Declarations, Initialization, and Scoping - SCJP Sun Certified Programmer for Java Platform SE6 - page 140

12. public Platypus(boolean hasFur) {
13. super(hasFur);
14. eggCount = 1;
15. }
16.
17. public static void main(String [] args) {
18. Platypus p = new Platypus(false);
19. p.eat(“leaves”);
20. p.breathe();
21. p.layEggs();
22.
23. }
24.}
The code compiles fi ne. The Platypus class correctly overrides the eat method in Mammal
and also declares a new method, layEggs , and a fi eld, eggCount . Inside main , the following
sequence of events occurs:
1. A new Platypus is instantiated on line 18, which is valid because Platypus is not
abstract. The constructor on line 12 is invoked.
2. Line 13 passes the hasFur boolean up to the Mammal constructor. Line 14 sets the
eggCount field to 1.
3. Invoking the eat method on line 19 executes the overridden eat method on line 8.
4. Invoking breathe on line 20 executes the breathe method in Mammal .
5. Invoking layEggs on line 21 invokes the layEggs method on line 4.
Therefore, the output is
Platypus is eating leaves
Mammal is breathing
Platypus is laying eggs
We use abstract parent classes all the time in Java to represent the common attributes
and behaviors of child objects. Now that you have seen how to declare an abstract class, we
can discuss the concept of an abstract method in Java.
Abstract Methods
An abstract method is an instance method of a class that does not contain a method body
and must be overridden by any nonabstract child classes. Use the abstract keyword to
declare a method as abstract. Instead of a method body, an abstract method simply has a
semicolon at the end of its declaration. For example, the java.io.InputStream declares the
following method:
public abstract int read() throws IOException;