How does an abstract class differ from a Java interface?
To begin with, all of the methods declared in a Java interface are abstract by definition. (Methods declared in a Java interface may not contain a body.) Therefore, any class that implements a Java interface must provide a concrete definition for all methods declared in the interface (or the class must be declared abstract). No default method behavior is inherited Therefore, when implementing a Java interface, the user doesn’t have the luxury of inheriting default definitions for some of the methods declared in the interface. In all cases, the programmer that implements the interface must provide concrete definitions for all methods declared in the interface, even if those concrete definitions are empty. A very specific example When writing event-driven programs that implement listener interfaces, the programmer often finds it necessary to define empty methods for events that are of no interest. In fact, the AWT provides a set of convenience classes, (such as the WindowAdapter class, f
