When an Abstract Class Implements an Interface

In the section on Interfaces, it was noted that a class that implements an interface must implement all of the interface's methods. It is possible, however, to define a class that does not implement all of the interface methods, provided that the class is declared to be abstract. For example,

abstract class X implements Y {
  // implements all but one method of Y
}

class XX extends X {
  // implements the remaining method in Y
}

In this case, class X must be abstract because it does not fully implement Y, but class XX does, in fact, implement