java - Cyclic inheritance when implementing inner interface in enum

Question

I have the following implementation that gives a compiler error:

public enum FusionStat implements MonsterStatBuilderHelper {
    ATTACK {
        @Override
        public MonsterCard.MonsterCardBuilder safeCreateBuilder(final MonsterCard baseMonsterCard, final MonsterCard fusedMonsterCard, final FusionCard fusionCard) {
            Objects.requireNonNull(baseMonsterCard);
            Objects.requireNonNull(fusedMonsterCard);
            Objects.requireNonNull(fusionCard);
            if (baseMonsterCard.equals(fusedMonsterCard)) {
                throw new IllegalArgumentException("baseMonsterCard and fusedMonsterCard need to be different");
            }
            return new MonsterCard.MonsterCardBuilder(baseMonsterCard)
                    .attack(baseMonsterCard.getAttack() + (fusionCard.getFusionPower() * fusedMonsterCard.getAttack()));
        }
    },

    HITPOINTS {
        @Override
        public MonsterCard.MonsterCardBuilder safeCreateBuilder(final MonsterCard baseMonsterCard, final MonsterCard fusedMonsterCard, final FusionCard fusionCard) {
            Objects.requireNonNull(baseMonsterCard);
            Objects.requireNonNull(fusedMonsterCard);
            Objects.requireNonNull(fusionCard);
            if (baseMonsterCard.equals(fusedMonsterCard)) {
                throw new IllegalArgumentException("baseMonsterCard and fusedMonsterCard need to be different");
            }
            return new MonsterCard.MonsterCardBuilder(baseMonsterCard)
                    .maximumHitpoints((int)(baseMonsterCard.getMaximumHitpoints() + (fusionCard.getFusionPower() / 100d * fusedMonsterCard.getMaximumHitpoints())))
                    .hitpoints((int)(baseMonsterCard.getHitpoints() + (fusionCard.getFusionPower() / 100d * fusedMonsterCard.getHitpoints())));
        }
    };

    protected interface MonsterStatBuilderHelper extends MonsterStatBuilder {
        default MonsterCard.MonsterCardBuilder safeCreateBuilder(final MonsterCard baseMonsterCard, final MonsterCard fusedMonsterCard, final FusionCard fusionCard) {
            return createBuilder(baseMonsterCard, fusedMonsterCard, fusionCard);
        }
    }
}

@FunctionalInterface
interface MonsterStatBuilder {
    MonsterCard.MonsterCardBuilder createBuilder(final MonsterCard baseMonsterCard, final MonsterCard fusedMonsterCard, final FusionCard fusionCard);
}

It gives a cyclic inheritance error on the first line involving FusionStat.

I don't exactly see what is going on. I had first implemented an abstract class and wanted to let the enum extend that, until I realized that enums cannot extend classes. Now I try to (ab)use the default methods in Java 8.

I am interested in the thought process about why my code does not compile, I was trying to remove code duplication (still have to do so), by pulling the duplicated code inside safeCreateBuilder.

Answer 1

This would be because you are implementing (coding) the interface you are implementing (inheriting) inside of the class that is inheriting from that class.

I wish I could make that sentence better...

But here is a visual example.

Class A implements Interface B {

    Interface B {
    }
}

As far as I know, this is not allowed. You need to define the interface outside of the class (in this case, an Enum).

Like so:

Interface B {
}

Class A implements Interface B {
}

Best practice is probably to break them up into different files.