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

Inspired by Tom Hawtin’s answer I created wrapper class holding Classes, but only those with signature <E extends Enum<E> & Field>:

public final static class FieldEnumWrapper<E extends Enum<E> & Field> {
  private final Class<E> clazz;
  private final ImmutableMap<String, E> index;

  public static <E extends Enum<E> & Field> 
      FieldEnumWrapper<E> of(final Class<E> clazz) {
    return new FieldEnumWrapper<E>(clazz);
  }

  private FieldEnumWrapper(final Class<E> clazz) {
    this.clazz = clazz;
    this.index = Maps.uniqueIndex(
        EnumSet.allOf(clazz), new Function<E, String>() {
          @Override
          public String apply(final E input) {
            return input.searchName();
          }
        });
  }

  public Class<E> clazz() {
    return clazz;
  }

  public Field valueOfSearchName(final String searchName) {
    return index.get(searchName);
  }
}

Now:

for (final FieldEnumWrapper<?> fieldEnum : ImmutableList.of(
    FieldEnumWrapper.of(EntityField.class),
    FieldEnumWrapper.of(AddressField.class),
    FieldEnumWrapper.of(PersonFunctionType.class))) {
  valueOfSearchName = fieldEnum.valueOfSearchName("POD_I_OS_PARTNER");
  // ...

is type-safe and inappropriate usage of FieldEnumWrapper‘s static factory:

FieldEnumWrapper.of(NotEnumAndFieldClass.class)

generates compile error.

Moreover, valueOfSearchName is now method of FieldEnumWrapper what make more sense that helper class.