How is values() implemented for Java 6 enums?

Basically, the compiler (javac) translates your enum into a static array containing all of your values at compile time. When you call values(), it gives you a . Clone'd() copy of this array Given this simple enum: public enum Stuff { COW, POTATO, MOUSE; } You can actually look at the code that Java generates: public enum Stuff extends Enum { /*public static final*/ COW /* = new Stuff("COW", 0) */, /*public static final*/ POTATO /* = new Stuff("POTATO", 1) */, /*public static final*/ MOUSE /* = new Stuff("MOUSE", 2) */; /*synthetic*/ private static final Stuff $VALUES = new Stuff{Stuff.

COW, Stuff. POTATO, Stuff. MOUSE}; public static Stuff values() { return (Stuff)$VALUES.clone(); } public static Stuff valueOf(String name) { return (Stuff)Enum.

ValueOf(Stuff. Class, name); } private Stuff(/*synthetic*/ String $enum$name, /*synthetic*/ int $enum$ordinal) { super($enum$name, $enum$ordinal); } } You can look at how javac 'translates' your classes by making a temporary directory and running: javac -d -XD-printflat filename.java.

Basically, the compiler (javac) translates your enum into a static array containing all of your values at compile time. When you call values(), it gives you a . Clone'd() copy of this array.

Given this simple enum: public enum Stuff { COW, POTATO, MOUSE; } You can actually look at the code that Java generates: public enum Stuff extends Enum { /*public static final*/ COW /* = new Stuff("COW", 0) */, /*public static final*/ POTATO /* = new Stuff("POTATO", 1) */, /*public static final*/ MOUSE /* = new Stuff("MOUSE", 2) */; /*synthetic*/ private static final Stuff $VALUES = new Stuff{Stuff. COW, Stuff. POTATO, Stuff.

MOUSE}; public static Stuff values() { return (Stuff)$VALUES.clone(); } public static Stuff valueOf(String name) { return (Stuff)Enum. ValueOf(Stuff. Class, name); } private Stuff(/*synthetic*/ String $enum$name, /*synthetic*/ int $enum$ordinal) { super($enum$name, $enum$ordinal); } } You can look at how javac 'translates' your classes by making a temporary directory and running: javac -d -XD-printflat filename.java.

1 for neatness factor. – nalroff Apr 27 at 17:09 Wouldn't a System. Arraycopy be faster?

– Gandalf Aug 3 at 18:43.

If you assign it to a local variable the only thing that you can modify is assigning another enum to this variable. This will not change the enum itself because you are only changing the object your variable references. It seems that the enums are in fact singletons so that only one element from each enum can exist in you whole program this makes the == operator legal for enums.So there is no performance problem and you can't accidentally change something in your enum definition.

1 I think the OP means modifying the array returned by values(). If this is the same array object that is kept internally (instead of a copy), then modifying it (e.g. Assigning one element to another, assigning null to an element, etc.) would mess it up, not only for the enum class, but for any future calls to values() – newacct Jul 22 '09 at 5:58 Yes you are right. If there wasn't a clone you could remove a Enum from the array and later calls from values would miss this value.

– Janusz Jul 22 '09 at 14:43.

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