Classes - The Java Language Specification - page 256

}
the class PackageOnly can be instantiated only within the package just , in which it is de-
clared.
8.9. Enums
An enum declaration specifies a new enum type.
EnumDeclaration:
ClassModifiers opt enum Identifier Interfaces opt EnumBody
EnumBody:
{ EnumConstants opt , opt EnumBodyDeclarations opt }
Enum types (§ 8.9 ) must not be declared abstract ; doing so will result in a compile-time error.
An enum type is implicitly final unless it contains at least one enum constant that has a class
body.
It is a compile-time error to explicitly declare an enum type to be final .
Nested enum types are implicitly static . It is permissible to explicitly declare a nested enum
type to be static .
This implies that it is impossible to define a local (§ 14.3 ) enum, or to define an enum
in an inner class (§ 8.1.3 ).
The direct superclass of an enum type named E is Enum< E > (§ 8.1.4 ).
An enum type has no instances other than those defined by its enum constants. It is a
compile-time error to attempt to explicitly instantiate an enum type (§ 15.9.1 ) .
The final clone method in Enum ensures that enum constants can never be cloned, and
the special treatment by the serialization mechanism ensures that duplicate instances
are never created as a result of deserialization. Reflective instantiation of enum types
is prohibited. Together, these four things ensure that no instances of an enum type ex-
ist beyond those defined by the enum constants.
8.9.1. Enum Constants
The body of an enum type may contain enum constants . An enum constant defines an in-
stance of the enum type.
EnumConstants: