Creating Applets

The main ideas for what's presented here are taken from Core Java 1.2, Volume 1.

To convert a GUI application to an applet you'll need to follow a few steps.

  1. To make things easier, test your program as an application. The easiest way to convert the application is to create your GUI class that extends JFrame so that it launches the whole program via a parameterless constructor. For example, suppose you have a class JoggleGui that extends JFrame. Create a class (it could be JoggleGui, or a separate class) so that the program is started and run with a very simple main: public static void main(String args[]) { JoggleGui jg = new JoggleGui(); }

    This shouldn't be too hard, the constructor can call other functions in other classes, but this will make conversion to an applet simplest.

  2. Copy the class that extends JFrame (assume it's JoggleGui for this discussion) to another file called JoggleApplet, change the name of the class, and make it extend JApplet instead of JFrame. Change the constructor of the new class so that instead of having the same name as the class, e.g., JoggleApplet it's a void function named init. This means you won't have a constructor, just a function named init.

  3. Remove calls to setSize, setTitle, pack, and any window listener calls, e.g., setDefaultCloseOperation. Compile the program---if something doesn't compile just comment it out for now.

  4. Create a simple web page with the following body. <APPLET CODE="JoggleApplet.class" ARCHIVE="joggle.jar" WIDTH=300 HEIGHT=300> Your browser does not support applets </APPLET>

  5. Put all the class files your program needs in a jar file, the name of the jar file is used in the ARCHIVE line of the web page, so in this example we use joggle.jar. To create a jar file use commands similar to tar/gtar. To create a jar file use the cf option as follows: jar cf joggle.jar *.class

    Don't forget to recreate the jar file every time you recompile any class that's used in the applet. Your application loads .class files from the file system, but the applet will load the .class files from the jar file.

    For information on reading files and images from jar files this resource page

  6. Use appletviewer to test the applet. WHen it's working you can use the Java Plug-in and converter to run the 1.2/Swing applet in a browser. If you have JDK1.3 (or 1.2) on your windows machine, you most likely already have the plug-in. To get the converter visit and download the converter. The converter transforms your html file into one that both Netscape and IE will understand.

    Note: on the acpub system which uses jdk1.4 or jdk1.3, there's a program called HtmlConvert (get the case right). You can run this using the format below to convert foo.html.

       HtmlConvert foo.html

  7. You can pass size and other parameters to your applet, see the Java tutorial or another book for details.

Owen L. Astrachan
Last modified: Thu Mar 22 13:26:01 EST 2001