Object-Oriented Concepts - SCJP Sun Certified Programmer for Java Platform SE6 - page 404

Using the references from Figure 6.5, the following statements are valid without
requiring any casting:
c.sleep();
c.breathe();
c.eat();
p.eat();
m.breathe();
Using the reference c , you can invoke all the methods of Cat , Pet , and Mammal . Using the
reference p , you can only invoke the eat method of Pet . Even though the corresponding Cat
object contains sleep and breathe methods, the p reference does not see those methods
because the p reference thinks it is pointing to a Pet object (not a Cat ). Similarly, using the
m reference, you can only invoke the breathe method, even though the object has an eat
and a sleep method. Casting is required if you want to invoke the “hidden” methods of the
Cat object using the m and p references.
Virtual Method Invocation
Now that we have discussed polymorphism, we can discuss the concept of a virtual
method. All methods in Java are virtual methods, meaning that if a method is overridden,
the overridden method is always invoked at runtime, even if the compiler sees the parent
class method at compile time. For example, suppose we have the following parent class
named ButtonListener with a single method named buttonClicked :
public class ButtonListener {
public void buttonClicked() {
System.out.println(“Inside ButtonListener”);
}
}
The following ChildListener class extends ButtonListener and overrides the
buttonClicked method:
public class ChildListener extends ButtonListener {
public void buttonClicked() {
System.out.println(“Inside ChildListener”);
}
}
Using polymorphism, the following statements are valid. Study them carefully and see if
you can determine the output:
7. ButtonListener listener = new ChildListener();
8. listener.buttonClicked();