Head First Java Chapter 07 Notes

Head First Java Chapter 07

This is the seventh Chapter learning of Head First Java by Kathy Sierra and Bert Bates,
This markdown file is my notes for the seventh chapter, not sure if any copyright issues are there.


  • A subclass extends a superclass.
  • A subclass inherits all public instance variables and methods of the superclass, but does not inherit the private instance variables and methods of the superclass.
  • Inherited methods can be overridden; instance variables cannot be overridden(although they can be redefined in the subclass).
  • Use the IS-A test to verify that your inheritance hierarchy is valid.
  • The IS-A relationship works in only one direction.
  • When a method is overridden in a subclass, and that method is invoked on an instance of the subclass, the overridden version of the method is called.
  • If a class extends A, and C extends B, class B IS-A class A, and class C IS-A class B, and Class C also IS-A class A.
  • Instance variable and Class has a HAS-A relationship.


  • Inheritance lets you guarantee that all classes grouped under a certain supertype have all the methods that the supertype has.
  • When you define a supertype for a group of classes, any subclass of that supertype can be substituted where the supertype is expected.


  • With polymorphism, the reference type can be a superclass of the actual object type.
  • We can have polymorphic aguments and return type.(Write the method arguments as a superclass type, and you can pass any subclass object at the runtime.)

Rules of overriding.

  • Arguments must be the same, and return types must be compatible.
  • The method can’t be less accessible.


  • Method overloading is nothing more than having two methods with the same name and different argument list.
  • The return type can be different.
  • You can’t change only the return type.
  • You can vary the access levels in any direction.

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s