Can I specify ordinal for enum in Java?

Question

The ordinal() method returns the ordinal of an enum instance.
How can I set the ordinal for an enum?

Answer 1

You can control the ordinal by changing the order of the enum, but you cannot set it explicitly like in C++. One workaround is to provide an extra method in your enum for the number you want:

enum Foo {
  BAR(3),
  BAZ(5);
  private final int val;
  private Foo(int v) { val = v; }
  public int getVal() { return val; }
}

In this situation BAR.ordinal() == 0, but BAR.getVal() == 3.

Answer 2

You can't set it. It is always the ordinal of the constant definition. See the documentation for Enum.ordinal():

Returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero). Most programmers will have no use for this method. It is designed for use by sophisticated enum-based data structures, such as EnumSet and EnumMap.

And actually - you should not need to. If you want some integer property, define one.

Answer 3

As the accepted answer points out, you can't set the ordinal. The closest you can get to this is with a custom property:

public enum MonthEnum {

    JANUARY(1),
    FEBRUARY(2),
    MARCH(3),
    APRIL(4),
    MAY(5),
    JUNE(6),
    JULY(7),
    AUGUST(8),
    SEPTEMBER(9),
    OCTOBER(10),
    NOVEMBER(11),
    DECEMBER(12);

    MonthEnum(int monthOfYear) {
        this.monthOfYear = monthOfYear;
    }

    private int monthOfYear;

    public int asMonthOfYear() {
        return monthOfYear;
    }

}

Note: By default, enum values start at 0 (not 1) if you don't specify values. Also, the values do not have to increment by 1 for each item.

Answer 4

You can update ordinal using reflection:

private void setEnumOrdinal(Enum object, int ordinal) {
    Field field;
    try {
        field = object.getClass().getSuperclass().getDeclaredField("ordinal");
        field.setAccessible(true);
        field.set(object, ordinal);
    } catch (Exception ex) {
        throw new RuntimeException("Can't update enum ordinal: " + ex);
    }
}

Answer 5

From http://download.oracle.com/javase/1.5.0/docs/api/java/lang/Enum.html

public final int ordinal()Returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero). Most programmers will have no use for this method. It is designed for use by sophisticated enum-based data structures, such as EnumSet and EnumMap.

Returns: the ordinal of this enumeration constant

If you have

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

then SUNDAY has an ordinal of 0, MONDAY is 1, and so on...

Answer 6

Check out the Java Enum examples and docs

Returns the ordinal of this enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero). Most programmers will have no use for this method. It is designed for use by sophisticated enum-based data structures, such as EnumSet and EnumMap.

Answer 7

The easy answer: just change the order of the constants. The first defined will be 0, the second will be 1, etc. However, this may not be practical if you have constantly changing code, or enums will many many values. You can define a custom method to work around the default ordinal, but MAKE SURE it is well documented to avoid confusion!

public enum Values
{
    ONE, TWO, THREE, FOUR;

    public int getCustomOrdinal()
    {
        if(this == ONE)
        {
            return 3;
        }
        else if(this == TWO)
        {
            return 0;
        }
        ...
    }
}