Iterator<String> iterator = asList("one", "two").iterator();
    IllegalArgumentException expected =
        assertThrows(IllegalArgumentException.class, () -> getOnlyElement(iterator));
    assertThat(expected).hasMessageThat().isEqualTo("expected one element but was: <one, two>");
  }

  public void testGetOnlyElement_noDefault_fiveElements() {
    Iterator<String> iterator = asList("one", "two", "three", "four", "five").iterator();

    IllegalArgumentException expected =
        assertThrows(
            IllegalArgumentException.class,
            () -> Maps.uniqueIndex(ImmutableSet.of("one", "uno"), Functions.constant(1)));
    assertThat(expected).hasMessageThat().contains("Multimaps.index");
  }

  /** Null values are not allowed. */
  public void testUniqueIndexNullValue() {
    List<@Nullable String> listWithNull = Lists.newArrayList((String) null);
    assertThrows(

    IllegalArgumentException expected =
        assertThrows(
            IllegalArgumentException.class,
            () -> Maps.uniqueIndex(ImmutableSet.of("one", "uno"), Functions.constant(1)));
    assertThat(expected).hasMessageThat().contains("Multimaps.index");
  }

  /** Null values are not allowed. */
  public void testUniqueIndexNullValue() {
    List<@Nullable String> listWithNull = Lists.newArrayList((String) null);
    assertThrows(