@Udf(description = "Converts a string representation of a date in the given format"
      + " into the number of days since 1970-01-01 00:00:00 UTC/GMT.")
  public int stringToDate(
      @UdfParameter(
          description = "The string representation of a date.") final String formattedDate,
      @UdfParameter(
          description = "The format pattern should be in the format expected by"
              + " java.time.format.DateTimeFormatter.") final String formatPattern) {
    // NB: We do not perform a null here preferring to throw an exception as
    // there is no sentinel value for a "null" Date.
    try {
      final DateTimeFormatter formatter = formatters.get(formatPattern);
      return ((int)LocalDate.parse(formattedDate, formatter).toEpochDay());
    } catch (final ExecutionException | RuntimeException e) {
      throw new KsqlFunctionException("Failed to parse date '" + formattedDate
          + "' with formatter '" + formatPattern
          + "': " + e.getMessage(), e);
    }
  }
@Test
  public void shouldThrowOnNullDate() {
    // When:
    final Exception e = assertThrows(
        KsqlFunctionException.class,
        () -> udf.stringToDate(null, "yyyy-MM-dd")
    );

    // Then:
    assertThat(e.getMessage(), containsString("Failed to parse date 'null' with formatter"));
  }