When should I use abstract classes and when should I use interfaces?

Hi Anand

Abstract classes should be used primarily for objects that are closely related, whereas interfaces are best suited for providing common functionality to unrelated classes. If you are designing small, concise bits of functionality, use interfaces. If you are designing large functional units, use an abstract class.

1) when you know scope of requirement use interface and when scope may change or extend use abstract class,

2) when you want all method to be public and force client to implement use interface and when you want different access modifier and do not force to implement on inheritance

3) when you use interface then you simply cannot change it in the end. Abstract classes always have extended compatibility and can still add the behavior without breaking the existing code.