sect. 14) Does Java support animated GIFs?

     [*] Java 1.0.2 and earlier releases use GIF and JPEG formats, and do
     not use the GIF89 animated GIF format. (An animated GIF is one that
     contains successive frames of an image, so when they are displayed in
     quick sequence the image appears to contain movement). When you display
     an animated GIF in Java 1.0.2, you will just get the first frame. There
     doesn't appear to be any easy way to get other frames from the image.
     The advantage of an animated GIF file is that there is only one file to
     download, and it is simple to do simple animations. The advantage of
     programmatic control over individual frames is that you control the
     rate and order of displaying them.
     Here's a surprise: JDK 1.1 supports the animated display of animated
     GIFs. For simple animations animated GIFs are a lot easier and
     lighter-weight than coding an animation explicitly.

  5. (Sect. 14) How do I create animated GIFs?

     [*] Use GIFanimator from ULead (said to be the best), or GIF Construction Set from Alchemy Mindworks

  6. (Sect. 14) How do I prevent animated GIFs from flashing while

     [*] The problem is most likely that in your paint method you have

         g.drawImage(img, ix, iy, this);

     You should change this to

         g.drawImage(img, ix, iy, getBackground(), this);

     This will change all the transparent regions of the image to the
     background color before painting to the screen. If you paint
     transparent images directly to the screen they flicker.
     If that does not solve it then check that imageUpdate is

     public boolean imageUpdate(Image img, int flags, int x, int y,
                   int width, int height) {
             if ((flags & (FRAMEBITS|ALLBITS))!= 0) repaint();
             return (flags & (ALLBITS|ABORT)) == 0;

     update is

         public void update(Graphics g) { paint(g); }

     If you have a background Image behind the partly transparent animated
     GIF you will have to double buffer. You can crop the backgound image so
     you won't have to double buffer the full app and waste too much memory.

  7. (Sect. 14) Does Java support transparent GIFs?

     GIF89a images with a transparent background show up as transparent
     without further filtering. This has been supported from 1.0 on. Java
     correctly displays both animated GIFs and transparent GIFs.
     Even better, you can fill the transparent pixels with a color (so they
     appear non-transparent in Java). Just pass the fill color explicitly:

         drawImage(img, x, y, w, h, fillcolor, this);

     Further, you can filter the pixels of an Image to turn any bits you
     wish transparent. However, the most you can do is reveal what is
     underneath the image. You cannot reveal what is underneath the applet
     (i.e. on the browser itself). By default applets have a plain grey
  8. (Sect. 14) How do I play video in Java?

     [*] Use the Java Media Framework Player API.
     Other sites:
        o The Java Media Framework Player API spec can be found at

        o Intel has released a SDK for the Java Media Framework Player API.
          The SDK is for Windows 95 and Windows NT. For more information,
        o SGI has released an implementation of JMF for IRIX: See

  9. (Sect. 14) How can I play *.au files from an application?

     [*] A new static method was introduced in JDK1.2 in the class Applet:

         public final static AudioClip newAudioClip(URL url) {

     The method turns a URL (pointing to an audio file) into an AudioClip.
     Since the method is static it can be used without any objects of class
     Applet needing to exist, e.g. in an application. Once you have an
     AudioClip, you can play it with:

     The Java Media Framework provides a richer API for playing sounds in an
     For code prior to JDK 1.2, you can use the AudioClip or AudioPlayer
     class in If you
     do this your code is no longer 100% pure Java, as it relies on a vendor


         URL url; ...
         AudioStream audiostream = new AudioStream(url.openStream());

     Also in the FAQ:
     Use the new Java Media Framework API, allowing a wide range of video
     and audio formats to be played back. See previous question.

 10. (Sect. 14) How do I read in an image file, in an application?


         Image img = Toolkit.getDefaultToolkit().getImage(fname);

 11. (Sect. 14) When I initialize a component,I call MyComponent.getImage()
     to get its image. createImage() returns null! I know the image works
     elsewhere. What's wrong?

     [*] A peer component needs to exist for your component before you can
     get its image. This is done by the method addNotify() (surely one of
     the most poorly named methods in all Java -- it doesn't mean "add a
     Notify". It means "Notify that the Component has been added to a
     Container". It tells the system, "you need to create the peer for this
     Component now"). addNotify will be called for you when you add your
     component to a container.
     Javasoft notes that most applications do not call addNotify() directly.
     It is called for you when you add the component to a container. If you
     have any code that requires peer resources, you can move it into a
     thread that is started from a conditional branch of the paint() or
     update() method. That way the peer will definitely exist when the code
     is executed.
     A common reason for seeming to require peer resources in a constructor,
     or alternatively in the getPreferredSize() method, (which is also
     usually before the peer is created) is to measure the area required for
     your window, in terms of font and image sizes. Font sizes may be
     obtained by calling
     Toolkit.getDefaultToolkit().getFontMetrics(somefont). This does not
     require a peer. Image sizes may be obtained by waiting for the relevant
     Image to load from the ImageProducer by using an ImageObserver, or a
     MediaTracker (see 8.15), also without requiring a peer. See 15.4 for
     more discussion of this problem.
     If you override addNotify(), don't forget to call super.addNotify() in
     your overriding version.

 12. (Sect. 14) How can I force a reload a fresh version of an image into my
     applet? My image file is changed periodically, and I want the applet to
     go and retrieve it, not cache it.

     [*] You need to turn off caching for the URL.

     URL url = null;
     URLConnection con;
     try {
      url = new URL(getDocumentBase(),"image.jpg");
      con = url.openConnection();
     } catch (MalformedURLException e1) {
       catch (IOException e2) {

     Note: Some programmers have reported that it caches anyway, even if
     they do this. That is a browser bug.
     One programmer reported that even after turning off caching and calling
     image.flush() before getImage(..), he was still seeing the same picture
     even though it had been changed on the server.
     He worked out a solution: access the image via a cgi script that
     returned a URL. This redirects the browser, and he put in an Expires:
     header as well to force the reload. Painful and laborious, but it got
     the result.

 13. (Sect. 14) How can I save an Image file to disk in JPG or GIF format?

        o If you have an Image and you want a JPG in a file, download James
          Weeks's code from
        o Sean Breslin also wrote a program that compresses a Java Image
          into a JPEG file.
        o If you just want to convert some file, a non-Java solution is to
          use the standard IJG 'cjpeg' utility. It supports GIF, PPM, BMP,
          PNG and Targa input files.
        o If you have an Image and you want a GIF or PPM in a file, download
          Jef Poskanzer's abstract ImageEncoder class at

     Also try the Java Image Management Interface (JIMI), which is free for
     non-commercial use. JIMI is a toolkit that lets your Java programs read
     and write many graphics file formats (PNG, JPG, BMP, GIF etc). JIMI is
     written in 100% Java, and best of all, it's a breeze to get started
     with. See

 14. (Sect. 14) What causes this problem:

     $ appletviewer m.html
     Premature end of JPEG file
     sun.awt.image.Im...Exception: JPEG datastream contains no image
     at sun.awt.image. ... .produceImage(
     at sun.awt.image.Inpu...mageSource.doFetch(

     [*] There's a known bug in early releases of the JDK which can cause
     the above failure when reading a JPEG across a slow connection. The
     failure only occurs if the JPEG contains a large application data block
     (APPn marker) - the problem is that the JPEG decoder is trying to skip
     over the APPn and failing if not all of the APPn has been received yet.
     The quoted error message is only one of several possible complaints,
     but they all stem from the same root.
     Photoshop is the most common source of JPEGs containing oversize APPn
     blocks. In particular, if you allow Photoshop 4 to save a thumbnail
     (preview) in a JPEG, the thumbnail plumps up Photoshop's private APPn
     marker to several K, which is usually enough to cause this problem.
     There are several possible workarounds:
        o Get a newer JDK - this problem is said to be fixed in 1.1. (If you
          are putting images up on the Web, this isn't much of a solution,
          because you can't assume visitors to your site have an up-to-date
          Java installation.)
        o When making JPEGs for Web use from Photoshop, make sure you have
          turned off the "save thumbnails" preference. (This is a good idea
          quite aside from bug workarounds, because the thumbnail is just a
          waste of download time as far as a Web browser is concerned.) You
          might still have a problem if you've got verbose comments or lots
          of paths being saved into the file, but 99% of the time, getting
          rid of the thumbnail will make Photoshop's APPn small enough to
          not trigger the Java bug.
        o Use a tool such as 'jpegtran' (from the Independent JPEG Group) to
          strip out the Photoshop APPn entirely without any loss of image
          quality. Recommended answer for the compulsive byte-trimmer.
        o (Last resort) Load and resave the image in a different image
          editor that won't insert any APPn or other overhead data. This
          implies a JPEG generational loss, so I don't recommend it if you
          are picky about image quality.
     Any large overhead marker will cause the same problem; 4K of comment
     text, say, in a COM marker. So Photoshop is not the only source of
     tickling this bug.

 15. (Sect. 14) How can I convert between GIF and JPEG formats?

     [*] In a word: don't.
     There's hardly any overlap between the set of images that JPEG works
     well on and the set that GIF works well on. Sometimes, with enough
     care, you can get an acceptable conversion...but most of the time
     GIF<->JPEG conversion will just turn your image to mush. It's better to
     pick the right format in the first place.
     Other sites:
        o If you're determined to convert formats anyway, try the GBM
          (Generalized Bitmap Module). The package is GNU licensed, in C and
          is very good. Find it at

          GBM does a good job converting to JPEG, and 'lossiness' is
          adjustable to 0%. It also converts to/from about 20 other formats,
          does cropping, sizing, color mapping, gamma correction,
          halftoning, everything you could want. GBM source doesn't support
          JPEG directly, but utilizes JPEG source from IJG called jpeg-6a
          and found at

        o For more info see the JPEG FAQ at

 16. (Sect. 14) If you have an InputStream (rather than a file) that
     contains an Image, how can you display it?

     [*] Use this method, and some adroit shuffling.

         Toolkit.getImage(URL url)

     Create a thread that pretends to be an http server. Make it listen to
     some port (8765 for example) for incoming requests. When the thread
     gets a request, it should simply whisk up the appropriate http headers
     and follow it by the InputStream. Thus the component that has the input
     stream and wants to do the getImage(url) can now invoke:


     The thread will act as a stream-to-url adapter, and send back the data
     It saves you from having to read 200K of JPEG data before you can begin
     drawing anything.

 17. (Sect. 14) How can I record sounds in Java?

     [*] The Java Media Framework will eventually support this, but it does
     not yet. JMF 1.0.1 only supports playback.
     JMF 1.0.1 is bundled with JDK 1.2, and available as a separate download
     for JDK 1.1 and Netscape Communicator 4 with Java 1.1.
     Other sites:
          In the meantime, there is a package for Win95/NT available at
 It supports 8, 16-bit, stereo, mono,
          11025, 22050, 44100 Hz record/play, load/save .WAV files. You
          could also interface to native code for your platform.

 18. (Sect. 14) Does Java have any built-in support for displaying HTML?

     [*] JDK 1.1 supports rendering HTML using the unbundled JFC 1.1 package
     known as Swing. The Swing package is bundled in JDK 1.2. It has an
     elementary (graphics, tables, text) HTML bean that is good enough for
     simple rendering (help files, email, etc).
     Other sites:
        o JavaBrowser Free
          source, free for use under GNU LGPL licence, HTML 2.0 (sort of).
        o ICE Browser - Java Bean Component
 Free binaries for use in free
          applications. Commercial licensing available including source -
          flat fee licence. Thin HTML client! Lightweight! HTML 3.2
        o HotJava HTML Component - Java Bean Component
 $195 for
          private use binary licence. HTML 3.2
        o HTML browser (free source)

        o Web Window Browser $139 -
          no sources.
        o jHelp ($20-650)
          jHelp is a HTML browser component written in Java, HTML 2.0

 19. (Sect. 14) I loaded an Image file from a JPEG/GIF file using the
     Toolkit/Applet.createImage(URL/String) method, and (the height and
     width are -1 / it will not draw to the screen). What is wrong?

     [*] The behaviour of the AWT on creating images in this way is to do
     nothing at all.
     When the image is first drawn using Component.drawImage(), or its size
     is requested, the image begins to load in a different Thread.
     As the image loads, the ImageObserver specified in the
     drawImage()/getHeight() call is periodically notified of the loading
     status of the image, by calls to its imageUpdate() method.
     In the case of Component.drawImage() call, the default behavior of
     Component.imageUpdate() is to schedule *another* repaint() call when
     the image has fully loaded. This means that, in particular the
     following code will not work:

         class MyComponent extends Component {
           public void paint(Graphics g) {
             ImageFilter cropper=new CropImageFilter(0,0,16,16);
             Image cropped_image=createImage(new
             g.drawImage(image,10,400,this);        // this line works
              // this line doesn't -

     The cropped_image will not be created in time to be painted, and when
     it is finally created, another call will be scheduled to paint, which
     will try to draw another one, etc.
     (Note also that creating objects like this in paint() methods is
     generally a very poor idea in Java, since they are called very
     frequently, and you will strongly offend the garbage collector.
     In order to get round this problem, you may i) add all such Images to a
     MediaTracker, and call the waitForAll() method. ii) implement your own
     ImageObserver interface, and wait for the imageUpdate() method to be
     called with the ALLBITS/FRAMEBITS value. i) is easier, but ii) is
     recommended, since there are reports of MediaTracker not working in
     some environments.
     Also in the FAQ:
        o See also Q13.12
        o See Q6.4 for examples of how to reuse objects.

 20. (Sect. 14) How can I record sound in an applet?

     [*] If you are using win95/nt, you could use SoundBite - Audio
     Recording in Applets. See
     It provides easy access to audio data in arrays:
     short[] left, right;

 21. (Sect. 14) Does Java support PNG? Yes. PNG - Portable Network Graphics
     - provides a patent-free replacement for GIF and TIFF. If you save a
     GIF, don't forget to pay the royalty to Unisys - see Unisys's web page
     at That patent is why GIFs
     are a poor choice for internet images.

     The PNG format is specified in RFCs 1950, 1951, and 2083, and is
     unencumbered by licenses or patents. See also the PNG-1.1 specification

     The PNG format is supported by the Java Advanced Imaging API which is
     part of the 1.2 media APIs.


15. Networking and Distributed Objects

                                    RMI Issues

     Note that there is an RMI FAQ at

  1. (Sect. 15) Should I use CORBA in preference to RMI? Or DCOM? Or what?

     [*] If your distributed programs are all in Java, then RMI provides a
     simpler mechanism that allows the transfer of code, pass-by-value of
     real Java objects, and automatic garbage collection of remote objects.
     If you need to connect to C++ (or other language) systems or you need
     CORBA-specific services, then CORBA is your choice.
     In July 1997, Sun announced that it was aligning RMI to work more
     closely with CORBA. Sun is simply adding an IIOP transport layer to RMI
     to support interoperability with CORBA. Java programs can then use RMI
     to access CORBA-based objects through IIOP, the OMG's CORBA-based
     protocol. This is very good news for those building heterogenous
     Enterprise systems, although it will take some additions to IIOP to
     support the pieces that RMI uses.
     In 1998 Microsoft spokespeople tried to promote DCOM by spreading
     misinformation that RMI is changing or being dropped. That is totally
     wrong. The RMI API continues unchanged in its current form. Using DCOM
     restricts your code to only ever run on Microsoft platforms using Intel
     hardware, and negates the "write once, run anywhere" Java philosophy.
     You would have to recompile your DCOM code to run it on other Microsoft
     platforms like Compaq's (formerly DEC's) alpha computer. Non-portable,
     single vendor code should be avoided. DCOM/DNA has limitations for use
     in the enterprise.
     Other sites:

          has a good intro to CORBA in the Java world.
 has a CORBA/RMI comparison.

  2. (Sect. 15) How do I do RMI into a different domain?"

     [*] Similar to the proxy answer in a section below; you must tell the
     program where to find the server. In this case start up the client with
     this commandline option: -Djava.rmi.server.hostname=hostname.domainname

  3. (Sect. 15) RMI seems to have stopped working for me in JDK 1.1. Why is

     [*] The rules for where the client looks for a stub class seem to have
     changed making it necessary to reset your class path on the client
     after starting the RMI registry. In particular, it looks like rmic was
     not updated to the new "don't need $CLASSPATH for current dir"
     convention as the compiler was. You are best off setting classpath
     Other sites:
     There are several very good sources available from Sun which cover many
     simple and advanced RMI problems.
        o The documentation, of course:

        o Dedicated FAQs on RMI and Object Serialization

        o Mailing list RMI-USERS@JAVASOFT.COM with archive at
 Visit the archive!

  4. (Sect. 15) After a number of RMI client to server connections (55 on my
     system), subsequent RMI clients trying to connect fail. Why?

     [*] You are hitting the default limit of 64 open file descriptors. Try
     increasing the limit in your OS.
     In addition there is currently a practical RMI connection limit imposed
     by the scalability of the VM and the performance of object
     serialization. This is addressed in JDK 1.2. The actual number of
     active clients you will be able to support will depend on the workload
     mix you have (i.e. the number of clients, how often they talk to the
     server, and how much work must be done per call).

  5. (Sect. 15) I'm using RMI on Win95, and the Naming.lookup() call is
     taking a long time, even for localhost. How do I fix it?

     [*] (See also the first answer in next section below, and note that
     this Windows workaround has never worked for some people). Try adding a
     definition for the machine in your "hosts" file. Typically, this file
     will be named c:\windows\hosts (if it doesn't exist, there should be a
     file called c:\windows\hosts.sam). The hosts file is searched by your
     TCP/IP stack before it resorts to DNS, so adding an entry in this file
     can speed up your lookups considerably. The hosts file is used to map
     IP addresses to symbolic addresses. To enter the name "localhost" with
     address (the IP loopback address), enter the following line
     in your hosts file. localhost

                                Windows Networking

  6. (Sect. 15) Why does < Windows RMI/my java debugger/IDE/other> hang for
     a couple of minutes if my Windows PC is not dialed up to the Internet?

     [*] Java has networking support built in. When the Java program starts
     the Winsock DLL automatically gets loaded. The first thing this does is
     to try to resolve the fully qualified domain name for your machine
     under the name "localhost". If your system doesn't have this name
     mapped, it will try to query a nameserver on the internet, which is
     typically (on a PC) your dialup ISP. So it either prompts you to
     connect to the ISP, or waits till the attempt times out.

     Some people say you can avoid the Win95 problem by giving your system
     another way to resolve DNS names. This tip has never worked for me.
     Edit the hosts file for your system so that localhost and the full
     domain name are both mentioned. On Windows 95 systems the hosts file
     is: %windir%\HOSTS (for example, C:\WINDOWS\HOSTS). On Windows NT
     systems the hosts file is: %windir%\System32\DRIVERS\ETC\HOSTS (for
     example, C:\WINNT\System32\DRIVERS\ETC\HOSTS).
     One gotcha under Win95 is that if the last entry in the hosts file is
     not concluded with a carriage-return/line-feed then the hosts file will
     not be read at all. So if my system is called change
     the hosts file from




     Showing more of the file:

          # Hosts file

     Another alternative is to dial up with a PPP connection to your ISP
     whenever you want to run networking programs.

     Fundamentally the experience of some people has been that networking is
     not completely satisfactory on Windows95 using Winsock 1.1, and is
     subject to sporadic unexplained failures. You could try downloading
     Winsock 2.0. To get Winsock 2.0, you need to drag in all the other junk
     from Microsoft Windows Sockets 2.0 Software Development Kit. This free
     software can be downloaded from the following addresses: or

     The patches needed to improve Win95 networking are already in Win98.

                             Other Networking Issues

  7. (Sect. 15) If I call the InetAddress.getByName() method with an
     IP-address-string argument, like "", get an
     UnknownHostException on some platforms, but not others. Code like

     Socket sock = new Socket("", 23);

     triggers the exception. Why?

     [*] This is a platform difference that arises out of different
     semantics in the underlying network libraries, and is [said to be, but
     subject to confirmation] fixed in JDK 1.1. On Solaris and Windows NT,
     the IP address string only works for IP addresses that have an
     associated hostname. On Linux and Windows 95, the IP address string
     works in all cases.
     When InetAddress is instantiated with an IP address, a reverse DNS
     lookup is done. If the IP address is not associated with a valid
     hostname, the instantiation will fail. This is part of anti
     DNS-spoofing, and in JDK 1.1 works because the reverse lookup will not
     occur until the hostname is asked for. So in JDK 1.1,

             InetAddress in = InetAddress.getByName("");

     [Note: this info is still to be confirmed. Net gurus?]
     Other sites:
          Microsoft has several network-related patches at its site

  8. (Sect. 15) I want to pass a class file to willing recipients who are
     using my applet. Any ideas how?

     [*] You could use a trick: put your .class file(s) in a .zip archive
     and use showDocument() on the URL. A person accessing this will get a
     dialog box put up asking them about saving the file to their local hard
     Other sites:
          You can see this in action and try it out yourself at:

  9. (Sect. 15) How do I get a URLConnection to work through proxy
     firewalls? I.e. How do you get your Java application to do its web
     accesses through a proxy?

     [*] This is typically needed for any net access to another domain. Tell
     the run time system what you are trying to do, by using these
     commandline arguments when you start the program.

     java -DproxySet=true -DproxyHost=SOMEHOST -DproxyPort=SOMENUM

     Note proxyPort is optional and it defaults to 80. Without this, you
     will see an exception like or

     The proxy settings work for both, and for

     Netscape's and IE's JVMs (at least in versions 4.x+) take the proxy
     settings for applets from the browser's proxy configuration. You can
     also do URL proxies in applications (not applets) with the following

         // set up to use proxy
         System.getProperties().put("proxySet", "true");
         System.getProperties().put("proxyHost", "");
         System.getProperties().put("proxyPort", "80");

     But how do I know the name of the proxy server?
     This code just tells you how you can get a URL connection to the
     outside. Since it is your proxy server, you are expected to know the
     name of it. There isn't any code that you can write that will allow
     arbitrary URL connections to be initiated from outside the firewall.
     Think about it! If there were, the firewall would not be doing its job.

     Also note there are corresponding socksProxyPort and socksProxyHost for
     when socks is used instead of proxy. The default socks port is 1080.

 10. (Sect. 15) What is "swizzle", as in "Swizzle this object?"

     [*] It means serialize. To swizzle an object is to recursively
     serialize or flatten composed objects.

 11. (Sect. 15) I have been using the Serializing capabilities in 1.1 to
     save some objects to disk. I added a new field to one of my objects
     that get serialized and now deserializing my old data no longer works.
     I get this exception: MacroData; Local class not compatible

     [*] You need to add a declaration such as

         static final long serialVersionUID = 4021215565287364875L;

     in the modified class. The actual value of this long is supplied by the
     "serialver" utilitity suppied with the JDK. Any versions of a class
     other than the first version require this static to be defined in the
     class. This is how versioning is achieved.

