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Java, A Beginner’s Guide, 5th Edition by Herbert Schildt

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Interfaces

In object-oriented programming, it is sometimes helpful to define what a class must do but not how it will do it. You have already seen an example of this: the abstract method. An abstract method defines the signature for a method but provides no implementation. A subclass must provide its own implementation of each abstract method defined by its superclass. Thus, an abstract method specifies the interface to the method but not the implementation. While abstract classes and methods are useful, it is possible to take this concept a step further. In Java, you can fully separate a class’ interface from its implementation by using the keyword interface.

Interfaces are syntactically similar to abstract classes. However, in an interface, ...

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