implements : Java Glossary


implements is a keyword used on class or interface statements. When a class provides the methods (possibly abstract) necessary to conform to some interface we say that class implements the interface. Implementing an interface is closely related to extending a class. A new class can extend at most one superclass, but it may implement several 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 too, but in the Java world that is called extending.

When you provide an abstract class to satisfy an interface, logically you have not really implemented anything, but in the Java world, that still counts as implementing.

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

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