public V2 apply(Entry<K, V1> entry) {
        return transformer.transformEntry(entry.getKey(), entry.getValue());
      }
    };
  }

  /** Returns a view of an entry transformed by the specified transformer. */
  static <V2 extends @Nullable Object, K extends @Nullable Object, V1 extends @Nullable Object>
      Entry<K, V2> transformEntry(