@Udf
  public <T> List<T> mapValues(final Map<String, T> input) {
    if (input == null) {
      return null;
    }
    return Lists.newArrayList(input.values());
  }
@Test
  public void shouldReturnNullForNullInput() {
    List<Long> result = udf.mapValues((Map<String, Long>) null);
    assertThat(result, is(nullValue()));
  }