Lab Exercise 1: Editing, Compiling, and Running

Using whatever programming environment you have available in your computing lab, edit, compile, and run the FirstApplet program. However, don't just keyboard the whole program and then compile it. Instead, use the stepwise refinement approach as outlined here.

• Stepwise Refinement: Stage 1. Begin by keyboarding the entire comment block at the beginning of the program, the import statements, and the following class definition:

  
  

  public class FirstApplet extends Applet implements ActionListener
  {
      public void actionPerformed (ActionEvent e)
      {
      }  // actionPerformed
  }  // End of FirstApplet
  

  
  

  This is an example of a stub class. A stub class has the basic outline for a class but no real content. In this case the definition must contain a stub definition of the actionPerformed() method, which is part of the ActionListener interface. After entering this, compile and run the program. It should compile correctly, but it won't do anything because it doesn't contain any executable code.

• Stepwise Refinement: Stage 2. Keyboard the declarations given at the beginning of the class definition and the stub method definition for the init() method. A stub method contains a correctly defined method header but an incomplete method body. The idea is that you will fill in the details of the body later. Your program should then contain the following code:

  
  

  public class FirstApplet extends Applet implements ActionListener
  {
      private Button clickMe;             // The button
  
      public void init()
      {
      }  // init()
  
      public void actionPerformed (ActionEvent e)
      {
      }  // actionPerformed
  }  // End of FirstApplet
  

  
  

  Recompile the program and run it again. It should compile correctly, but it still won't really do anything. But you've accomplished a lot, because you've now correctly coded the basic structure of the program.

• Stepwise Refinement: Stage 3. Complete the coding of the init() method by keyboarding all of the executable statements contained within its body. Then recompile and run the program. Now when the program runs you should see a Button on the applet. Note that if any errors occur as you are coding this portion of the program, you know that they must be located in the init() method, because you haven't touched the rest of the program. You've now successfully implemented the applet's user interface.

• Stepwise Refinement: Stage 4. Complete the coding of the actionPerformed() method by keyboarding all of the statements in its method body. Then recompile and run the applet. It should now have its full functionality. Every time you click on the button, it should toggle its label. Cool, eh!?

Hopefully, going through this exercise has illustrated some of the advantages of the stepwise refinement approach to writing Java code.

Programming Tip: Stepwise Refinement. Stepwise refinement is a coding and testing strategy that employs the divide-and-conquer principle. Keyboard and test small segments of the program in a step-by-step fashion. In addition to breaking up the task into more manageable subtasks, this approach helps to localize any problems that arise.