When to use abstract class or interface? [closed]

Interface is used when you only want to declare which methods and members a class MUST have. Anyone implementing the interface will have to declare and implement the methods listed by the interface.

If you also want to have a default implementation, use abstract class. Any class extending the abstract class will have to implement only its abstract methods and members, and will have some default implementation of the other methods of the abstract class, which you may override or not.

--EDIT - forgot to mention, Earwicker reminded me

Finally, you can implement as many interfaces as you want, but only extend one class (being it abstract or not). Keep that in mind before choosing.

The key difference is that you can implement multiple interfaces in a class, but only extend a single abstract class. This is because an abstract class can also define fields that store data, whereas an interface cannot.