@Udf(description = "Returns a masked version of the input string. All characters except for the"
      + " first n will be replaced according to the default masking rules.")
  @SuppressWarnings("MethodMayBeStatic") // Invoked via reflection
  public String mask(
      @UdfParameter("input STRING to be masked") final String input,
      @UdfParameter("number of characters to keep unmasked at the start") final int numChars
  ) {
    return doMask(new Masker(), input, numChars);
  }
@Test
  public void shouldReturnNullForNullInput() {
    final String result = udf.mask(null, 5);
    assertThat(result, is(nullValue()));
 }