Accessing Same Class' enum value inside an enum declaration within a lambda doesn't compile

Question

I am trying to access an enum value inside the declaration of another enum value in the same class. Following is the way I found to get that done .

Java enum- Cannot reference a field before it is defined

which compiles just fine but since it involves a lot of formal code, I tried to replace the interface implementations with a Lambda but it doesn's compile with an error such as

Cannot reference a field before it is defined

Following is the lambda replacement I did.

package test;
public enum Baz {

yin(() -> {
    return Baz.yang;//doesnt compile. ->Cannot reference a field before it is defined
}),
yang(new OppositeHolder() {
    @Override
    public Baz getOpposite() {
        return yin;
    }
}),
good(new OppositeHolder() {
    @Override
    public Baz getOpposite() {
        return evil;//BUT THIS COMPILES JUST FINE, EVEN THOUGH IT ACCESSES evil WHICH HAS BEEN DECLARED LATER.
    }
}),
evil(new OppositeHolder() {
    @Override
    public Baz getOpposite() {
        return good;
    }
});

private final OppositeHolder oppositeHolder;

private Baz(OppositeHolder oppositeHolder) {
    this.oppositeHolder = oppositeHolder;
}

private static interface OppositeHolder {
    public Baz getOpposite();
}

}

Is there any way I could do this with the help of lambda expressions, since I could be seeing a lot of enums in this class and i don't really want to repeat the boiler plate code.

EDIT I know that the error means that I cannot access an Enum that has been declared later. Also, I cannot really have the enum name stored as a string since that would be a maintenance nightmare for me. Also, I am curious why the example in the link compiles but mine does not.

Answer 1

This cannot be done by simply replacing anonymous interface implementations by corresponding lambda expressions, because they are not the same thing.

A lambda body does not have its own context. This is defined in the Java Language Specification, § 15.27.2:

Unlike code appearing in anonymous class declarations, the meaning of names and the this and super keywords appearing in a lambda body, along with the accessibility of referenced declarations, are the same as in the surrounding context (except that lambda parameters introduce new names).

^{(Emphasis mine)}

That means you have effectively the same problem as the OP of your linked question.

---

I doubt if I would follow your approach. I personally prefer the code from this answer.

enum Baz {

    YIN,
    YANG,
    GOOD,
    EVIL;

    private static final Map<Baz, Baz> OPPOSITES;
    static {
        Map<Baz, Baz> m = new EnumMap<>(Baz.class);
        m.put(YIN, YANG);
        m.put(YANG, YIN);
        m.put(GOOD, EVIL);
        m.put(EVIL, GOOD);
        OPPOSITES = Collections.unmodifiableMap(m);
    }

    public Baz opposite() {
        return Objects.requireNonNull(OPPOSITES.get(this));
    }
}

Answer 2

the issue is that your using variable before they are defined even if it's a lambda call. A workaround can be to use valueOf

public enum Baz {

        yin(() -> Baz.valueOf("yang")),
        yang(() -> Baz.valueOf("yin")),
        good(() -> Baz.valueOf("evil")),
        evil(() -> Baz.valueOf("good"))
        ;

        private final OppositeHolder oppositeHolder;

        private Baz(OppositeHolder oppositeHolder) {
            this.oppositeHolder = oppositeHolder;
        }

        private static interface OppositeHolder {
            public Baz getOpposite();
        }
    }

    public static void main(String... args){
        System.out.println(Baz.yin);
        System.out.println(Baz.yin.oppositeHolder.getOpposite());

    }

output is

yin
yang