Enumeration Types - The Java Programming Language - page 223

An enum cannot be declared to extend another type because all enums
implicitly extend java.lang.Enum , which we discuss a little later. Nor can
any other type extend an enum (not even another enum), because all
enum types act as if they are implicitly final . An enum can declare that
it implements one or more interfacesin fact all enums are implicitly Seri-
alizable (see " Object Serialization " on page 549 ) and Comparable .
The possible members of an enum include all the class members: fields,
methods, and nested types, including nested enumsthough it should be
rare to require such sophistication: The main attraction of enums is their
simplicity. The enum constants themselves are implicitly static fields
with the same type as the enum.
Every enum type E has two static methods that are automatically gen-
erated for it by the compiler:
public static E [] values()
Returns an array containing each of the enum constants in the
order in which they were declared.
public static E valueOf(String name)
Returns the enum constant with the given name. If the name
does not match an enum constant name exactly then an Il-
legalArgumentException is thrown.
Note that the length of the array returned by values tells you how
many enum constants there are in the enum, so the getSize method we
demonstrated is not needed in practice.
An enum type is not allowed to override the finalize method from Ob-
ject . Enum instances may never be finalized (see " Finalization " on page
449 ) .