Java Language Specification
Third Edition

8.1.1.2 final Classes
A class can be declared final if its definition is complete and no
subclasses are desired or required. A compile-time error occurs if the
name of a final class appears in the extends clause (§8.1.4) of another
class declaration; this implies that a final class cannot have any
subclasses. A compile-time error occurs if a class is declared both
final and abstract, because the implementation of such a class could
never be completed (§8.1.1.1).

Because a final class never has any subclasses, the methods of a final
class are never overridden (§8.4.8.1).

http://java.sun.com/docs/books/jls/third_edition/html/classes.html#54727