code: public static <K0 extends Enum<K0>> MultimapBuilderWithKeys<K0> enumKeys(
        final Class<K0> keyClass) {
      checkNotNull(keyClass);
      return new MultimapBuilderWithKeys<K0>() {
        @SuppressWarnings("unchecked")
        @Override
        <K extends K0, V> Map<K, Collection<V>> createMap() {
          // K must actually be K0, since enums are effectively final
          // (their subclasses are inaccessible)
          return (Map<K, Collection<V>>) new EnumMap<K0, Collection<V>>(keyClass);
        }
      };
    }
docs: Uses an {@link EnumMap} to map keys to value collections.
  
  @since 16.0