Java Reference
In-Depth Information
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
Co-
loredPoint
when the class of
this
is
ColoredPoint
, as shown by the output of this test pro-
gram:
class Test1 {
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 programmer a powerful way of ex-
tending abstractions and is a key idea in object-oriented programming.
Example 15.12.4.4-2. Method Invocation Using
super
An overridden instance method of a superclass may be accessed by using the keyword
super
to access the members of the immediate superclass, bypassing any overriding
declaration in the class that contains the method invocation.
