assertThrows(
        NullPointerException.class,
        () -> Maps.uniqueIndex((List<String>) listWithNull, Functions.constant(1)));
  }

  /** Null keys aren't allowed either. */
  public void testUniqueIndexNullKey() {
    List<String> oneStringList = Lists.newArrayList("foo");
    assertThrows(
        NullPointerException.class,
        () -> Maps.uniqueIndex(oneStringList, Functions.constant(null)));
  }