Learning Java

Interfaces

Java expands on the abstract method concept with its interfaces scheme. It’s often desirable to specify the prototypes for a set of methods and provide no implementation. In Java, this is called an interface. An interface defines a set of methods that a class must implement (i.e., some or all of the class’s behavior). A class in Java can declare that it implements an interface and then go about implementing the required methods. A class that implements 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 of the methods listed in the interface can declare 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 even specify that the return type of a method is an interface type. In each of these cases, what is meant is that any object that implements the interface (i.e., wears the right merit badge) can fill that spot. In this sense, ...