@Udf
  public <T> List<String> mapKeys(final Map<String, T> input) {
    if (input == null) {
      return null;
    }
    return Lists.newArrayList(input.keySet());
  }
@Test
  public void shouldGetKeys() {
    final Map<String, String> input = new HashMap<>();
    input.put("foo", "spam");
    input.put("bar", "baloney");
    assertThat(udf.mapKeys(input), containsInAnyOrder("foo", "bar"));
  }