 12. (Sect. 15) My socket code looks good, but is broken!

     [*] When using sockets you typically open both inward and outward
     streams. A TCP connection is full duplex, but either the send or
     receive side may be closed independently. By default, the remote end
     will take the close as indicating that the connection has simply been
     closed, and will close its end as well. Check whether this is happening
     for you, by adding the matched pair. Use tcpdump to check this.

 13. (Sect. 15) How do I map between IP address and hostname?

     [*] In Java 1.1 (earlier releases were buggy) use:

         String host = InetAddress.getByName("").getHostName();

 14. (Sect. 15) How do I embed an anchor in a URL? Just putting it as part
     of the string in the constructor doesn't work.

     [*] Like this:

         URL url = new URL("");
         URL anchor = new URL(url, "#section2");

 15. (Sect. 15) How do I POST to a CGI script from an applet?

     [*] Let's start by noting that this is more troublesome than it might
     seem at first, and that GET is preferred. For an untrusted applet, the
     CGI script can only be on the server that served the applet. POSTing to
     the server involves sending key/value pairs, not just a message. Also
     values must be encoded. To send "words to the server" you might try:

     StringBuffer sb = new StringBuffer();
     String str="words to the server";

     URLConnection cn = url.openConnection();

     // set properties

     // send parameters
     PrintWriter out = new PrintWriter(cn.getOutputStream());

     // read stuff

     BufferedReader in = new BufferedReader(
                           new InputStreamReader(

     sb = new StringBuffer();
     String inputLine;

     while ((inputLine = in.readLine()) != null){


         uresp = new URL(getDocumentBase(),"respond.html");
         getAppletContext().showDocument(uresp); }

     The CGI has to write its output to respond.html so that it can be
     displayed by the browser. But it may still fail, because respond.html
     could be overwritten by a subsequent request to the same CGI before the
     results of the first POST are read back.
     To get an acceptable solution takes quite a lot of effort. In general
     you should prefer GET to POST for CGI access from Java. As it says on
     the Javaworld page, the answers to the question are really: you can't,
     don't POST (use GET), use a bean, or cheat.

     Note, if you request a URL via the URLConnection/HttpURLConnection, the
     server sets the content type, and your applet can use
     URLConnection.getContentType() to get the type. Alternatively, use
     setRequestProperty to set it, like this:

         url = new URL(cgiUrl);
         urlc = url.openConnection();

     Other sites:
          There's a pretty good explanation at

     See also the Marty Hall book Core Web Programming (Prentice Hall ISBN:
     0-13-625666-X). It has comprehensive coverage of writing Java programs
     that interface to CGI scripts. Better still, use servlets, write
     everything in Java, and discard your CGI code.

 16. (Sect. 15) How can I write CGI programs in Java?

     [*] CGI (the Common Gateway Interface for web servers) is an API for
     writing programs that use the web as its user interface. By far, the
     most popular language for this task is Perl, because of its powerful
     text handling capabilities, and excellent resources available for
     making the jobs of CGI programmers easier. CGI programs can be written
     in any language, including Java.
     Unfortunately, the interface between the web server and the CGI program
     uses environment variables extensively. Use of environment variables
     has always been deprecated in Java, because of portability issues (not
     all systems have environment variables). The way to get around this is
     to write a "wrapper" for the Java program in a language that supports
     environment variables, and can then invoke the Java program with the
     appropriate environment data passed in as Java properties.
     Because the Java runtime environment is not a lightweight process, it
     might take a moment for the CGI program to get started before anything
     happens. This is particularly true on operating systems, like NT, that
     have a large overhead to spawning new processes.
     In preference to using Java for CGI on the server, you might consider
     using the Java servlet API in Netscape's Enterprise Server. This allows
     you to develop server-side programs in Java without suffering the same
     performance restrictions and other limitations of the CGI API.
     Other sites:
     query/eval_guide/enterprise/advantage.html for more details.

 17. (Sect. 15) How can I write the "ping" program in Java?

     [*] You can't do it directly. Quoting from the Java Networking FAQ,

          Ping requires ICMP packets. These packets can only be created
          via a socket of the SOCK_RAW type. Currently, Java only
          allows SOCK_STREAM (TCP) and SOCK_DGRAM (UDP) sockets. It
          seems unlikely that this will be added very soon, since many
          Unix versions only allow SOCK_RAW sockets to be created by
          root, and winsock does not address ICMP packets (win32
          includes an unsupported and undocumented ICMP.DLL).

     You could write a program that handshakes with a remote system, to
     simulate ping, or you could use native code for the raw socket. Other
          You can find the Java Networking FAQ at

          There is an additional Java Networking FAQ (which is being
          modified and reposted regularly) at

 18. (Sect. 15) Why, when I make a call to DatagramPacket.getlength() does
     it returns the wrong size?

     [*] You are probably reusing a Datagram packet, and it has no way to
     grow its buffer if a longer packet comes in. Take care of it by calling
     setLength(max) before reuse.

     Failing to do this is a very common mistake.

 19. (Sect. 15) Why do my networking programs freeze on the Mac?

     [*] You shouldn't println to a socket. This is the subject of Apple
     Technical Note 1157 at
     The problem is that many socket protocols expect CR+LF to terminate a
     string, but println delivers a platform-specific EOL mark. On the Mac,
     it is just CR. Therefore the server hangs waiting for the Mac to send a
     LF. It never does. But the program works fine on platforms that send

 20. (Sect. 15) How do I "timeout" a socket read, as the "select" function
     in Unix does?

     [*] There are two timeouts at issue here.
     Use the ServerSocket.setSoTimeout(int millis) for timing out your
     'accept()' calls.

     For a select() like mechanism, you can poll InputStreams of your
     connections for available() > 0

     URLConnection doesn't have a method for this, but you can do it by
     using multiple threads. Create a timer thread that would just track
     time and close the connection after your specified timeout period.
     Check the URLConnection to see if it's got any data flowing through it
     (calling URLConnection.connected() is probably a sufficient test). The
     URLConnection should be in a separate thread.


16. Security

  1. (Sect. 16) What is a "trusted applet"?

     [*] JDK 1.1 introduced the notion of a "trusted applet" which is one
     that has been cryptographically signed to guarantee its origin and make
     it tamper-resistant. Trusted applets can be granted more system access
     privileges than untrusted applets.
     You preconfigure your browser with a list of whose X.509 certificate
     you trust, and then applets arrive with X.509's attesting to their
     keys. It's easier than it sounds.

  2. (Sect. 16) What is the story with Java and viruses? What is the
     blackwidow virus?

     [*] Java was designed with security in mind. The security features make
     it very difficult, probably impossible, to attach a virus (self-copying
     code) to a Java applet. There has never been a Java virus carried from
     system to system by applets.
     There has been mention of a "Java virus" called "BlackWidow" in the
     media (it was mentioned in Unigram in late 1996, and obliquely on the
     RISKS newsletter in February 1997). A request to the editor of Unigram
     for more information brought the answer that there was no more
     information, it was just a report of a rumor. As far as is known, this
     story exists only as rumors reported on by the press. There is no
     actual Java virus or blackwidow virus (there was a legitimate
     commercial product of that name, since renamed).
     In spring 1998 there were press reports of a "Java applet" called White
     Ghost. It turns out that it relies on security flaws in ActiveX, and
     the only susceptible systems are Microsoft's Active Desktop. If anyone
     has a URL for a copy of this code, and an analysis of it, please
     contact the FAQ author.

     In August 1998, Symantec had some information about a Java program
     (application) that could append itself to some other Java program. They
     termed it "Strange Brew". That is just normal file processing, no
     different to a program written in C, C++, Fortran, COBOL, etc. What
     makes PC viruses dangerous is when they have a hidden means of
     travelling between places, such as being attached to code that is
     automatically executed, like boot sector code, or Word macro
     initialization files. No one has yet produced a virus that does this in

     In March 1999, reports surfaced of a virus called BeanHive. Again this
     was not Java-related, but relied on the browser user explicitly
     accepting an unknown certificate as trusted when prompted. If the user
     allowed that, then any code from that source will be uploaded and run
     without further input from the user. Don't remove security for
     untrusted code simply because it asks for it, is the lesson here.
     If anyone has more concrete information about a virus that can attack a
     Java applet (again, this is thought to be impossible), please contact
     the FAQ author.

  3. (Sect. 16) Why do I get the warning string "Unsigned Java Applet
     Window" at the bottom of popup windows in my applets?

     [*] This is a security feature, to make certain that users can always
     tell that a window asking for their password and credit card details
     (or whatever) is from an applet. There should be no way for an
     untrusted applet to work around this message.
     Also in the FAQ:
     See also the answer to Q12.7.

  4. (Sect. 16) Where can I find information on signing applets?

     [*] Please take a look at the "Code Signing for Java Applets" page at The page explains how to
     sign your Java applet so that it can be used in both
     Navigator/Communicator and Internet Explorer.

  5. (Sect. 16) Where can I find crypto libraries for Java?

     [*] Cryptographic libraries are not part of the Java release because US
     Government policy classifies strong cryptography under the same rules
     as munitions. Its export is regulated under the International Traffic
     in Arms Regulations. Many people regard this as a Kafka-esque (and
     futile) attempt to stem the use of cryptography inside the US.
     Other sites:
        o The comprehensive and free "Cryptix" crypto library written in
          Java is at:

          See also
        o Another free crypto library for Java is at:

          It includes Blowfish, CRC16, CRC32, DES, DES3, IDEA, RC4, ROT13
          (can they really call that "crypto"?), and more.
        o Another pure Java Cryptography toolkit is at
          for personal use).
        o One commercial Java encryption source (from Ireland) is:

        o A complete crypto API for Java (with HTML documentation) at:

          The library provides comprehensive and complete range of crypto
          library and functions covering DES, 3DES, IDEA, Blowfish ...and
          RSA, DH, DSA and PGP access to Java programmers. The crypto
          functions are based on the C cryptlib, by Peter Gutmann. It would
          be illegal to export this under current US government rules, but
          the author of the code is outside the US, and not subject to US
          export regulations. Download it today before it becomes illegal.
        o Some Java crypto available with source under GPL is at

        o Also, data about Sun's Java Cryptography Extension (JCE) is
          available at:

          (This may not be exported outside the USA and Canada).
        o An actual port of PGP v2.6.3i to Java is at:

        o A list of free crypto libraries, not necessarily in Java, and of
          course the FBI will be all over you like hair on an ape if you try
          to export any of these, is at:

  6. (Sect. 16) How do I find out what these terms mean?

     [*] Read Bruce Schneier's excellent book "Applied Cryptography 2nd Ed."
     for more info on what these terms mean. Read David Kahn's excellent
     (and exhaustive) book "The Codebreakers" for more info on the history
     and background of encryption.

  7. (Sect. 16) Where is Javasoft's Security FAQ?

     [*] Javasoft's security FAQ can be found at:
     Other sites:

  8. (Sect. 16) Where is that online book on Java Security?

     [*] Gary McGraw and Ed Felten's book Securing Java can be found on the
     web at


17. For C and C++ Afficionados

  1. (Sect. 17) How do I translate C/C++ into Java or vice-versa?

