Extending Classes - The Java Programming Language - page 143

3.6. Marking Methods and Classes final
Marking a method final means that no extended class can override the
method to change its behavior. In other words, this is the final version of
that method. Entire classes can also be marked final :
final class NoExtending {
// ...
}
A class marked final cannot be extended by any other class, and all the
methods of a final class are themselves effectively final .
Final classes and methods can improve security. If a class is final ,
nobody can declare a class that extends it, and therefore nobody can vi-
olate its contract. If a method is final , you can rely on its implementation
details (unless it invokes non- final methods, of course). You could use
final , for example, on a validatePassword method to ensure that it does
what it is advertised to do instead of being overridden to always return
TRue . Or you can mark as final the class that contains the method so that
it can never be extended to confuse the implementation of validatePass-
word .
Marking a method or class final is a serious restriction on the use of the
class. If you make a method final , you should really intend that its beha-
vior be completely fixed. You restrict the flexibility of your class for other
programmers who might want to use it as a base from which to add func-
tionality to their code. Marking an entire class final prevents anyone else
from extending your class, limiting its usefulness to others. If you make
anything final , be sure that you want to create these restrictions.
In many cases, you can achieve the security of marking a whole class
final by leaving the class extensible and instead marking each method
in the class as final . In this way, you can rely on the behavior of those
methods while still allowing extensions that can add functionality without