Java Tech - Chapter 4

In these demos we use the StartApplet3.java starter, which includes a class called Test and its subclass Test1, which in turn is subclassed by Test2.

In the first demo, each subclass overrides a method in its corresponding superclass. In the Applet class init() we use an array of the Test type to reference instances of Test and the two subclasses.

Cycling through the the Test array in a loop we invoke the aMethod. You will see in the printout that the overriding aMethod of the particular subclass object is invoked, not the aMethod in the Test base class.

This demonstrates the powerful, so-called, polymorphic quaility of objects in that an object can be referenced as its superclass yet act according to its own class.

OverrideApplet.java
(Output goes to browser's Java console.)

public class OverrideApplet extends java.applet.Applet
{
  public void init () {

     Test [] array = new Test[3];

     array[0] = new Test ();

     array[1] = new Test1 ();

     array[2] = new Test2 ();

     for (int i=0; i<3; i++) {
         array[i].aMethod ();
     }
  }

  // Paint message in Applet window.
  public void paint (java.awt.Graphics g) {
     g.drawString ("OverrideApplet",10,20);
  }

} // class OverrideApplet

/** Use Test to examine various aspects of class design. **/
class Test
{
  void aMethod () {
    System.out.println ("In Test  aMethod");
  }
}

/** Test 1 is a sub-class of Test. **/
class Test1 extends Test
{
  void aMethod () {
    System.out.println ("In Test1 aMethod");
  }
}

/** And Test2 is a sub-class of Test1. **/
class Test2 extends Test1
{
  void aMethod () {
    System.out.println ("In Test2 aMethod");
  }
}

In the second demo, each of the "Test#" classes overloads its aMethod() and each of the subclasses also overrides one of the versions of aMethod() in its corresponding superclass.

In the Applet class init() we create an instance of the Test2 type and then call each of the different aMethod() versions.

Before looking at the printout on the Java console, write down what you think it will show.

RideLoadApplet.java
(Output goes to browser's Java console.)

public class RideLoadApplet extends java.applet.Applet
{
  public void init () {

    Test2 test2 = new Test2 ();

    test2.aMethod ();
    test2.aMethod (4.5);
    test2.aMethod (4);
    test2.aMethod (true);
  }

  // Paint message in Applet window.
  public void paint (java.awt.Graphics g)  {
     g.drawString ("RideLoadApplet", 20, 20);
  }
}

/** Base class. **/
class Test
{
  void aMethod () {
    System.out.println ("In Test  aMethod");
  }

  void aMethod (double d) {
    System.out.println ("In Test  aMethod (double d)");
  }
}

/** Test1 is a subclass of Test. **/
class Test1 extends Test
{
  void aMethod (int i) {
    System.out.println ("In Test1 aMethod (int i)");
  }

  void aMethod (boolean b) {
    System.out.println ("In Test1 aMethod (boolean b)");
  }
}

/** And Test2 is a subclass of Test1. **/
class Test2 extends Test1
{

  void aMethod () {
    System.out.println ("In Test2 aMethod ()");
  }

  void aMethod (boolean b) {
    System.out.println ("In Test2 aMethod (boolean b)");
  }
}

Note that you must be careful with integer and floating point arguments in overriding and overloading due to the different rules regarding conversions from integer to floating point and floating point to integer. (See Chapter 2: Casting and Mixing)

In this case, if the aMethod(double d) had been put into Test1 and
void aMethod(int i) in Test, the compiler would complain of an ambiguity:

The test2.aMethod(4) invocation would fit the argument list of the
aMethod(double d) in Test1 since an integer can convert to a double without a cast. But then as the compiler looked further in the method stack, it would find also the aMethod(int i) in Test.

As currently composed, the compiler first finds the aMethod(int i) in Test1 and since the match is exact, doesn't look farther.