extend : Java Glossary


When you derive a class from a base class we say the derived subclass extends the base class. An interface can also extend another interface. In contrast, when a class provides the methods (possibly abstract) necessary to conform to some interface we say that class implements the interface. Extending a class is closely related to implementing an interface. A new class can extend at most one superclass, but it may implement several interfaces.

In interface may extend several other interfaces (though a class implements an interface). You sometimes do this to give you a short same to stand for the requirement implementing half a dozen related interfaces.

Extending means adding new method definitions. Implementing means satisfying the existing interface contract by writing the proscribed method bodies. Logically, when you extend an abstract class and fill in the method bodies, that should be called implementing, but in the Java world that is called extending.

When a class or abstract class implements or extends, those methods are are available in all descendants. All descendant classes automatically implement that inteface too. You you can refer to descendant class objects by the base class name or the interface name.

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