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java – From whence cometh Enum.values()?

Posted by: admin June 15, 2020 Leave a comment

Questions:

I was looking through the documentation and source code, because I wanted to be certain that values() would always return an array in the order in which the Enum values are declared. Turns out, it’s not in the documentation as far as I can tell.

I checked the source code for the Enum class, and no luck (there is a related, private “getValues” method).

So I’m guessing that some compiler/interpreter-foo is already going on to create a class that extends Enum out of a declaration like:

public static enum MyEnum

So is values() also statically translated into a hardcoded array during compilation? Or is it actually a method called at runtime, and if so, where is it defined?

How to&Answers:

The values() method is part of the definition of the enum type. Not to be confused with the Enum base class. The formal definition is in the Section 8.9 of the JLS which does specify the order returned matches the order in which they are declared.

Answer:

As you can see from the bytecode below obtained by disassemlbing an enum, the values() method on the enums simply return a copy of the private static array which contains all the declared enum constant. This array ENUM$VALUES is filled in a static initilization block.

DaysOfTheWeek.java

public enum DaysOfTheWeek {
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY
}

DaysOfTheWeek.java disassembled

After the static block, the bytecode marked 0-92 initializes the enum constants, the bytecode marked 94-139 puts those constants in an array and the bytecode marked 140 assigns the array to the ENUM$VALUES static field of the class. The code in the values() method simply creates a copy of the array assigned to the ENUM$VALUES field by calling System.arraycopy on it and returns the copy.

Compiled from "DaysOfTheWeek.java"
public final class DaysOfTheWeek extends java.lang.Enum{
public static final DaysOfTheWeek MONDAY;

public static final DaysOfTheWeek TUESDAY;

public static final DaysOfTheWeek WEDNESDAY;

public static final DaysOfTheWeek THURSDAY;

public static final DaysOfTheWeek FRIDAY;

public static final DaysOfTheWeek SATURDAY;

public static final DaysOfTheWeek SUNDAY;

static {};
  Code:
   0:   new #1; //class DaysOfTheWeek
   3:   dup
   4:   ldc #18; //String MONDAY
   6:   iconst_0
   7:   invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   10:  putstatic   #23; //Field MONDAY:LDaysOfTheWeek;
   13:  new #1; //class DaysOfTheWeek
   16:  dup
   17:  ldc #25; //String TUESDAY
   19:  iconst_1
   20:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   23:  putstatic   #26; //Field TUESDAY:LDaysOfTheWeek;
   26:  new #1; //class DaysOfTheWeek
   29:  dup
   30:  ldc #28; //String WEDNESDAY
   32:  iconst_2
   33:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   36:  putstatic   #29; //Field WEDNESDAY:LDaysOfTheWeek;
   39:  new #1; //class DaysOfTheWeek
   42:  dup
   43:  ldc #31; //String THURSDAY
   45:  iconst_3
   46:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   49:  putstatic   #32; //Field THURSDAY:LDaysOfTheWeek;
   52:  new #1; //class DaysOfTheWeek
   55:  dup
   56:  ldc #34; //String FRIDAY
   58:  iconst_4
   59:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   62:  putstatic   #35; //Field FRIDAY:LDaysOfTheWeek;
   65:  new #1; //class DaysOfTheWeek
   68:  dup
   69:  ldc #37; //String SATURDAY
   71:  iconst_5
   72:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   75:  putstatic   #38; //Field SATURDAY:LDaysOfTheWeek;
   78:  new #1; //class DaysOfTheWeek
   81:  dup
   82:  ldc #40; //String SUNDAY
   84:  bipush  6
   86:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   89:  putstatic   #41; //Field SUNDAY:LDaysOfTheWeek;
   92:  bipush  7
   94:  anewarray   #1; //class DaysOfTheWeek
   97:  dup
   98:  iconst_0
   99:  getstatic   #23; //Field MONDAY:LDaysOfTheWeek;
   102: aastore
   103: dup
   104: iconst_1
   105: getstatic   #26; //Field TUESDAY:LDaysOfTheWeek;
   108: aastore
   109: dup
   110: iconst_2
   111: getstatic   #29; //Field WEDNESDAY:LDaysOfTheWeek;
   114: aastore
   115: dup
   116: iconst_3
   117: getstatic   #32; //Field THURSDAY:LDaysOfTheWeek;
   120: aastore
   121: dup
   122: iconst_4
   123: getstatic   #35; //Field FRIDAY:LDaysOfTheWeek;
   126: aastore
   127: dup
   128: iconst_5
   129: getstatic   #38; //Field SATURDAY:LDaysOfTheWeek;
   132: aastore
   133: dup
   134: bipush  6
   136: getstatic   #41; //Field SUNDAY:LDaysOfTheWeek;
   139: aastore
   140: putstatic   #43; //Field ENUM$VALUES:[LDaysOfTheWeek;
   143: return

public static DaysOfTheWeek[] values();
  Code:
   0:   getstatic   #43; //Field ENUM$VALUES:[LDaysOfTheWeek;
   3:   dup
   4:   astore_0
   5:   iconst_0
   6:   aload_0
   7:   arraylength
   8:   dup
   9:   istore_1
   10:  anewarray   #1; //class DaysOfTheWeek
   13:  dup
   14:  astore_2
   15:  iconst_0
   16:  iload_1
   17:  invokestatic    #51; //Method java/lang/System.arraycopy:(Ljava/lang/Object;ILjava/lang/Object;II)V
   20:  aload_2
   21:  areturn

public static DaysOfTheWeek valueOf(java.lang.String);
  Code:
   0:   ldc #1; //class DaysOfTheWeek
   2:   aload_0
   3:   invokestatic    #59; //Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
   6:   checkcast   #1; //class DaysOfTheWeek
   9:   areturn

}

Answer:

I’ll add to Devon’s answer, that the values() method is added by the compiler, by definition.
From the enums tutorial:

The compiler automatically adds some
special methods when it creates an
enum. For example, they have a static
values method that returns an array
containing all of the values of the
enum in the order they are declared.

Answer:

Your link you provided (where there’s a private getValues method) is from Apache Harmony (version 6, which is an open source Java SE). They have a different implementation to Oracle’s Enum class (which has no private getValues method).

The Oracle java (version 1.6.0-21 at the time of writing) has the valueOf(Class<T> enumType, String name). Here’s its implementation:

 /**
     * Returns the enum constant of the specified enum type with the
     * specified name.  The name must match exactly an identifier used
     * to declare an enum constant in this type.  (Extraneous whitespace
     * characters are not permitted.) 
     *
     * @param enumType the <tt>Class</tt> object of the enum type from which
     *      to return a constant
     * @param name the name of the constant to return
     * @return the enum constant of the specified enum type with the
     *      specified name
     * @throws IllegalArgumentException if the specified enum type has
     *         no constant with the specified name, or the specified
     *         class object does not represent an enum type
     * @throws NullPointerException if <tt>enumType</tt> or <tt>name</tt>
     *         is null
     * @since 1.5
     */
    public static <T extends Enum<T>> T valueOf(Class<T> enumType,
                                                String name) {
        T result = enumType.enumConstantDirectory().get(name);
        if (result != null)
            return result;
        if (name == null)
            throw new NullPointerException("Name is null");
        throw new IllegalArgumentException(
            "No enum const " + enumType +"." + name);
    }

In essence, it’s how Apache implemented their valueOf that’s different.


Enum (or enum) have a public static method called values() that returns the enumerated values constants declared inside the enum. That’s populated by the compiler.