EXPRESSIONS

Runtime Evaluation of Method Invocation

15.11.4

color = 0;

}

}

the subclass

ColoredPoint

 extends the

clear

 abstraction defined by its super 

class

Point

. It does so by overriding the

clear

 method with its own method,

which invokes the

clear

 method of its superclass, using the form

super.clear

.

This method is then invoked whenever the target object for an invocation of

clear

 is a

ColoredPoint

. Even the method

move

 in

Point

 invokes the

clear

method of class

ColoredPoint

 when the class of

this

 is

ColoredPoint

, as

shown by the output of this test program:

class Test {

public static void main(String[] args) {

Point p = new Point();

System.out.println("p.move(20,20):");

p.move(20, 20);

ColoredPoint cp = new ColoredPoint();

System.out.println("cp.move(20,20):");

cp.move(20, 20);

p = new ColoredPoint();

System.out.println("p.move(20,20), p colored:");

p.move(20, 20);

}

}

which is:

p.move(20,20):

Point clear

cp.move(20,20):

ColoredPoint clear

Point clear

p.move(20,20), p colored:

ColoredPoint clear

Point clear

Overriding is sometimes called  late bound self reference ; in this example it

means that the reference to

clear

 in the body of

Point.move

 (which is really

syntactic shorthand for

this.clear

) invokes a method chosen  late  (at run time,

based on the run time class of the object referenced by

this

) rather than a method

chosen  early  (at compile time, based only on the type of

this

). This provides

the Java programmer a powerful way of extending abstractions and is a key idea in

object oriented programming.

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