java enum illegal forward reference with constant

Question

I have the following simple enum in which I want to declare and use a few constants (as the constructor arguments):

public enum MyEnum {

    A(FOO), B(BAR);

    MyEnum(String foo) {
    }

    static final String FOO = "foo";
    static final String BAR = "bar";
}

However, the compiler gives me the following error:

Error:(3, 7) java: illegal forward reference
Error:(3, 15) java: illegal forward reference

If I try to refactor the original enum in an IDE like Intellij IDEA, it extracts the constructor arguments into constants of an inner class, and the following code compiles ok:

SNIPPET 1:

public enum MyEnum {

    A(Constants.FOO), B(Constants.BAR);

    MyEnum(String foo) {
    }

    private static class Constants {
        public static final String FOO = "foo";
        public static final String BAR = "bar";
    }
}

I also found out that even without an inner constants class, the code compiles ok if the constants are specified using fully classified names:

SNIPPET 2:

public enum MyEnum {

    A(MyEnum.FOO), B(MyEnum.BAR);

    MyEnum(String foo) {
    }

    static final String FOO = "foo";
    static final String BAR = "bar";
}

Questions:

1. Could anybody please explain why the code in snippet 2 compiles while the original one does not? Is it some linking issue?

2. When choosing between solutions of snippet 1 and 2, which approach is better (cleaner)? To me, snippet 1 (inner class with constants) looks cleaner, but I'm concerned with an extra cost of an inner class: there's gonna be 20 enums in the app +20 inner classes in that case.

Thanks!