* override {@link #containsKey} to forward to this implementation.
   *
   * @since 7.0
   */
  protected boolean standardContainsKey(@CheckForNull Object key) {
    return Maps.containsKeyImpl(this, key);
  }

  /**
   * A sensible implementation of {@link Map#values} in terms of the following methods: {@link
   * ForwardingMap#clear}, {@link ForwardingMap#containsValue}, {@link ForwardingMap#isEmpty},

   * override {@link #containsKey} to forward to this implementation.
   *
   * @since 7.0
   */
  protected boolean standardContainsKey(@CheckForNull Object key) {
    return Maps.containsKeyImpl(this, key);
  }

  /**
   * A sensible implementation of {@link Map#values} in terms of the following methods: {@link
   * ForwardingMap#clear}, {@link ForwardingMap#containsValue}, {@link ForwardingMap#isEmpty},

    } catch (ClassCastException | NullPointerException e) {
      return null;
    }
  }

  /** An admittedly inefficient implementation of {@link Map#containsKey}. */
  static boolean containsKeyImpl(Map<?, ?> map, @CheckForNull Object key) {
    return Iterators.contains(keyIterator(map.entrySet().iterator()), key);
  }

  /** An implementation of {@link Map#containsValue}. */