checkEntryNotNull(key, value);
      enumMap.put(key, value);
    }
    return ImmutableEnumMap.asImmutable(enumMap);
  }

  /**
   * Returns a {@link Collector} that accumulates elements into an {@code ImmutableMap} whose keys
   * and values are the result of applying the provided mapping functions to the input elements. The
   * resulting implementation is specialized for enum key types. The returned map and its views will