Inner Classes

•  Trivial class can be defined inside a method

 public class DataSetTester3
{
   public static void main(String[] args)
   {
      class RectangleMeasurer implements Measurer
      {
         . . .
      }
      Measurer m = new RectangleMeasurer();
      DataSet data = new DataSet(m);
      . . .
   }
}

• If inner class is defined inside an enclosing class, but outside its methods, it is available to all methods of enclosing class

• Compiler turns an inner class into a regular class file:
  DataSetTester$1$RectangleMeasurer.class

Click here to see a sample.

Why would you use an inner class instead of a regular class?

   Answer: Inner classes are convenient for insignificant classes.
   Also, their methods can access variables and fields from the
   surrounding scope.

How many class files are produced when you compile the DataSetTester3 program?

   Answer: Four: one for the outer class, one for the inner class,
   and two for the DataSet and Measurer classes.