Provide definitions for all of the abstract methods of its base class. Sometimes, a non-abstract class is called a concrete class. Such a class would specify a set of behaviors, without at all defining how they are to be carried out. In Java, such an entity is called an interface , rather than a class. An interface can extend any number of other interfaces, but cannot extend a class.
Interfaces can also declare static constants.
Any variable declared in an interface is implicitly public, static, and final, and must be initialized at the point of declaration. An interfaces is used when one wants to specify that a class inherits a set of behaviors, without inheriting their implementation. Interfaces provide a way of dealing with the restriction that a class can extend at most one other class. A class is allowed to extend one class and implement any number of interfaces. ASK The various types of awt listener are interfaces.