Question

Possible Duplicate:
Cannot access static field within enum initialiser

My situation:

enum Attribute { POSITIVE, NEGATIVE }
enum Content {
    C1(Attribute.POSITIVE),
    C2(Attribute.POSITIVE),
    ... // some other positive enum instances.
    Cm(Attribute.NEGATIVE),
    ... // some other negative enum instances.
    Cn(Attribute.NEGATIVE);

    private final Atrribute a;
    static int negativeOffset = 0;
    private Content(Atrribute a) {
        this.a = a;
        if ( a.compareTo(Attribute.POSITIVE) == 0 ) {
               negativeOffset ++;
        }
    }

    public static int getNegativeOffset() { return negativeOffset; }
} 

My intention is to add negativeOffset by one whenever I add a new enum(with POSITIVE attribute), then I could call the getNegativeOffset() to get the start point of negative
enum and do whatever I want.

But comlier complains that

Cannot refer to the static enum field Content.negativeOffset within an initializer

Answer 1

You can use this “trick”:

private static class IntHolder {
    static int negativeOffset;
}

then refer to the variable like this:

IntHolder.negativeOffset ++;

and

return IntHolder.negativeOffset; 

The reason this works is that the JVM guarantees that the variable is initialized when the IntHolder static inner class is initialized, which doesn’t happen until it’s accessed.

The whole class will then be as follows, which compiles:

enum Attribute { POSITIVE, NEGATIVE }
enum Content {
    C1(Attribute.POSITIVE),
    C2(Attribute.POSITIVE),
    ... // some other positive enum instances.
    Cm(Attribute.NEGATIVE),
    ... // some other negative enum instances.
    Cn(Attribute.NEGATIVE);

    private final Attribute a;

    private static class IntHolder {
        static int negativeOffset;
    }

    private Content(Attribute a) {
        this.a = a;
        if ( a == Attribute.POSITIVE) {
               IntHolder.negativeOffset ++;
        }
    }

    public static int getNegativeOffset() { return IntHolder.negativeOffset; }
}

Note the corrections for spelling errors and the simpler comparison with the Attribute enum value using == instead of compareTo()