Hi,

whats the difference between interface and a complete abstract class(no variables declared).
other than the way of using multiple inheritance do we have any other difference between this two.

when do we go for both ...

If an interface also comes with a default abstract implementation, when you extend, you only need to implement the methods that are truly needed in your situation, and leave the rest alone - this simplifies your code.

SAX deafult event handler is a good example.