@Udf(description = "Returns the hex-encoded md5 hash of the input string")
  public String md5(
      @UdfParameter(description = "The input string") final String s
  ) {
    if (s == null) {
      return null;
    }
    return DigestUtils.md5Hex(s);
  }
@Test
  public void shouldReturnNullForNull() {
    assertThat(udf.md5(null), is(nullValue()));
  }