interface

Why do we need interface ?

it is used to declare methods and it gives an abstraction of a class,
excepting this any other use of interface.

interface can be used for many purposes.you know in java there is no multiple inheritence,so only one class can be extended but we can implement more interfeces at a time.if you do not have interfeces only one class can be extended by your class.if you want to extend Thread class and Exception class at a time what can you do?
use Exception class and Runnable interface...this is one purpose.Interfaces have so many uses...

Tell me other uses of interface in java
Please tell me

You'll get more out of your studies if you do some research and then come back and tell us.

What have you read about them so far?

i want to know that how does an interface intrect with our system ?

like a Runable interface has only one method that is run, how does it kwon that it is time to invoke run method ?
even Thread class also implements Runable interface?

actully interface is some tough to understand to me.

try to look it this way. interface is like a contract between classes. when one class is implementing interface, this implementation guaranties that this class has certain methods that other classes can call.
if you want to make a thread you need a class that implements runnable interface. this way you know you can call start on instance of that class. and it will do whatever is designed in its run method.
hope this helps a bit