Anilrgowda

Extending a class does what it says... it takes a class and applies its methods and constructor to another class. Extending classes is a powerful way of creating efficient programs.
What this means is that when a class extends to another, the class that receives the extension can use all methods and constructors of the class that extends.
For a class to receive an extension you must use the following class header:
public class ClassName extends ClassExtender
First to establish some terminology, the ClassName is a sub class and the ClassExtender is a super class.
In java, unlike C programming, a class can receive only one extension from another class.

In the example, ClassName can access all of ClassExtender's constructors and methods in this case. If ClassExtender had a method called q, an instance (object) of ClassName could execute the process.
It becomes more interesting if ClassName also had a method of the name q. If you called a method in with both the super class and the sub class, the sub class' method would execute.
The opposite situation, where the sub class contains the method and the super class doesn't is complicated also. If a sub class object was cast into the subclass variable, it would be able to execute the method specific only to the subclass.
But if the object was of a subclass cast to a super class, the object couldn't execute the classes method,
Case 1
ClassName objectQ = new ClassName();
aMethod(); // aMethod is a method only to the ClassName class
// This would work fine
Case 2
ClassExtender objectQ = new ClassName();
aMethod(); // aMethod is a method only to the ClassName class
// This would NOT work
To fix this, use the following
ClassExtender objectQ = new ClassName();
((ClassName)objectQ). aMethod(); // aMethod is a method only to the ClassName class
// This would work
If you want to access a super class' method, its simply super.method()
Also, when you create a constructor of a sub class, you should always have a line at the top of the constructor with the following.
super();