How are enums implemented in Java?

Question

I'm just now realizing their power and utility over using a Constants class... but I'm very interested to know how they're implemented under the hood. They seem to work alot like a static method or static constant in that you just import the file that "owns" the enum and you can make reference to them by using enumName.valueName. The Javadocs online seem to suggest they're a class but it seems weird to have an "unnamed" class on demand. (In Java at least...)

Answer 1

i believe each instance of the enum is an anonymous final subclass of the enum.

decompile:

public enum Color {
    r(0xff0000), g(0x00ff00), b(0x0000ff);
    Color(int rgb) {
        this.rgb=rgb;
    }
    final int rgb;
}

and you can see the instances being made:

D:\home\ray\dev\indigoapps\so8032067enumimpl\bin>javap -c Color
Compiled from "Color.java"
public final class Color extends java.lang.Enum{
public static final Color r;

public static final Color g;

public static final Color b;

final int rgb;

static {};
  Code:
   0:   new     #1; //class Color
   3:   dup
   4:   ldc     #16; //String r
   6:   iconst_0
   7:   ldc     #17; //int 16711680
   9:   invokespecial   #18; //Method "<init>":(Ljava/lang/String;II)V
   12:  putstatic       #22; //Field r:LColor;
   15:  new     #1; //class Color
   18:  dup
   19:  ldc     #24; //String g
   21:  iconst_1
   22:  ldc     #25; //int 65280
   24:  invokespecial   #18; //Method "<init>":(Ljava/lang/String;II)V
   27:  putstatic       #26; //Field g:LColor;
   30:  new     #1; //class Color
   33:  dup
   34:  ldc     #28; //String b
   36:  iconst_2
   37:  sipush  255
   40:  invokespecial   #18; //Method "<init>":(Ljava/lang/String;II)V
   43:  putstatic       #29; //Field b:LColor;
   46:  iconst_3
   47:  anewarray       #1; //class Color
   50:  dup
   51:  iconst_0
   52:  getstatic       #22; //Field r:LColor;
   55:  aastore
   56:  dup
   57:  iconst_1
   58:  getstatic       #26; //Field g:LColor;
   61:  aastore
   62:  dup
   63:  iconst_2
   64:  getstatic       #29; //Field b:LColor;
   67:  aastore
   68:  putstatic       #31; //Field ENUM$VALUES:[LColor;
   71:  return

public static Color[] values();
  Code:
   0:   getstatic       #31; //Field ENUM$VALUES:[LColor;
   3:   dup
   4:   astore_0
   5:   iconst_0
   6:   aload_0
   7:   arraylength
   8:   dup
   9:   istore_1
   10:  anewarray       #1; //class Color
   13:  dup
   14:  astore_2
   15:  iconst_0
   16:  iload_1
   17:  invokestatic    #43; //Method java/lang/System.arraycopy:(Ljava/lang/Obj
ect;ILjava/lang/Object;II)V
   20:  aload_2
   21:  areturn

public static Color valueOf(java.lang.String);
  Code:
   0:   ldc     #1; //class Color
   2:   aload_0
   3:   invokestatic    #51; //Method java/lang/Enum.valueOf:(Ljava/lang/Class;L
java/lang/String;)Ljava/lang/Enum;
   6:   checkcast       #1; //class Color
   9:   areturn

}

Answer 2

Enums are statically created when the enum class is first loaded and are immutable.

You must have a constructor in the enum class if you want to assign different values to your enum. After the constructor was finished you cannot change the enums value (immutable as said).

You don't need to use equals() method on enums: the == operator will work just fine.

I have written a tutorial in my blog showing some nice uses of the enum class, that are not trivial from just reading the enum reference.

if you're interested, heres the link. Blog