     [*] In general it is not simple to translate C/C++ into Java, as Java
     lacks the arbitrary pointer arithmetic of those languages. If your C
     code does not use pointer arithmetic, automatic translation gets a lot
     simpler. Try these URLs:
 (search for X-Designer 4.6: Java edition).

     Going the other way there are currently three freely-available tools to
     translate Java into C. It seems that these have been done for hacking
     value, rather than practical purposes.
        o j2c from Japan,
        o Toba from the Sumatra research project, translates 1.0.2 .class
          files into .c source code
        o JCC from Nik Shaylor.

     None of them support the AWT yet, and both j2c and JCC have additional

     There's a product to convert Visual Basic to Java. Details at
 (also a Delphi-to-Java source converter)

     There's a product to translate COBOL source to Java source, see

     This program dumps info about the class file:

     Chuck McManis was one of Sun's original Java implementors.

  2. (Sect. 17) How are finalizers different from C++ destructors?

     [*] Java objects are not explicitly deleted and do not have
     destructors. Instead they are implicitly garbage collected when the JVM
     realizes your program can no longer access them. Typically this
     technology is _not_ based on reference counting and _will_ cope with
     circular references.

     Every object has a routine called finalize() which will be called
     before the object is collected. This is Java's nearest equivalent to
     C++'s destructor. However, it is not a good idea to rely on
     finalization for the timely freeing of resources.

     This is because garbage collection and hence finalization may be
     arbitrarily delayed, and may never happen at all if the program
     terminates before it runs out of memory. You should instead provide
     your objects with methods similar to Graphics.dispose() to free
     resources, and call the dispose() method explicitly when you have
     finished using them - typically within the "finally" clause of a
     "try/catch" block. You may then call your dispose() method from within
     your finalize() method as a last-ditch attempt to free the resource if
     someone forgets.

     Alas, all this means the C++ idiom of "object construction is resource
     aquisition" does not translate well to Java. However, note that 90% of
     destructors in C++ are there to free memory, and the GC means you don't
     need to do that in Java. As well as fixing an important source of bugs,
     the GC is essential to Java's security model; without it you could
     forge object references by preserving the reference after the object
     has been deleted.

     If your program appears to be crashing due to running out of some
     system resource (like File, Window or Graphics handles), it probably
     because the system is running out of handles before it has run out of
     memory. Check that you have called the dispose() method (or equivalent)
     on every object that uses system resources. You can help the GC a
     little bit more by explicitly NULLing out references that you've
     finished with.

  3. (Sect. 17) What's the Java equivalent of sizeof()?

     [*] There isn't one. sizeof() in C and C++ is used in three main
       1. To check on the size of a primitive type. In Java, the sizes of
          primitive types are fixed in the language specification (a short
          is _always_ 16 bits; an int is _always_ 32 bits, etc), so this is
          no longer necessary.
       2. In memory allocation (i.e. malloc (32 * (sizeof(int));) In Java
          you always allocate a specific type of object, rather than a block
          of raw memory that you will fill as you like. The system always
          knows the size of the kind of objects you are allocating. So
          sizeof is not needed.
       3. In pointer arithmetic (i.e. p += sizeof (int)) Pointer arithmetic
          of this type is not allowed in Java, so this isn't necessary,

     For all these reasons, there is no need for a Java sizeof() operator.
     by having the object serialize itself to a ByteArrayOutputStream, and
     looking at the bytearray.length.

     That won't work because a lot of additional data is written when an
     object is serialized. The additional data includes a description of the
     class, any objects referenced by the serialized object, null references
     (written as a single byte), etc. If you write another instance of the
     same class, the amount of data written can differ dramatically. And if
     you serialize the same object again, it isn't written at all -- even if
     its data fields have changed! Instead, a one byte token and a four byte
     "sequence number" that refer to the first writing are output. Using
     Object Serialization to determine the size of an object does not
     (except by coincidence) give the right answer.

  4. (Sect. 17) Does Java have the equivalent of "const" arguments in C and

     [*] Java 1.1 adds the ability to use the "final" keyword to make
     arguments constant. When used to qualify a reference type, however,
     this keyword indicates that the reference is constant, not that the
     object or array referred to is constant. For example, the following
     Java code:

          void foo(final MyClass c, final int a[]) {
              c.field = 7; // allowed
              a[0] = 7; // allowed
              c = new MyClass(); // final means this is NOT allowed
              a = new int[13]; // final means this is NOT allowed

     is roughly equivalent to the following C/C++ code:

          void foo(MyClass * const c, int * const a) {
              c->field = 7; // allowed
              a[0] = 7; // allowed
              c = new MyClass(); // const means this is NOT allowed
              a = new int[13]; // const means this is NOT allowed

     Java does not have any equivalent to the following C/C++ function

          void foo(const MyClass *c); // a pointer to a const class
          void foo(const int *a); // a pointer to a const int
          void foo(const int a[]); // a pointer to an array of const ints

  5. (Sect. 17) Are there any hacks around this?

     [*] Certainly! There are always hacks around stuff. One way of
     enforcing constant values is to have two interfaces, a constant one and
     a non-constant one, e.g.

          public interface ConstFoo {
             int getValue();

          public interface Foo extends ConstFoo {
              int getValue();
              void setValue(int i);

     Then when you want to receive a parameter that cannot be modified you

          void noChange(ConstFoo foo);

     For a parameter that can be modified

          void change(Foo foo);

  6. (Sect. 17) How can I write C/C++ style assertions in Java?

     [*] The two classes shown below provide an assertion facility in Java.
     Set Assert.enabled to true to enable the assertions, and to false to
     disable assertions in production code. The AssertionException is not
     meant to be caught--instead, let it print a trace. Since the exception
     is not meant to be caught, we just extend Error instead of
     RuntimeException. As with RuntimeException, a method does not need to
     declare that it throws Error. In addition programmers are less likely
     to write "catch(Error) ..." than "catch(RuntimeException)".

     With a good optimizing compiler there will be no run time overhead for
     many uses of these assertions when Assert.enabled is set to false.
     However, if the condition in the assertion may have side effects, the
     condition code cannot be optimized away. For example, in the assertion

          Assert.assert(size() <= maxSize, "Maximum size exceeded");

     the call to size() cannot be optimized away unless the compiler can see
     that the call has no side effects. C and C++ use the preprocessor to
     guarantee that assertions will never cause overhead in production code.
     Without a preprocessor, it seems the best we can do in Java is to write

          Assert.assert(Assert.enabled && size() <= maxSize, "Too big");

          Alternatively, use

              if (Assert.enabled)
                  Assert.assert( size() <= maxSize, "Too big" );

     In this case, when Assert.enabled is false, the method call can always
     be optimized away totally, even if it has side effects. The relevant
     sections of the JLS are Section 13.4.8, final Fields and Constants and
     Section 14.19, Unreachable Statements. 13.4.8 requires that primitive
     constants ("a field that is static, final, and initialized with a
     compile-time constant expression") be inlined. So everywhere
     Assert.enabled is refered it is replaced at compile time with its
     value. Writing:

           if (Assert.enabled) Assert.assert(size() <= maxSize, "Too big");

     is exactly the same as writing:

           if (false) Assert.assert(size() <= maxSize, "Too big");

     ... assuming Assert.enabled is false at compile time. Section 14.19
     discusses compiling away such dead code. To sum up: the inlining of the
     primitive constant is required by the spec. The subsequent optimization
     of not generating code masked by (what turns into) an "if (false) ..."
     is not required but is implemented by many existing Java compilers.

          public class AssertionException extends Error {
              public AssertionException(String s) {

          public final class Assert {
              public static final boolean enabled = true;
              public static final void assert(boolean b, String s) {
                  if (enabled && !b)
                      throw new AssertionException(s);

  7. (Sect. 17) How do I do stuff like scanf and sscanf in C/C++? And how do
     I do stuff like sprintf, e.g.

     float x = 12345.6789;
     printf("%6.3f/n", x);

     [*] You can break a string like "5 loaves 2 fishes" into its parts by
     using java.util.StringTokenizer. This is the Java equivalent of

     StreamTokenizer does a similar thing on a file or any stream (i.e, what
     scanf() and fscanf() do in C).

     To do formatted character output, create a format string, and then use
     that to format your binary value, e.g.

          import java.text.*;

          float fi = 1234.56789F;
          DecimalFormat mydf = new DecimalFormat( "###0.000" );
          mydf.setMinimumIntegerDigits(3);  // for example
          System.out.println( mydf.format(fi) );


     If you want to see a float print out as "0.0000001" instead of "1E-7",

     java.text.DecimalFormat myFmt = new

     There are lots of different characters you can feed to the
     DecimalFormat constructor, not just "0" and "#". See
     $JAVAHOME/src/java/text/ source for details.

     The always excellent Acme site has an sprintf() package written in Java

  8. (Sect. 17) What is the Java equivalent of C++'s "friend"?

     [*] The keyword "friend" in C++ is a hack to allow a piece of code to
     access the private member declarations of another class. In Java, you
     would do this by labelling, not the friend, but the private members.
     Instead of making them private, make them either protected or package
     (no keyword) or public.

     The four different Java protection levels are: private, package,
     protected, and public.
        o private members can only be accessed by the containing class and
          internal classes.
        o package (specified by omitting other keywords) is the default
          level of protection; members are accessible from any class within
          the package of the containing class.
        o protected is package-level-access plus access to sub-classes of
          the containing class. So "protected" is less protected than the
        o public fields in public classes are accessible from all classes.

  9. (Sect. 17) Does anything like the C++ Standard Template Library exist
     for Java?

     [*] Yes, only it's better and simpler to use in Java. It's called the
     Java Generic Library. This library (JGL) is freely downloadable from

     It includes about a dozen nice data structures (including sets and
     bags) and algorithms like unions, searching, and sorting.

     It has over 100,000 users and 11 OEM distributors. [Some Java vendors
     are bundling it with their next release]

 10. (Sect. 17) What happens to post-increment when an exception is thrown?

     [*] If you have the code:

         array[i++] = foo();

     and foo() throws an exception, i will be incremented anyway. This can
     cause problems if sometimes foo() throws an exception and you don't
     want i incremented in cases when it does.

     This is a consequence of JLS 15.25.1 and 15.6.1 "the left-hand operand
     of a binary operator appears to be fully evaluated before any part of
     the right-hand operand is evaluated." (assignment is taken as a binary
     operator). Note that this is not how C++ behaves.


18. Java Idioms

     See also the list of Java Design Patterns at

  1. (Sect. 18) What are the naming conventions in Java?

     [*] The naming conventions are straightforward:
        o Package names are guaranteed uniqueness by using the Internet
          domain name in reverse order: com.javasoft.jag - the "com" or
          "edu" (etc.) part used to be in upper case, but now lower case is
          the recommendation.
        o Class and interface names are descriptive nouns, with the first
          letter of each word capitalized: PolarCoords. Interfaces are often
          called "something-able", e.g. "Observable", "Runnable",
        o Object and data (field) names are nouns/noun phrases, with the
          first letter lowercase, and the first letter of subsequent words
          capitalized: currentLimit.
        o Method names are verbs/verb phrases, with the first letter
          lowercase, and the first letter of subsequent words capitalized:
        o Constant (final) names are in caps: UPPER_LIMIT.
     Other sites:
        o Check out the section "Naming Conventions" in the language

        o Also take a look at Doug Lea's draft coding standard:

  2. (Sect. 18) How do I convert a String to an int?

     [*] There are several ways. The most straightforward is:

         String mystring = numString.trim();
         int i = Integer.parseInt(myString);
         long l = Long.parseLong(myString)


         String mystring = numString.trim();
         i = Integer.parseInt(myString,myIntRadix);

     Note 1: There is a gotcha with parseInt - it will throw a
     NumberFormatException for String values in the range "80000000" to
     "ffffffff". You might expect it to interpret them as negative, but it
     does not. The values have to be "-80000000" .. "-ffffffff" to be
     properly recognized as negative values. This is true for all radixes.
     According to Bug Parade bug report 4068580, the proper way to generate
     negative-valued hex Strings for eventual use by parseInt() is with
     Integer.toString(i, 16). Once that high "sign bit" is on, without the
     accompanying character, parseInt() says "too big".
     Note 2: There are similar methods for Byte, Short, and Long. Use
     myString.trim() to get rid of unwanted spaces before the conversion.
     Some of the parse methods can cope with spaces, others can't. They were
     written by two different people.

         int i = Integer.valueOf(my_str).intValue();

     also works but involves the creation of an extra object. Note: the
     pre-FCS JDK 1.2 documentation at one point said that parseDouble and
     parseFloat methods were to be introduced, but this does not seem to be
     the case (see bug 4160672). JDK 1.2.

         float f = Float.valueOf(my_str).floatValue();
         double d = Double.valueOf(my_str).doubleValue();

  3. (Sect. 18) How do I convert an int to a string?

     [*] Try any of these:

         String s = String.valueOf(i);


         String s = Integer.toString(i);


         String s = Integer.toString(i, radix);


         // briefer but may result in extra object allocation.
         String s = "" + i;

     Note: There are similar classes for Double, Float, Long, etc.

  4. (Sect. 18) How do I print the hex value of an int?

     [*] You can print the hex equivalent of an int with:

         int i = 0xf1;
         System.out.println("i is hex " + Integer.toHexString(i) );

     OK, how do I read a hex string into an int?

         int i = Integer.valueOf(myHexString, 16).intValue();

  5. (Sect. 18) How can you send a function pointer as an argument?

     [*] Simple answer: use a "callback". Make the parameter an interface
     and pass an argument instance that implements that interface.

         public interface CallShow { public void Show( ); }

         public class ShowOff implements CallShow {
             public void Show( ) { .... }

         public class ShowOff2 implements CallShow {
             public void Show( ) { .... }

         public class UseShow {
             CallShow savecallthis;

             UseShow( CallShow withthis ) {
                 savecallthis = withthis;

             void ReadyToShow( ) { savecallthis.Show( ); }

         // in some other class that uses all this stuff:
         UseShow use_1 = new UseShow( new ShowOff() );
         UseShow use_2 = new UseShow( new Showoff2() );

     and then the ReadyToShow() method on use_1 or use_2 will call the
     appropriate method, as if you had stored a pointer to the method.

  6. (Sect. 18) How do I execute a command from Java?

     [*] Use

         Runtime.getRuntime().exec( myCommandString )

     where myCommandString is something like "/full/pathname/command". An
     applet will need to be signed in order to allow this.

     Note, there is a gotcha associated with reading output from commands.
     When the runtime exec's the process, it passes to it 3 streams, for
     stdin, stdout, and stderr; the out and err are buffered but the buffer
     size isn't very big. When your process runs, it reads (if needed) from
     in, and writes to out and err. If it doesn't write more than the
     buffer-size, it can run to completion.

     But if it tries to write more data to one or the other stream than the
     buffer can hold, the write blocks, and your process hangs, waiting for
     you to empty the buffer so it can write some more.

     So after the exec call, get the streams, and read from them in a loop
     until they both hit end-of-stream (don't block on either one, just read
     whatever is available from each, each loop iteration). Then when the
     streams have ended, call the process.waitFor() method to let it finish

  7. (Sect. 18) How do I do I/O redirection in Java using exec()?

     [*] This solution works on Unix platforms using either JDK 1.0.2, or
     JDK 1.1. The trick is to use an array of Strings for the command line:

         String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};

     If you don't do this, and simply use a single string, the shell will
     see the -c and /bin/ls and ignore everything else after that. It only
     expects a single argument after the -c.

         import java.util.*;

         class IoRedirect {
         public static void main(String Argv[]) {
             try {
                 String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};
                 Process p = Runtime.getRuntime().exec(command);
                 System.out.println("return code: "+ p.exitValue());
             } catch (IOException e) {
                 System.err.println("IO error: " + e);
             } catch (InterruptedException e1) {
                 System.err.println("Exception: " + e1.getMessage());

  8. (Sect. 18) So why can't I exec common DOS commands this way (as in

     [*] The reason is that many of the DOS commands are not individual
     programs, but merely "functions" of There is no DIR.EXE or
     COPY.EXE for example. Instead, one executes the command processor
     (shell) explicitly with a request to perform the built-in command, like
     Runtime.getRuntime().exec(" /c dir") for example. On NT, the
     command interpreter is "cmd.exe", so the statement would be
     Runtime.getRuntime().exec("cmd /c dir") And you could bring the command
     output into the program with code like:

     Process p = runtime.exec ("cmd /c dir");
     DataInputStream procIn = new DataInputStream(p.getInputStream());
     while ( true ) {
        String line = procIn.readLine();
        if ( line == null ) break;
        // do something with lin

     This situation occurs on any OS where some commands are actually
     interpreted directly by the shell.

  9. (Sect. 18) OK, how do I read the output of a command?

     [*] As above (18.6, 18.7), adjusted like this:

         BufferedReader pOut= new BufferedReader(
         new InputStreamReader(p.getInputStream()));
         try {
         String s = pOut.readLine();
         while (s != null) {
             s = pOut.readLine();
         } catch (IOException e) { }

     Another possibility is to read chunks of whatever length as they come


         p = r.exec(cmd);
         InputStream is = p.getInputStream();
         int len;
         byte buf[] = new byte[1000];
         try {
             while( (len = != -1 ) {
                 String str = new String(buf,0,0,len);
                 System.out.println( "Process out: " + str );
         } catch( eof ) { ...
         } catch( ioe ) { ...  }

     However, you cannot read output from some DOS commands on Windows
     95/98. This is bug 4211683 at Sun, but it is actually a Microsoft
     bug/poor design. Windows programs that explicitly use a handle to the
     console ("CON" or "CON:") instead of stdin and stdout cannot be used as
     subprocesses to Java. The Win95 FTP program is an example of such a
     program. Windows provides no convenient way for Java to redirect the
     input and output of such processes from the console to input and output
     pipes. The only workaround is to use a different implementation of the
     program that does not use the console directly.

     Microsoft documents a difference in DOS Console stream handling between
     95 and NT: see

     So capturing output from GNU ls on Win95 works, but the output from
     " /c dir" is not captured.

 10. (Sect. 18) How do I compile code which has a cyclic dependency, i.e
     class pkg1.X contains a reference to class pkg2.Y ?

     [*] You throw both classes at the compiler at the same time.
     javac pkg1/ pkg2/

 11. (Sect. 18) What is the point of creating the temporary reference to

     [*] This code is from the 1.0 AWT, and the programmer was probably
     pretty skilled.

         public synchronized void layout() {
              LayoutManager layoutMgr = this.layoutMgr;
              if (layoutMgr != null) {

     The code makes a local copy of a global variable for one or both of two
     The first reason is that accessing local variables can be faster than
     accessing (non final) member variables. It's good for loops or where
     there are many references in the source.
     The second reason is so that even if other threads update the global,

         this.layoutMgr = someOtherLayoutMgr;

     This method will still have a pointer to the original layoutMgr.
     If the local variable were omitted, and another thread used the
     setLayout() method to change layoutMgr to null between when the layout
     method checked for null and when it invoked layoutMgr's layoutContainer
     method, a NullPointerException would result.
     Note that the synchronized keyword on the layout method doesn't help
     any, since setLayout (which could make such a dire change) isn't
     synchronized. Synchronized methods only lock out other synchronized
     methods on this object. (The unhelpful synchronized keyword on the
     layout method is gone in JDK 1.1.)
     There are two alternative solutions. One would be to make setLayout
     synchronized and make layoutMgr private, so that it can't be set other
     ways. This provides a stronger form of thread serialization, in that
     you would never be able to see an old layout manager being used after
     it had been replaced. However, it is slower. Another option that
     provides no increase in thread serialization over the original would be
     to catch the NullPointerException.
     Threaded programming is hard! This idiom was probably put in place by
     someone who got really bitten by this in the past.

 12. (Sect. 18) What is the difference between "a & b" and "a && b" ?

     [*] "a & b" takes two boolean operands, or two integer operands. It
     always evaluates both operands. For booleans, it ANDs both operands
     together producing a boolean result. For integer types, it bitwise ANDs
     both operands together, producing a result that is the promoted type of
     the operands (i.e. long, or int). "|" is the corresponding bitwise OR
     operation. "^" is the corresponding bitwise XOR operation.

     "a && b" is a "conditional AND" which only takes boolean operands. It
     always avoids evaluating its second operand if possible. If a is
     evaluated to false, the AND result must be "false" and the b operand is
     not evaluated. This is sometimes called "short-circuited" evaluation.
     "||" is the corresponding short-circuited OR operation.

     Possible mnemonic: The longer operators "&&" or "||" try to shorten
     themselves by not evaluating the second operator if they can.

 13. (Sect. 18) If I create a thread, and then null out the reference to it,
     what happens to the thread? Does it get interrupted or what?

     [*] The code looks like this:

         Thread t = new Thread( my_runnable_obj );
         t = null; // what happens to the thread?

     The answer is that you may no longer have a reference to the thread,
     but the JVM still does. Once a thread is started, and as long as it
     keeps running, it is a root object. Root objects are the starting
     points for "things in use" that the garbage collector uses.

 14. (Sect. 18) How can a Java program determine the level of JDK support
     given by the underlying VM? I.e. is it running in a JDK 1.0.2 or 1.1

     [*] Look at the java.version system property with:

         String ver = System.getProperty("java.version");

     There isn't a lot of standardization on the string contents however.
     Another possibility is to try { ... } to load a class that is unique to
     one release, like this:

     boolean isJDK1_1 = true;
     try {
         // java.awt.Cursor is available only in the 1.1.x JDK
         Class cls = Class.forName("java.awt.Cursor");
     } catch (Exception e)    {
         // we should have written 'ClassNotFoundException e',
         // but Communicator generates security exception instead.
         isJDK1_1 = false;

     This approach has the advantage that it can be compiled by any version

 15. (Sect. 18) How can I set a system property?

     [*] JDK 1.2 has

         System.setProperty( "property", "new value" );

     Until then, you can get all the properties, and set just the one you
     want with code like this:

         System.getProperties().put("property", "new value" );

 16. (Sect. 18) How can I clone using serialization?

