Question

In Java, you can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. That being the case, can you use abstract classes instead of interfaces?

Answer 1

Not always:

• a class can extend only one class
• a class can implement more than one interface

Sun docs make a more detailed comparison:

Abstract Classes versus Interfaces

Unlike interfaces, abstract classes can contain fields that are not static and final, and they can contain implemented methods. Such abstract classes are similar to interfaces, except that they provide a partial implementation, leaving it to subclasses to complete the implementation. If an abstract class contains only abstract method declarations, it should be declared as an interface instead.

Multiple interfaces can be implemented by classes anywhere in the class hierarchy, whether or not they are related to one another in any way. Think of Comparable or Cloneable, for example.

By comparison, abstract classes are most commonly subclassed to share pieces of implementation. A single abstract class is subclassed by similar classes that have a lot in common (the implemented parts of the abstract class), but also have some differences (the abstract methods).