More Features of the Java Language

Writing Final Classes and Methods

You can declare that your class is final, that is, that your class cannot be subclassed. There are (at least) two reasons why you might want to do this: to increase system security by preventing system subversion, and 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 then substitute their class 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 in the java.lang package is a final class for just this reason. This class is so vital to the operation of the compiler and the interpreter that the Java system must guarantee that whenever a method or object uses a String it gets exactly a java.lang.String and not some other 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 bytecode verifier 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 such as the following:

Chess.java:6: Can't subclass final classes: class ChessAlgorithm
class BetterChessAlgorithm extends ChessAlgorithm {
      ^
1 error

Final Methods

Does creating a final class seem heavy-handed for your needs? Do you really just want to protect some of your class's methods from being overridden? You can use the final keyword in a method declaration to indicate to the compiler that the method cannot be overridden by subclasses. As just shown, 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 instead to make the nextMove method final:

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

More Features of the Java Language