When to use abstract classes?

Abstract classes in Java are just like regular Java classes but you cannot instantiate them directly. They may not completely implement all methods defined in classes. This incomplete implementation may be different in different context. Derived class implements the abstract class in its context.

When you have a common implementation, abstract classes shine. Using abstract classes you can enforce an implementation hierarchy and avoid duplicate code. Using abstract classes, however, should not affect your decision to use interfaces to define your behavior.

An abstract class allows to add new methods without breaking compatibility. The derived class can just provide working implementation of a subset of the methods defined in the abstract class. While if you add new methods to an interface, you'd need to update all the classes that implements the interface.