public static MonthsWindows months(int number) {
    return new MonthsWindows(number, 1, DEFAULT_START_DATE, DateTimeZone.UTC);
  }
@Test
  public void testDefaultWindowMappingFn() {
    MonthsWindows windowFn = CalendarWindows.months(2);
    WindowMappingFn<?> mapping = windowFn.getDefaultWindowMappingFn();

    assertThat(
        mapping.getSideInputWindow(
            new BoundedWindow() {
              @Override
              public Instant maxTimestamp() {
                return new Instant(100L);
              }
            }),
        equalTo(windowFn.assignWindow(new Instant(100L))));
    assertThat(mapping.maximumLookback(), equalTo(Duration.ZERO));
  }