Implementation

The implementation of this program is left to you as a lab (or programming) exercise, but here are some hints and suggestions:

• Stepwise refinement . A good first step for writing this program would be to create the TextField and a TextArea components and implement the init() and actionPerformed() methods. The first version of the applet should merely display in the TextArea whatever number the user types into the TextField, without testing for primality.

  Once you have successfully written and tested a simple interface, define the Primes class and write and test the isPrime() method. In order to write this method, you will have to complete the development of the algorithm described in the previous section. Use pseudocode to help you lay out the algorithm. You can use your interface to help you test the method. Have the user input an integer, and then just test whether that integer is prime.

  Once you have a correct isPrime() method, write and test the findPrimes() methods. This last step should be very simple.

• Null pointer error. Beware of ``null pointer''  errors which result when you forget to instantiate (create) an object before using it. A null pointer error occurs when you attempt to refer to an object using a reference that has a value of null. When you declare a reference variable, it is given a default value of null. If you forget to instantiate it, you'll get this error. The typical sequence is declare, create, and use, although these don't always happen in the same place in the program. For example, the TextArea display can be declared in the global portion of the program, instantiated in init() and used in actionPerformed():

      TextArea display;              // Declare a variable
      .
      .
      display = new TextArea();      // Create an instance (init() )
      .
      .
      display.appendText ("blah") ;  // Use it (actionPerformed() )
  

  If you forget to instantiate in init() the use of display in actionPerformed() will cause a null pointer exception.