This applet uses a simple GUI, and exemplifies the Model-View-Controller design pattern. That is, it divides the problem in three parts and assigns behaviors to each, as follows:

• The Model. This is the part that does the actual simulation--in this example, a bouncing ball. The ball is not actually animated (that requires Threads), but only moves one step per click of the button. This part is almost completely independent of the rest of the program.
   
• The Controller, which extends the Applet. It initializes things and puts up the controls (one Button). Each time the button is clicked, the Controller tells the Model to do something, then tells the View to display the result.
   
• The View. This accesses the Model and displays whatever it thinks should be displayed. The View doesn't necessarily need to know about the Controller, but in this example it wants to display extra information in the status line, which is part of the Applet (and therefore part of the Controller). Notice the lines in init() that make the Model and Controller available to the View:

      view.model = model;
      view.controller = this;

This clean separation should make it a lot easier for you to adapt my code to your needs.