The Java Language Specification, Third Edition

8.9 Enums CLASSES
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An enum constant may be preceded by annotation (§9.7) modifiers. If an
annotation a on an enum constant corresponds to an annotation type T, and T has
a (meta-)annotation m that corresponds to annotation.Target, then m must have
an element whose value is annotation.ElementType.FIELD, or a compile-time
error occurs.
An enum constant may be followed by arguments, which are passed to the
constructor of the enum type when the constant is created during class initialization
as described later in this section. The constructor to be invoked is chosen
using the normal overloading rules (§15.12.2). If the arguments are omitted, an
empty argument list is assumed. If the enum type has no constructor declarations,
a parameterless default constructor is provided (which matches the implicit empty
argument list). This default constructor is private.
The optional class body of an enum constant implicitly defines an anonymous
class declaration (§15.9.5) that extends the immediately enclosing enum type. The
class body is governed by the usual rules of anonymous classes; in particular it
cannot contain any constructors.
DISCUSSION
Instance methods declared in these class bodies are may be invoked outside the enclosing
enum type only if they override accessible methods in the enclosing enum type.
Enum types (§8.9) must not be declared abstract; doing so will result in a
compile-time error. It is a compile-time error for an enum type E to have an
abstract method m as a member unless E has one or more enum constants, and all
of E’s enum constants have class bodies that provide concrete implementations of
m. It is a compile-time error for the class body of an enum constant to declare an
abstract method.
DRAFT
An enum type is implicitly final unless it contains at least one enum constant
that has a class body. In any case, it is a compile-time error to explicitly
declare an enum type to be final.
Nested enum types are implicitly static. It is permissable to explicitly
declare a nested enum type to be static.