Interface Vs Abstract class

[ParagS Kulkarni]

Can anybody tell me in Java, exactly in what kind of situation we have to use interface & not an abstract class?
The typical answer is to achieve multiple Inheritance?
But practically is it the only anwer for that?

[Bhushan Damle]

Originally posted by Parag S. Kulkarni:
Can anybody tell me in Java, exactly in what kind of situation we have to use interface & not an abstract class?
The typical answer is to achieve multiple Inheritance?
But practically is it the only anwer for that?

Besides this, there is another answer also. That when u implement an interface in several unrelated classes, u can interact with these two classes although there is no physical connection between those(viz. subclass/parent-child relationship). In fact here the term interface comes into existence. It is the interface between two unrelated classes so that they can communicate.
Is it satisfactory?

[Vj Kamath]

The purpose of an interface is to define only the structure but not the implementation of a class. This is used to define the WHAT and leave the HOW to the implementing class.
But an abstract class can have a partial implementation.

[Jeroen Wenting]

more correctly, an interface defines behaviour through a contract while an abstract class defines a relation through inheritance.

There are theoretical as well as practical differences between the two.

Saying you implement an interface tells calling classes that "yes, I know how to do this".
Inheriting from an abstract class says "yes, I AM an XXX and therefore I will indeed know how to do this".
It's a far stronger link.