Difference between abstract class with all method abstract and interface?

Question

I had an interview where interviewer asked me first what is the difference between abstract class with all the methods abstract and an interface.

I replied that if

Answer 1

I can't say what your interviewer had in mind, but an interface is more of a "contract" whereas an abstract base class, while it can play that role too, is more geared towards hierarchies or IS-A relationships. E.g. an Apple IS-A Fruit, a Pear IS-A Fruit, etc. But you're right, they could well be used interchangeably for practical purposes in that context, but an OO purist might not want to use an abstract class unless they were expressing IS-A relationship(s).