Learning Java, 4th Edition

Interfaces

Java expands on the concept of abstract methods with interfaces. It’s often desirable to specify a group of abstract methods defining some behavior for an object without tying it to any implementation at all. In Java, this is called an interface. An interface defines a set of methods that a class must implement. A class in Java can declare that it implements an interface if it implements the required methods. Unlike extending an abstract class, a class implementing an interface doesn’t have to inherit from any particular part of the inheritance hierarchy or use a particular implementation.

Interfaces are kind of like Boy Scout or Girl Scout merit badges. A scout who has learned to build a birdhouse can walk around wearing a little sleeve patch with a picture of one. This says to the world, “I know how to build a birdhouse.” Similarly, an interface is a list of methods that define some set of behavior for an object. Any class that implements each method listed in the interface can declare at compile time that it implements the interface and wear, as its merit badge, an extra type—the interface’s type.

Interface types act like class types. You can declare variables to be of an interface type, you can declare arguments of methods to accept interface types, and you can specify that the return type of a method is an interface type. In each case, what is meant is that any object that implements the interface (i.e., wears the right merit badge) can fill that role. In this sense, ...