ImmutableEnum{Set,Map} Collectors

Add collectors for enum-specialized ImmutableSet and ImmutableMap:

• toImmutableEnumSet()
• toImmutableEnumMap(keyFunction, valueFunction)
• toImmutableEnumMap(keyFunction, valueFunction, mergeFunction)

Providing these collectors in Guava would be a matter of convenience. Users could implement functionally-equivalent collectors themselves, but I think most users would not do so. I think enum-specialized collections are underused and that it is worthwhile to promote their usage and remove friction / barriers to entry.

For example, if a user has code like this:

ImmutableSet<Month> months = stream.collect(ImmutableSet.toImmutableSet());

… then it would be cool if they could optimize their code like this:

ImmutableSet<Month> months = stream.collect(ImmutableSet.toImmutableEnumSet());

… and it would be lame if they had to do this (it would be so lame that they wouldn’t bother):

ImmutableSet<Month> months = stream.collect(SomeHelper.toImmutableEnumSet(Month.class));

// Elsewhere... javadoc and tests omitted...
public static <E extends Enum<E>> Collector<E, ?, ImmutableSet<E>>
toImmutableEnumSet(Class<E> elementType) {
  Objects.requireNonNull(elementType);
  return Collectors.collectingAndThen(
      Collectors.toCollection(() -> EnumSet.noneOf(elementType)),
      Sets::immutableEnumSet);
}

[lowasser]

I’ve put this into Guava’s API review queue. I’ve put together a different implementation, though, that avoids quite so much redundant copying. I’ll keep this thread updated on the status of the API review.

[Stephan202]

D’oh I was looking in the wrong place. You’re right! Nice.