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Writing Final Classes and Methods

Final Classes

You can declare that your class is final, that is, that your class cannot be subclassed. You might want to do this for two reasons: (1) to increase system security by preventing system subversion, and (2) for reasons of good object-oriented design.

• Security: One mechanism that hackers use to subvert systems is to create a subclass of a class and to then substitute the subclass for the original. The subclass looks and feels like the original class but does vastly different things, possibly causing damage or getting into private information. To prevent this kind of subversion, you can declare your class to be final and thereby prevent any subclasses from being created. The String class is a final class for just this reason. This class is so vital to the operation of the Java platform that it must guarantee that whenever a method or an object uses a String, it gets exactly a java.lang.String and not another kind of string. This ensures that all strings have no strange, inconsistent, undesirable, or unpredictable properties.

  If you try to compile a subclass of a final class, the compiler prints an error message and refuses to compile your program. In addition, the Java runtime system ensures that the subversion is not taking place at the bytecode level. It does this by checking to make sure that a class is not a subclass of a final class.

• Design: You may also wish to declare a class as final for object-oriented design reasons. You may think that your class is "perfect" or that, conceptually, your class should have no subclasses.

To specify that your class is final, use the keyword final before the class keyword in your class declaration. For example, if you wanted to declare your (perfect) ChessAlgorithm class as final, its declaration should look like this:

final class ChessAlgorithm {
    ...
}

Any subsequent attempts to subclass ChessAlgorithm will result in a compiler error.

Final Methods

If declaring an entire class final is too heavy-handed for your needs, you can declare some or all of the class's methods final instead. Use the final keyword in a method declaration to indicate that the method cannot be overridden by subclasses. The Object class does this; some of its methods are final, and some are not.

You might wish to make a method final if it has an implementation that should not be changed and it is critical to the consistent state of the object. For example, instead of making your ChessAlgorithm class final, you might want the nextMove method to be final instead:

class ChessAlgorithm {
    ...
    final void nextMove(ChessPiece pieceMoved,
                        BoardLocation newLocation) {
        ...
    }
    ...
}

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