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Thinking in Java
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Abstract base classes and interfaces

Often in a design, you want the base class to present only an interface for its derived classes. That is, you don’t want anyone to actually create an object of the base class, only to upcast to it so that its interface can be used. This is accomplished by making that class abstract by using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents it. This is a tool to enforce a particular design.

You can also use the abstract keyword to describe a method that hasn’t been implemented yet—as a stub indicating “here is an interface method for all types inherited from this class, but at this point I don’t have any implementation for it.” An abstract method may be created only inside an abstract class. When the class is inherited, that method must be implemented, or the inheriting class becomes abstract as well. Creating an abstract method allows you to put a method in an interface without being forced to provide a possibly meaningless body of code for that method.

The interface keyword takes the concept of an abstract class one step further by preventing any method definitions at all. The interface is a very handy and commonly used tool, as it provides the perfect separation of interface and implementation. In addition, you can combine many interfaces together, if you wish, whereas inheriting from multiple regular classes or abstract classes is not possible.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire