Java Reference
In-Depth Information
The output produced by this program is shown here:
The version of
show( )
in
B
takes a string parameter. This makes its signature different
from the one in
A
, which takes no parameters. Therefore, no overriding (or name hiding)
takes place.
Overridden Methods Support Polymorphism
While the examples in the preceding section demonstrate the mechanics of method overrid-
ing, they do not show its power. Indeed, if there were nothing more to method overriding
than a namespace convention, then it would be, at best, an interesting curiosity but of little
real value. However, this is not the case. Method overriding forms the basis for one of
Java's most powerful concepts:
dynamic method dispatch
. Dynamic method dispatch is the
mechanism by which a call to an overridden method is resolved at run time rather than
compile time. Dynamic method dispatch is important because this is how Java implements
run-time polymorphism.
Let's begin by restating an important principle: a superclass reference variable can refer
to a subclass object. Java uses this fact to resolve calls to overridden methods at run time.
Here's how. When an overridden method is called through a superclass reference, Java de-
termines which version of that method to execute based upon the type of the object being
referred to at the time the call occurs. Thus, this determination is made at run time. When
different types of objects are referred to, different versions of an overridden method will be
called. In other words,
it is the type of the object being referred to
(not the type of the ref-
erence variable) that determines which version of an overridden method will be executed.
Therefore, if a superclass contains a method that is overridden by a subclass, then when
different types of objects are referred to through a superclass reference variable, different
versions of the method are executed.
Here is an example that illustrates dynamic method dispatch:
