*       {@code Objects.equal(transformer.transform(k1, v1), transformer.transform(k2, v2))}.
     * </ul>
     *
     * @throws NullPointerException if the key or value is null and this transformer does not accept
     *     null arguments
     */
    @ParametricNullness
    V2 transformEntry(@ParametricNullness K key, @ParametricNullness V1 value);
  }