Extending Classes - The Java Programming Language - page 130

public static void main(String[] args) {
ExtendShow ext = new ExtendShow();
SuperShow sup = ext;
sup.show();
ext.show();
System.out.println("sup.str = " + sup.str);
System.out.println("ext.str = " + ext.str);
}
}
There is only one object, but we have two variables containing referen-
ces to it: One variable has type SuperShow (the superclass) and the other
variable has type ExtendedShow (the actual class). Here is the output of
the example when run:
Extend.show: ExtendStr
Extend.show: ExtendStr
sup.str = SuperStr
ext.str = ExtendStr
For the show method, the behavior is as you expect: The actual class
of the object, not the type of the reference, governs which version of
the method is called. When you have an ExtendShow object, invoking show
always calls ExtendShow 's show even if you access it through a reference
declared with the type SuperShow . This occurs whether show is invoked ex-
ternally (as in the example) or internally within another method of either
ExtendShow or SuperShow .
For the str field, the type of the reference, not the actual class of the ob-
ject, determines which class's field is accessed. In fact, each ExtendShow
object has two String fields, both called str , one of which is hidden by
ExtendShow 's own, different field called str :