     [*] Look at the code below, submitted by expert programmer John Dumas.
     It uses serialization to write an object into a byte array, and reads
     it back to reconstitute a fresh copy. This is a clever hack!


     public class Cloner {
        private Cloner() {}

        public static Object cloneObject(Object o) throws Exception {
           ByteArrayOutputStream bOut = new ByteArrayOutputStream();
           ObjectOutputStream out     = new ObjectOutputStream(bOut);


           ByteArrayInputStream bIn =
                    new ByteArrayInputStream(bOut.toByteArray());
           ObjectInputStream in     = new ObjectInputStream(bIn);


        public static void main(String args[]) throws Exception {
           java.util.Vector v = new java.util.Vector();
           v.addElement(new StringBuffer("Hello"));

           java.util.Vector vClone =

           // Changing the StringBuffer int the cloned vector has no
           // effect on the original StringBuffer object --
           // demonstrating that we have indeed done a deep copy

           ((StringBuffer)vClone.elementAt(0)).append(" world");

           StringBuffer sb = (StringBuffer)v.elementAt(0);

           sb = (StringBuffer)vClone.elementAt(0);

           int array[] = { 1, 2, 3, 4, 5 };

           int arrayClone[] = (int [])Cloner.cloneObject(array);

           // Again, changes to an element in the cloned array do not
           // have any effect on the original



     The main() routine is just a driver. All the cleverness is in the very
     brief cloneObject(). It does a "deep" clone, which is what you usually
     want (though Java gives you a "shallow" clone by default).


19. Java GOTCHA'S

  1. (Sect. 19) What is a "GOTCHA" (for non-English native speakers)?

     [*] It is an abbreviation of "Got you!" It is the triumphant
     exclamation that a bug or programming idiom makes as it traps the
     unwary programmer. This section details some of the popular "gotcha's"
     of Java.

     See also the list of Java Gotcha's at

  2. (Sect. 19) Why can't I filter filenames with the accept() method?

     [*] It's a known bug. FileDialog doesn't call FilenameFilter.accept().
     The bug id is 4031440, and it can be seen at the Java Developer

     There is no way to implement FilenameFilter support with the current
     reliance on the Win32 common file dialog. To support FilenameFilter,
     the FileDialog needs to issue a callback for each file it wants to
     display, which the FilenameFilter can veto. But the Win32 common
     FileDialog doesn't have any way to issue callbacks. Instead it accepts
     simple wildcard patterns for choosing files which match a certain
     pattern. That's a reasonable alternative to FilenameFilters, but that
     model isn't supported by the current Java API.

     JFC has a JFileChooser class that is a better bet for you to use for
     file dialogs. It works fine.

  3. (Sect. 19) I changed a final value, and recompiled just the file that
     it was in, and the entire rest of the program used the old value!

     [*] This is the "expected" behavior. If you have this in one file

     class Flags { final static boolean debug = true; }

     and you change it to, and recompile just this file:

     class Flags { final static boolean debug = false; }

     Then the rest of your Java .class files will still see it as "true".

     When you declare a "static final int" (or any other primitive), the
     compiler turns that into a compile time constant whose value can be
     substituted wherever it is used in your program. If you update the
     value in the source file, you'll need to recompile every class that
     uses it.

     See Java Language Specification, section 13.4.8 "final Fields and
     Constants": "We call a field that is static, final, and initialized
     with a compile-time constant expression a primitive constant. If a
     field is a primitive constant, then deleting the keyword final or
     changing its value will not break compatibility with pre-existing
     binaries by causing them not to run, but they will not see any new
     value for the constant unless they are recompiled."

  4. (Sect. 19) What is the "substring trap"?

     [*] The "substring trap" is the name for a mistake that is all too easy
     to make when using the substring() method of class String. The method
     signature is:

     public String substring(int beginIndex, int endIndex)

     The name "endIndex" suggests that is the index where the Substring

     But in fact, the substring extends only to the character at position
     (endIndex-1)! It seems to be done this way so that
     s.substring(0,s.length()) is equal to s. If so, the name of the second
     parameter should be something like endInxLessOne or Length. But not the
     confusing and misleading endIndex. Beware the substring trap.

  5. (Sect. 19) Why does getGraphics() return null on my offscreen image?

     [*] The following code

     class MyFrame extends Frame {
       MyFrame() {
         Image offscreen = createImage(100,100);
         Graphics offg = offscreen.getGraphics();

     will usually not work, since the peer will not exist at this time.
     Without the peer for the Frame, you cannot succeed in creating
     offscreen Images. (There's no problem creating Produced Images without
     a peer. Trying to draw them, of course, is another matter).

     One "standard" form of offscreen code looks like this: (note the reuse
     of the Graphics and Image objects for as long as possible)

     class Gumble extends java.awt.Something {
       private Image offi;
       private Graphics offg;

       public void update(Graphics g) {
         if (g == null) return; // Paranoia
         Dimension size = size();
         if (    offi == null
              || offi.getWidth()!=size.width
              || offi.getHeight()!=size.height  ) {
           if (offg!=null) offg.dispose();
           offi = createImage(size.width, size.height);
           offg = offi.getGraphics();
           // Regenerate offi here...
         // If you use getClipBounds() here,
         // check that for being null, too!
         // several implementations have been known to pass them....

       public void paint(Graphics g) {

     See also Question 8.7

  6. (Sect. 19) The dynamic type of a method argument doesn't seem to be
     used to choose an overridden method at runtime.

     [*] Correct. Generally, if you invoke a method on an object, the
     object's actual runtime type, not the type of the reference that you
     used to reference it, determines which method is invoked. This is
     regular polymorphism.

     It's not the same for object parameters: the compiler decides at
     compile time, depending on the types of the parameter expressions,
     which method signature to use, and this is "hardwired" into the
     bytecode. The compiler does not look at the object argument at runtime
     and say "ah, this is a derived type, so I will choose the method that
     takes the derived type as an argument."

     This is best seen in a code example:

     class Base { }
     class Derived extends Base { }

     public class foo {

        public static void method(Base b) {
          System.out.println("In the base method...");

        public static void method(Derived d) {
          System.out.println("In the derived method...");

        public static void test(Base b) {
          if (b instanceof Derived)
            System.out.print("Derived:  ");
            System.out.print("Base:     ");

          method(b);   // which method?  method(base) or method(derived)?

        public static void main(String args[]) {
          Base b = new Base();
          Derived d = new Derived();

          System.out.println("test calls.");

     Running the program gives an output of

     test calls.
     Base:     In the base method...
     Derived:  In the base method...

     See JLS section and 15.11.3:
     "If class S contains a declaration for a method named m with the same
     descriptor (same number of parameters, the same parameter types, and
     the same return type) required by the method invocation as determined
     at compile time then this is the method to be invoked."

  7. (Sect. 19) Why did I lose my updates when I changed data fields in a
     graph that I was serializing?

     [*] Quoting from the object serialization specification at:
          The writeObject method serializes the specified object and
          traverses its references to other objects in the object graph
          recursively to create a complete serialized representation of the

          Within a stream, the first reference to any object results in the
          object being serialized or externalized and the assignment of a
          handle for that object. Subsequent references to that object are
          encoded as the handle.

     In other words, changing an object and then writing it again does not
     really write it twice. Instead it just writes a reference back to the
     first occurrence, losing any fields that have changed in the meantime.

     There are three ways around this: (1) (inefficient) Reset (or close and
     reopen) the stream, and start again by writing the new value of the
     object. This is drastic -- you are throwing away all the serialization
     that you have already done.
     (2) (kludgey) Create a new object and write that.
     (3) (could be a lot of work) Write your own protocol for object
     serialization. Have something like a data stream where the contents of
     an object are marked by special identifiers. Each "end" of the stream
     can decide whether it will use a new object each time or reuse an
     existing object.

  8. (Sect. 19) When I click on a Java window frame, it doesn't close!

     [*] You need to add the code to listen for a window closing event, and
     take the appropriate action (hide the window, exit the program if the
     top level frame, etc).

     The window closing event handler is simple:

     Frame mf = new Frame("binky");
     mf.addWindowListener( new WindowAdapter() {
         public void windowClosing(WindowEvent we) {
             System.exit(0); // or setVisible(false); etc.
         } });

     This really should be the default behavior of an AWT Frame. So you'll
     be delighted to hear that JavaSoft has "made it so" for the JFrame
     Swing component. That leads to a slightly different problem. See
     Question 4.3.3. See also Question 13.9.

  9. (Sect. 19) What's the deal with "super"? How far back into parent
     classes can I go?

     [*] The most common use of super is the call "super()" to invoke a
     constructor in the superclass. The keyword "super" is also used to
     access fields of any superclass (not just the immediate superclass)
     that are hidden by an identically named feature in the current class.

     However there is no way to "chain" several super's together, and reach
     back higher into the parent class hierarchy. E.g. do not think that
     "super.x" means the "x of parent" and "super.super.x" means the "x of
     grandparent". This is a very common mistake. There is no
     "super.super.x". Looking at the generated byte code, if you have

     class Parent {        }

     class Child extends Parent {       }

     then, in Child "super.someParentMethod();" means "invokespecial
     X.someParentMethod()" in the JVM, not "invokevirtual".
     Invokespecial means "call the exact method I am telling you."
     Invokevirtual means "call the right method for whatever object this

 10. (Sect. 19) When I change some component (e.g. a new label on a button)
     I don't see the change on the screen immediately even if I repaint().

     [*] You need to add a call to the paintImmediately(x,y,w,h) method of
     JComponent. That repaints the component completely before continuing
     execution. On pre-JDK 1.2 systems, use:


     They cause the component hierarchy to be marked as needing to be laid
     out again, and the validate causes that to be done. It may be
     expensive, but is a way of getting the peers to recalculate size and to
     do what is needed to bring the display up to date. It has limitations:
     it doesn't cause an immediate screen update when invoked from an event
     handler, where paintImmediately() does.

 11. (Sect. 19) Why aren't popup menus working cross-platform for me?

     [*] On Windows, the pop-up trigger is a mouse release (except in
     certain programs like Netscape Communicator). On Unix, the pop-up
     trigger is a mouse press.

     Therefore you need to ask the question isPopupTrigger() in both the
     mousePressed() and mouseReleased() methods when implementing the
     MouseListener interface. Alternatively override Component's
     processMouseEvent as a central place for handling mouse input.

 12. (Sect. 19) Why aren't newlines working cross-platform for me?

     [*] Code like this:

     if (c == '\n')
          fin = true;

     is not cross-platform. On Unix the line terminator is "\n", on Windows,
     it is frequently "\r\n", on the Mac it is "\r".

     The call System.getProperty("line.separator") will return a string
     containing the platform-specific line separator character(s), and you
     then need to compare it according to how your data is formatted (e.g.
     compare 2 characters or one). There is also a property for the
     separator character in file pathnames, and other values too.

     This can screw-up your networking programs too. Most ASCII based
     protocols like HTTP expect \r\n to terminate a request. They will hang
     on a request from a Mac that only sends \r.

 13. (Sect. 19) Why didn't my text display in my GUI? Is the Inset wrong?

     [*] The most common Inset problem is not an Inset problem at all, but
     rather that people just assume the x,y location of a
     Graphics.drawString() actually refers to the top left part of the
     string image. In fact it refers to the baseline. So you'll need to take

     g.drawString("Hello World",0,getFontMetrics(getFont()).getAscent());

 14. (Sect. 19) Why did my polygon come out the wrong shape?

     [*] This question and answer comes directly off, and deserves to be immortalized for
     When I use fillPolygon with the following points I get two inverted
     triangles instead of a rectangle. Why?

     int xPoints[] = {71, 78, 71, 78};
     int yPoints[] = {147, 147, 130, 130};
     g.fillPolygon(xPoints, ypoints, xPoints.length);

     Developer Felix Pahl supplied the answer in limerick form:
        o A developer (for details bored her)
          didn't follow the polygon's border
          so instead of right angles
          she got two triangles
          'cause the endpoints were in the wrong order!

     You must put the points in the order you would encounter them in if you
     went round the polygon's border. The filling algorithm is doing the
     right thing! Try drawing the points on paper to see:

     71,130   78,130
       |        |
       |        |
       |        |
     71,147   78,147

     Under JDK1.1, the two endpoints are connected automatically and you
     would order the array elements as:

     int xPoints[] = { 71,  78,  78,  71};
     int yPoints[] = {130, 130, 147, 147};

     Under JDK1.0.2, you have to explicitly connect the two endpoints, and
     you would write the array elements as:

     int xPoints[] = { 71,  78,  78,  71,  71};
     int yPoints[] = {130, 130, 147, 147, 130};

 15. (Sect. 19) Why can't I see all the components I added to a Frame?

     [*] If you have code like:

     Frame myframe = new Frame("Child Frame");
     myframe.add(new Label("Child"));;

     The default layout manager for Frame is BorderLayout. Components
     positioned with a BorderLayout should include a positioning constant to
     be correct. If you don't include one, "-1" is assumed, which causes the
     component to go to the end of the list, and possibly be buried (in
     terms of z-order) under any other components you add. So, change the
     add to

     myframe.add("Center", new Label("Child"));

     and all will be well.

 16. (Sect. 19) Why do I get the wrong results when I compare two Strings

     if (s1 == s2)

     is giving me funny results.

     [*] The comparison using "==" on objects, like Strings, is asking the
     question "do these two objects have the same reference?". That is, do
     they have the same address, and hence are not two object but one? What
     you most probably meant is "do these two Strings have the same
     contents?" which you can express this way:

     if ( s1.equals(s2) )

     This is a very, very easy mistake to make and impossible to spot until
     you have had it explained to you.

     People talk about "interning" a String. That means calling the intern()
     method on a String. This places the String in the runtime constant pool
     if it was not already there. The compiler is required to intern() all
     literal Strings. If you intern() all your Strings as well then all
     duplicates are shared and comparisons can be done by the (much faster)
     address comparison rather than content comparison. It's a performance
     optimization. See also Q3.22.

     Note that this comparison error also occurs with other objects, not
     just Strings. The code:

             if (getBackground() ==

     is a test for object identity, rather than content identity. It will
     work if you originally setBackground( To avoid difficult
     debugging in the future, you almost certainly want to say

             if (getBackground().equals( ) )

     or even (in this visual case) compare the darkness of the RGB values of
     the pixels.

 17. (Sect. 19) Why doesn't final prevent my object from changing?

     [*] You have code like this

     final StringBuffer s = new StringBuffer("don't change me");
      // ...
     s.append(", but I did");

     And the new value of s is "don't change me, but I did". The reason is
     that the "final" modifier makes the reference variable (here, s) final,
     not the object that s points to. It means that the reference variable
     cannot be changed to point to some other StringBuffer. The state of the
     StringBuffer can still be modified by calling methods on it or directly
     assigning to its public fields.

     The right way to think about final is that it prevents you assigning to
     that particular variable. The only way to make the fields of an object
     constant (unchanging) is to make all its data fields private, and not
     provide any set methods for them, only get methods. Even that won't
     stop other objects of the same class adjusting it.

 18. (Sect. 19) Why can't the compiler find my package?

     [*] When trying to compile a file in a package you get a compiler error
     like: Class database.Table not found in type declaration.

     The file and are in the same directory. They
     both have "package database;" at the top of the file. The current
     directory is included in the classpath.

     The reason is that when compiling packages, you have to be at the 'top'
     of the directory/package hierarchy. So to compile both and, you have to be in the directory that contains the database
     directory (i.e. where the package hierarchy starts), and just:

            javac database/
            javac database/

     and it should all compile fine.

 19. (Sect. 19) I have a program with keyboard input and a button. When the
     user hits the space bar, the button gets pressed as it is in focus!

     [*] The VM sets the focus on the first traversible object in the UI. If
     you want the button not to be assigned focus by default, you must
     subclass the button and override the isFocusTraversable() method to
     return false.

     Another approach is to manually set the focus on some other component
     (say the Frame) when you show the window. To do so you have to jump
     through hoops to outsmart the VM that is trying to set it on the
     button. One approach is to listen for the windowActivated event and set
     a Swing Timer to do a requestFocus() on the frame about 0.1 seconds
     after the activated message. This seems convoluted, but it is the only
     thing found that consistently works cross platform.

     Another reader suggests that if the frame normally gets the focus
     first, you can override its gotFocus() event and set the focus to the
     component you want. Don't forget to return true!

 20. (Sect. 19) What's the hidden size limitation of String serialization?

     [*] If you wish to Serialize a string, be alert to the restriction
          The size of the String, when UTF-encoded, must be < 64Kb
     So for robust code you have to examine the String once to ensure that
     it will be less than 64Kb after encoding, and then have the JVM
     effectively repeat that work in the process of encoding, when you write
     it to an ObjectOutputStream.

     A possible workaround that is to strip the string down to "byte[]" and
     pass it around in RMI that way. The code with this restriction is in

         public final void writeUTF(String str)
         [perform the size-after-conversion-to-UTF computation]
             if (utflen > 65535)
                 throw new UTFDataFormatException();

     RMI relies on serialization, so RMI has the same String size

 21. (Sect. 19) When I change a field in just one object in my array, that
     field changes in all the objects in my array!

     [*] Here the problem is probably that you have initialized the array
     with N references to the same one object.

     This is easy to overlook, because arrays in Java only contain
     references to objects, not objects. (Or they can contain primitives).

 22. (Sect. 19) Do DrawRect and FillRect work on rectangles of the same

     [*] No. java.awt.Graphics.drawRect draws a rectangle that's 1 pixel
     wider and 1 pixel taller than a rectangle drawn by fillRect.

 23. (Sect. 19) My executable Jar files don't work!

     [*] It is possible to exactly follow the instructions given in the
     documentation and yet still get the following error message when
     attempting to run the .jar file: "Failed to load Main-Class manifest
     attribute from myapp.jar"

     There has to be a carriage return after the Main-Class definition in
     the manifest file, otherwise it does not work! Example, in a file
     called manifest.txt:

     Main-Class: MyClass  // does not work

     Main-Class: MyClass
     // does work

     Then, when the manifest file is merged into the jar file like this:

     jar cmfv manifest.txt myapp.jar *.class

     it works fine.

 24. (Sect. 19) Why do I get the error message "Wrong number of arguments in

     [*] Almost certainly, you have written your constructor with a return
     type or void or something. I.e. you have written:

          public MyClass Myclass() { ...}

     instead of:

          public Myclass() { ...}

     Giving a constructor a return type makes it into a regular method that
     happens to have the same name as the class. The language should ideally
     outlaw this, but it doesn't.


20. Further Resources

  1. (Sect. 20) Will Java ever be fast enough for games like Quake?

     [*] Some people originally thought not.

     Please see the site which has the Frag
     Island game (a quake-style game) written in 100% Java. You play it as
     an applet, by browsing the above site. Watch out -- it's noisy!

  2. (Sect. 20) Are there any commercial/shareware/free Java libraries?

     [*] Take a look at the Java Collection Framework, a group of classes
     that are part of Java 1.2. These classes implement general-purpose data
     structures, and they will become widely used.

     The documentation for JDK 1.2 explains that the Collection Framework
     defines three kinds of things:
        o Standard interfaces representing data structures of various kinds
          for you to implement. Since these are interfaces, you can use them
          in your code before you have implemented them.
        o Partial implementations of those interfaces, saving you some work.
        o Complete implementations, ready to use for data in your programs.

     The standard interfaces are Collection, Set, List and Map, plus the
     more specialised SortedSet and SortedMap. Lists have duplicate elements
     whereas Sets do not. Finer distinctions such as immutability are
     defined in the implementor classes, enforced by throwing runtime
     exceptions. See the JDK 1.2 documentation for a full discussion.

     For more about sorting prior to JDK 1.2, look at the class SortDemo in
     the demo directory of the JDK. Alternatively, use one of the several
     classic sorts available from Roedy Green. They are supplied free with
     heavily commented Java source code.

     See "QuickSort", "HeapSort" and "RadixSort" in the Java glossary at

     Also, try the Java Generic Library. This library (JGL) is freely
     downloadable from

     Also Visual Engineering has JChart at: No licensing

     Visual Numerics has its Java Numeric Library available for download at They offer the JNL as
     a proposed standard library for numerical functions missing from Java.

  3. (Sect. 20) Why doesn't somebody write a shell in Java? Then they could
     use it on all platforms!

     [*] Somebody has done just that. Look at

  4. (Sect. 20) Are there any URLs for other libraries?

     [*] Indeed, there are. The Java3D Repository

  5. (Sect. 20) Are there any URLs for regular expression handlers in Java?

     [*] There is one from ORO Inc. They dissolved as a corporation, but one
     of the founders maintains the software at
     For other sources, see

     And don't forget to check out Lava -- a set of Java classes designed to
     support programmers who develop console-mode applications and/or C
     programmers who are converting to Java. The first release of Lava has
     printf and other text formatting, encryption, parsing and miscellaneous
     I/O. Lava can be downloaded from

     Also consider the Java version of the Unix find command. It offers
     Regex filename matching, mindepth, maxdepth, symlink follow / no
     follow, file type matching all cross-platform. The package is at

  6. (Sect. 20) Are there any installers for Java? Preferably
     platform-neutral ones.

     [*] There are several possibilities.
        o InstallAnywhere 2 from ZeroG software. See

          They have a free version for shareware authors.
        o InstallShield makes a Java version of their installation package.
        o IBM offers a comprehensive Java installer through its alphaworks
          site http//
        o Denova offers the J'Express installer. There is a free trial
          version available for download at
          Freeware authors can receive a completely free license to
          J'Express Standard. Simply fill in the application at

  7. (Sect. 20) What is "Jazilla"?

     [*] Jazilla is Mozilla (Netscape Communicator free source) ported to
     Java. In other words, a free source browser that supports Java and
     Javascript, written in Java!

     You can get more information, and volunteer to help with the project at

  8. (Sect. 20) Where can I get Java for my Palm Pilot PDA?

     [*] There is a translator allowing you to compile Java programs for the
     Palm Pilot PDA! This is an astonishing piece of work as the Pilot has
     such a small memory footprint. The translator is in an early stage of
     development, but is available at:

     Try it, or even better, volunteer to help with the project. Details at
     the website above.

     There is a Java-PalmPilot Project called "jSyncManager" which allows
     PalmPilot synchronization and jConduit development in pure Java. See
     If you can run Java 1.1 and Sun's Communications API, you can run

  9. (Sect. 20) What is "Dippy Bird" and where can I get it?

     [*] Dippy Bird is Java documentation in WinHelp format, which can be
     used directly on Windows desktops, and has a searching utility. The
     developer of the Dippybird project, Bill Bercik, has stopped further
     work on the project due to lack of time and funds. Instead you can use which has a more up to
     date Java WinHelp doc.
     You can get still get the Dippy Bird download at (JDK 1.1). Note that on NT 4.0 you
     need to change the generated shortcut to point to NT's 32-bit WinHelp.

 10. (Sect. 20) Are there any Java graphing tools?

     [*] Take a look at GraphMaker -- a complete full-featured Java
     application for creating and searching graphs. It is available under
     GPL with source, and uses the latest Swing JFC features. See

 11. (Sect. 20) Where can I get icons for use with Java?

     [*] Public spirited programmer and Java supporter Dean S. Jones has
     created a collection of over 100 icons for use in Java freeware. They
     are available on the Java Lobby site at

 12. (Sect. 20) What is "UML"?

