}
        };

    Multimap<Integer, String> index = Multimaps.index(badGuys, stringLengthFunction);

    assertEquals(expectedIndex, index);
  }

  public void testIndex_nullValue() {
    List<@Nullable Integer> values = Arrays.asList(1, null);
    assertThrows(
        NullPointerException.class,
        () -> Multimaps.index((List<Integer>) values, Functions.identity()));
  }