# as is just because it's an instance of a subclass of a more limited class
            # e.g. UserInDB (containing hashed_password) could be a subclass of User
            # that doesn't have the hashed_password. But because it's a subclass, it
            # would pass the validation and be returned as is.

   */
  public static <K extends @Nullable Object, V extends @Nullable Object> SortedMap<K, V> filterKeys(
      SortedMap<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
    // TODO(lowasser): Return a subclass of Maps.FilteredKeyMap for slightly better
    // performance.
    return filterEntries(unfiltered, Maps.<K>keyPredicateOnEntries(keyPredicate));
  }

  /**

   */
  public static <K extends @Nullable Object, V extends @Nullable Object> SortedMap<K, V> filterKeys(
      SortedMap<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
    // TODO(lowasser): Return a subclass of Maps.FilteredKeyMap for slightly better
    // performance.
    return filterEntries(unfiltered, Maps.<K>keyPredicateOnEntries(keyPredicate));
  }

  /**

  final Segment<K, V>[] newSegmentArray(int ssize) {
    return (Segment<K, V>[]) new Segment<?, ?>[ssize];
  }

  // Inner Classes

  /**
   * Segments are specialized versions of hash tables. This subclass inherits from ReentrantLock
   * opportunistically, just to simplify some locking and avoid separate construction.
   */
  @SuppressWarnings("serial") // This class is never serialized.