     [*] UML is the Unified Modeling Language. It is unified in the sense
     that it draws together ideas from a couple of earlier software design
     languages. UML is an emerging standard for diagrams of object-oriented
     classes. It was devised by Grady Booch, Ivar Jacobsen, and James
     Rumbaugh, and it unifies several popular existing notations.

     A UML product is available from Rational Software, who also offer a
     tutorial CD for free. See There
     are some whitepapers too, but there don't seem to be any free online

     See for information on Together/J. That is a
     platform-independent UML product that supports round-trip engineering
     for Java. Whiteboard version is free.

 13. (Sect. 20) Where can I get info on Java college courses?

     [*] The JCampus site at has links and
     connections to Java CS Dept. courses, assignments, academic papers and
     Java-related events. JCampus is a non-profit, online Community for CS
     Dept. professors, students and staff who are teaching, learning and
     using the Java programming language.

 14. (Sect. 20) What is the Java IFAQ?

     [*] It is the Java list of Infrequently Answered Questions, a FAQ
     maintained by Peter Norvig, author of the book "Artificial Intelligence
     - A Modern Approach". Take a look at the Java IFAQ at There's a lot of good information
     in that document.

 15. (Sect. 20) Are there any Java tools for PDF?

     [*] PDF (Portable Document Format) is the text publishing format
     defined by Adobe. Acrobat is the technology to display and print PDF
     files. Abode supplies the client (document reading) software for free.
     There is a PDF toolkit written in Java at Even
     better it is GPL'd. It is more a toolkit for programmers embedding PDF
     in their products, than an end-user technology though. It doesn't have
     a GUI for displaying PDF for example.

 16. (Sect. 20) Are there any Java info search tools?

     [*] IBM has a very good search engine for java developers

 17. (Sect. 20) What other languages compile to bytecode?

     [*] Quite a lot of languages compile to Java bytecode, more than 60 at
     the last count. See the webpage

 18. (Sect. 20) Has anyone written a Java-to-RPC interface, to talk to
     legacy code?

     [*] See It implements a subset of RPC, and is a
     commercial, supported product. You can review RFCs 1831 and 1832 for
     information on the full protocol. Java uses the same endianness as
     RPC's external data representation (network byte order), so all the
     Java file reads/writes can be used directly.

     Netbula recently released Java RPC, a fully compatible port of ONC RPC
     to Java. The binary package can be downloaded for evaluation from

     The specifications are in RFC 1831 (the RPC protocol spec) and RFP 1832
     (the XDR spec).

 19. (Sect. 20) Are there any automated tools for Javadoc?

     [*] Yes. See
     for a Java development tool called DocWiz. It is the easiest way to add
     JavaDoc comments to your Java code.

 20. (Sect. 20) Is there any information on XML and Java?

     [*] Yes. See
     It shows you how to use:
        o the Sun parser (Sun ProjectX EA2)
        o the DOM (org.w3c.dom.*) interfaces
        o JFC/Swing with XML
        o Servlets with XML
     and more.

 21. (Sect. 20) How can I find the format of a .class file/a JPG/a PNG
     file/any file?

     [*] There is a great website that maintains descriptions and links to
     descriptions of hundreds of file formats. The site is at:
     It shows you how the files are structured, and makes it a lot simpler
     for you to write code that creates/decodes such a file.

 22. (Sect. 20) Why not start up one copy of the JVM and share it among many
     Java programs?

     [*] If the JVM takes about 15MB (say) in overhead, and a program takes
     3MB, then starting up a fresh JVM for each program is slow and
     wasteful. If a single copy of the JVM and libraries could instead be
     shared among multiple Java applications there would be less overhead
     and everything would run faster.

     That observation was the guiding force behind the echidna project to
     support a JVM that could run multiple applications. See the site for more details.

 23. (Sect. 20) How can I edit sound files?

     [*] The JaWavedit java code lets you edit .wav and .au files. It can be
     found at It's free to use.

     If you are inspired to write freeware like this yourself, many file
     formats are explained and described at


21. Acknowledgements

A jolly little song that explains how to solve commonly-encountered problems
in Java.

                    The FAQ Melody
                  by Antranig Basman.

On the First Day of Christmas, my true-love said to me:
Read the F-A-Q.

On the Second Day of Christmas, my true-love said to me:
My Image isn't drawing;
Read the F-A-Q.

On the Third Day of Christmas, my true-love said to me:
My Pixels are not grabbing,
My Image isn't drawing,
Read the F-A-Q.

On the Fourth Day of Christmas, my true-love said to me:
My Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.

On the Fifth Day of Christmas, my true-love said to me:
Null - Pointer - Exception!
My Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.

On the Sixth Day of Christmas, my true-love said to me:
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.

On the Seventh Day of Christmas, my true-love said to me:
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.

On the Eighth Day of Christmas, my true-love said to me:
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.

On the Ninth Day of Christmas, my true-love said to me:
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.

On the Tenth Day of Christmas, my true-love said to me:
Time-zone's in Pacific,
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.

On the Eleventh Day of Christmas, my true-love said to me:
Docs are not specific,
Time-zone's in Pacific,
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing,
Read the F-A-Q.

On the Twelfth Day of Christmas, my true-love said to me:
File I/O's horrific,
Docs are not specific,
Time-zone's in Pacific,
Threads they are a-blocking,
J++ don't mind it,
Netscape will not run it,
Null - Pointer - Exception!
Layout is not laying,
Pixels are not grabbing,
Image isn't drawing;

You Should Read The-e F-A-Q!

FAQ copyright 1997, 1998, 1999 by Peter van der Linden. Contributions and
help from:

Matt Kennel, Patric Jonsson, Brad Van Tighem, Tony Hursh, Glenn L
Vanderburg, Peter Jones, John McDowall, Jim Driscoll, Uday, Dave Harris,
Bill Wilkinson, Tom Valesky, Dan Drake, Giles Thomas, Mitch Baltuch, Guy
Ruth Hammond, Gordon Keith, Jason Brome, Shani Kerr, Steve Chapel, Timothy
Wolters, Robert Lynch, Jake Cormier, Sean C Sullivan, Joseph A. Millar, Jim
Frost, Jim Balter, Jeff Bauer, John Kochmar, Carl Burke, William Stubbs,
Mark Smith, Volker Turau, Real Gagnon, Russell Gold, Max Hailperin, Bill
Tschumy, Marco Nijdam, Marc Pawlowsky, Laurence Vanhelsuwe, Ian Macgregor,
Mike Faulkner, Rich Koch, Will Clark, Govind Seshadri, Rich Simkin, Ian
Stiles, Kieren, Darren Christie, Tom Lane, Michael Jungmann, Rob Mayoff,
George Ruban, Tom McCann, David Hopwood, Thomas Phan, Kai Stuke, Rolf
Howarth, Derek Snider, David Boydston, Andy Godwin, John F. Dumas, Doug
Bell, David J. Biesack, Tiger Quimpo, Martin Hugh Rogers, Brian Krahmer, Ian
Burrell, Nikki Locke, Bin Li, Jackson Thompson, Steve Odendahl, Greg Smith,
Jeffrey C. Ollie, Mark Halvin, Jeremy Cook, Lak Ming Lam, Peter S. Morris,
Mark Halvin, Juergen Keil, Alex Stewart, Mike Abney, Rodney Stephenson, Mark
Gritter, Satish Talim, Tamminen Eero, Alexander Gridnev, Eric Hodges, Jamey
Graham, Will Lockhart, Scott Plante, Tom Sanfilippo, Jan Newmarch, Sean
Breslin, Stuart D. Gathman,, C Matthew Curtin, Tor Iver
Wilhelmsen, A.N.Pryke, Phil Race, David Holmes, David Rodal, Dominique
Plante, Trent Jarvi, Ingrid Biery, Gopal Unni Krishnan, Grant Lewis, Tov Are
Jacobsen, Gary McGath, Marty Hall, Will Forster, Colin Mummery, Darin
McBride, Mayank Shah, Jens Alfke, Glen Stampoultzis, Philip Brown, Peter
Steiner, Kurt Spaugh, Rasmus Ekman, Jonathan Revusky, Ken Kalish, Dave
Sanders, Bill Hyden, James Cloughley, Philip "diodes" Gustafson, Paul
Kinnucan, Juan Valdéz, Antranig Basman, Felix Pahl, David N. Still, Simon
Arthur, Mark Hammond, Dan Kegel, Thomas Weidenfeller, Pavel Shvartsman,
Christen Monberg, George Reese, Ian Macgregor, John Sublett, David
Zimmerman, Tony Dahlman, Druid, Chris Kelly, Patricia Shanahan, Paul Hill,
Lyne Lamoureux, Don Kennedy, Alec Muffett, Andrew Mickish, Pavel Shvartsman,
Neil of Parkway Consultants, Chris Thiessen, David Michaels, Bob Sutherland,
Michael Allen Latta, Joshy, Eric Albert, Wes Isberg, Lisa Retief, Michael
Park, Dave Postill, Thomas Weidenfeller, Konstantin Laufer, Hĺkan
Gustavsson, James Stauffer, Reuben Firmin, David Lim, Eamonn Maher, Craig
West, Pavel Shvartsman, Jay Dunning, Kevin Swan, Grant Gainey, Dan Schmitt,
Benjamin Goldberger, Jake Hamby, Yaakov Itzhaki, Robert Lynch, Laura Werner,
Tomm Hoeft, King Dale, Joe Halpin, Daniel M. Pomerantz, Kevin Kelley,, Peter Seibel, Werner Zsolt, Hank Stuck, David M.
Karr, John J. Bartholdi, III, Garry Turkington, John Bokma, Frank McCreedy,
Nat Pryce, Jeff Luszcz, Brent Callaghan, Neil Allen, Joe Preston, Tim Bell,
Rajesh, Jeffrey Galyan, Nick Matsakis, Larry Hamel, David Lamb, Mr Tines,
Bill McHardy, Phillip Lord, Jon K. Hellan


I am maintaining a FAQ list to address specifically programming issues
(not a general tutorial on Java). Please mail suggested FAQ entries
including answer to faqidea on the site
Question with answer gets you a credit in the FAQ.
Peter van der Linden, Sun Certified Java Programmer.

Cross references

Most cross reference links inside this document are still to be filled in
after the great FAQ re-org. If you'd like to contribute a few, send me the
new text for the NAME= and the HREF=, and I'll fold them in as time permits.
Look at the FAQ source for the style to follow.


Copyright (c), 1997,1998,1999 Peter van der Linden. Permission to copy all
or part of this work is granted for individual use, and for copies within a
scholastic or academic setting. Copies may not be made or distributed for
resale. The no warranty, and copyright notice must be retained verbatim and
be displayed conspicuously. You need written authorization before you can
include this FAQ in a book and/or a CDROM archive, and/or make a
translation, and/or publish/mirror on a website (scholastic and academic use
excepted). If anyone needs other permissions that aren't covered by the
above, please contact the author.

No Warranty

This work is provided on an "as is" basis. The copyright holder makes no
warranty whatsoever, either express or implied, regarding the work,
including warranties with respect to merchantability or fitness for any
purpose. Furthermore the author has been known to wear socks that don't
match his pants, and to commit other egregious lapses of good fashion sense.


User Contributions:

Comment about this article, ask questions, or add new information about this topic:


[ Usenet FAQs | Web FAQs | Documents | RFC Index ]

Send corrections/additions to the FAQ Maintainer: (Peter van der Linden)

Last Update March 27 2014 @ 02:11 PM