Using generics with collection of enum classes implementing same interface

Question

I am trying to do reverse lookup on few enum classes implementing same Field interface by iterating through list of Classes using Guava's Maps.uniqueIndex:

  Field valueOfSearchName = null;
  for (final Class<? extends Enum<?>> clazz : ImmutableList.of(
      EntityField.class,
      AddressField.class,
      PersonFunctionType.class)) {
    valueOfSearchName = Fields.valueOfSearchName(clazz, term.field()); // error
    if (valueOfSearchName != null) {
      // do something...
      break;
    }
  }

I don't want to repeat same code (for making index and doing lookup) in all enum classes, so I use helper static class Fields containing Fields.valueOfSearchName method:

  public static <E extends Enum<E> & Field> Field valueOfSearchName(
      final Class<E> clazz, final String searchName) {
    // TODO: cache the index
    final ImmutableMap<String, E> index = Maps.uniqueIndex(
        EnumSet.allOf(clazz), GET_SEARCH_NAME_FUNCTION);
    return index.get(searchName);
  }

Unfortunately, Eclipse shows an error:

Bound mismatch: 
The generic method valueOfSearchName(Class<E>, String) of type Fields is not 
applicable for the arguments (Class<capture#1-of ? extends Enum<?>>, String).
The inferred type capture#1-of ? extends Enum<?> is not a valid substitute
for the bounded parameter <E extends Enum<E> & Field>

---

The problem is Class<? extends Enum<?>> clazz in for-each loop (not matching Field), but I don't know how to deal with this case (obviously I cannot add & Field to clazz).

Answer 1

Consider Class<? extends List<?>. Class<? extends List<?> has two wildcards whereas <E extends List<E>> Class<E> only has generic parameter. The former will admit Class<ArrayList<String>>. So without doing something extra special for enums, the types are not compatible.

How to fix? An extra layer of indirection!

public final class MetaEnum<E extends Enum<E>> {
    private final E clazz;
    public static <E extends Enum<E>> MetaEnum<E> of(E clazz) {
        return clazz;
    }
    private MetaEnum(E clazz) {
        this.clazz = clazz;
    }
    public E clazz() {
        return clazz;
    }
    // ...
}

for (final MetaEnum<?> meta : ImmutableList.of(
    MetaEnum.of(EntityField       .class),
    MetaEnum.of(AddressField      .class),
    MetaEnum.of(PersonFunctionType.class)
)) {
    Field valueOfSearchName = Fields.valueOfSearchName(
        meta.clazz(), term.field()
    );
    ...

(Usual Stack Overflow dislaimer: Not so much as attempted to compile.)