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.