Applets

• Differences between Java applications and applets
  • Because applets run within a larger program, they do not have a class with a main() method. Instead they must have a class which extends the built-in class JApplet. (This requires an import statement: import javax.swing.JApplet;.) This class in turn extends the graphical class Panel, so your applet class can call or redefine all of the methods that Panel has.
  • The JApplet class does not need a constructor. Instead you can put things that would appear in a constructor in the class's init() method, which is automatically executed when the applet is accessed.
  • To be run, an applet must be embedded in an HTML file. Here is an example of how to do this for a class called "NNApplet".
    
    <applet code="NNApplet.class" width=300 height=100> </applet>
  • An applet is run either in a Web browser or a special program for viewing applets, usually called "appletviewer". In either case, the HTML file in which the applet is embedded is used.
• The main JApplet methods are the following. Each can be defined in a class that extends Applet.
  • init(). This gets called when the applet is first created and loaded by the underlying software. It performs one-time operations.
  • start() This gets called when the applet is visited, for example, when the user goes to the webpage with the applet. After start() executes, the applet's paint method gets called.
  • stop(). This staps the applet, but does not get rid of it, whenever the user leaves the webpage with the applet.
  • destroy(). This is called when the browser exits. It's the place to put clean-up operations.
• Two examples of applets: NNApplet, AdjApplet

© 2004. Indiana University and Michael Gasser. www.indiana.edu/~gasser/Q530/Notes/applets.html 1 Dec 2004