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

 Use Interface, when:

• Design changing often or when various implementations only share method signatures.
• When we need some classes to use some methods which we don't want to be included in class.

Use Abstract Class, when:

• Several implementations are of the same kind and use common behavior.
• Enabling with generalized form of abstraction and leave implementation task with inheriting subclass.
• creating planned inheritance hierarchies