* #entriesOnlyOnLeft()}, {@link #entriesOnlyOnRight()}, {@link #entriesInCommon()} and {@link
   * #entriesDiffering()} of the two instances are equal.
   */
  @Override
  boolean equals(@Nullable Object object);

  /**
   * Returns the hash code for this instance. This is defined as the hash code of
   *
   * {@snippet :
   * Arrays.asList(entriesOnlyOnLeft(), entriesOnlyOnRight(),
   *     entriesInCommon(), entriesDiffering())

   * #entriesOnlyOnLeft()}, {@link #entriesOnlyOnRight()}, {@link #entriesInCommon()} and {@link
   * #entriesDiffering()} of the two instances are equal.
   */
  @Override
  boolean equals(@Nullable Object object);

  /**
   * Returns the hash code for this instance. This is defined as the hash code of
   *
   * {@snippet :
   * Arrays.asList(entriesOnlyOnLeft(), entriesOnlyOnRight(),
   *     entriesInCommon(), entriesDiffering())

    MapDifference<Integer, Integer> diff = Maps.difference(EMPTY, EMPTY);
    assertTrue(diff.areEqual());
    assertEquals(EMPTY, diff.entriesOnlyOnLeft());
    assertEquals(EMPTY, diff.entriesOnlyOnRight());
    assertEquals(EMPTY, diff.entriesInCommon());
    assertEquals(EMPTY, diff.entriesDiffering());
    assertEquals("equal", diff.toString());
  }

  public void testMapDifferenceEmptySingleton() {

    MapDifference<Integer, Integer> diff = Maps.difference(EMPTY, EMPTY);
    assertTrue(diff.areEqual());
    assertEquals(EMPTY, diff.entriesOnlyOnLeft());
    assertEquals(EMPTY, diff.entriesOnlyOnRight());
    assertEquals(EMPTY, diff.entriesInCommon());
    assertEquals(EMPTY, diff.entriesDiffering());
    assertEquals("equal", diff.toString());
  }

  public void testMapDifferenceEmptySingleton() {

            && entriesOnlyOnRight().equals(other.entriesOnlyOnRight())
            && entriesInCommon().equals(other.entriesInCommon())
            && entriesDiffering().equals(other.entriesDiffering());
      }
      return false;
    }

    @Override
    public int hashCode() {
      return Objects.hash(
          entriesOnlyOnLeft(), entriesOnlyOnRight(), entriesInCommon(), entriesDiffering());

            && entriesOnlyOnRight().equals(other.entriesOnlyOnRight())
            && entriesInCommon().equals(other.entriesInCommon())
            && entriesDiffering().equals(other.entriesDiffering());
      }
      return false;
    }

    @Override
    public int hashCode() {
      return Objects.hash(
          entriesOnlyOnLeft(), entriesOnlyOnRight(), entriesInCommon(), entriesDiffering());