assertMapEquals(mapTwo, "one", 1, "two", 2, "three", 3, "four", 4);
    assertMapEquals(mapTwo.inverse(), 1, "one", 2, "two", 3, "three", 4, "four");
  }

  public void testBuilderPutNullKey() {
    Builder<String, Integer> builder = new Builder<>();
    assertThrows(NullPointerException.class, () -> builder.put(null, 1));
  }

  public void testBuilderPutNullValue() {

    assertMapEquals(mapOne, "one", 1, "two", 2);
    assertMapEquals(mapTwo, "four", 4, "one", 1, "three", 3, "two", 2);
  }

  public void testBuilderPutNullKey() {
    Builder<String, Integer> builder = ImmutableSortedMap.naturalOrder();
    assertThrows(NullPointerException.class, () -> builder.put(null, 1));
  }

  public void testBuilderPutNullValue() {

    assertThat(multimap.get("foo")).containsExactly(1, 2, 1, 6, 7, 2).inOrder();
    assertThat(multimap.get("bar")).containsExactly(4, 5, 4).inOrder();
    assertEquals(9, multimap.size());
  }

  public void testBuilderPutNullKey() {
    Multimap<@Nullable String, Integer> toPut = LinkedListMultimap.create();
    toPut.put(null, 1);
    ImmutableListMultimap.Builder<String, Integer> builder = ImmutableListMultimap.builder();

    assertThat(multimap.get("foo")).containsExactly(1, 2, 1, 6, 7, 2).inOrder();
    assertThat(multimap.get("bar")).containsExactly(4, 5, 4).inOrder();
    assertEquals(9, multimap.size());
  }

  public void testBuilderPutNullKey() {
    Multimap<@Nullable String, Integer> toPut = LinkedListMultimap.create();
    toPut.put(null, 1);
    ImmutableListMultimap.Builder<String, Integer> builder = ImmutableListMultimap.builder();

    builder.putAll(toPut);
    ImmutableSetMultimap<String, Integer> multimap = builder.build();
    assertEquals(4, multimap.size());
  }

  public void testBuilderPutNullKey() {
    Multimap<@Nullable String, Integer> toPut = LinkedListMultimap.create();
    toPut.put(null, 1);
    ImmutableSetMultimap.Builder<String, Integer> builder = ImmutableSetMultimap.builder();