});
  }

  public void testUniqueIndex_nullKey() {
    assertThrows(
        NullPointerException.class, () -> fluent(1, 2, 3).uniqueIndex(Functions.constant(null)));
  }

  public void testUniqueIndex_nullValue() {
    assertThrows(
        NullPointerException.class,
        () -> {
          ImmutableMap<Object, Integer> unused =
              fluent(1, null, 2)
                  .uniqueIndex(