code: public final void createCalendarSource() {
          Callback<CreateCalendarSourceParameter, CalendarSource> factory = getCalendarSourceFactory();
          if (factory != null) {
              CreateCalendarSourceParameter param = new CreateCalendarSourceParameter(this);
              CalendarSource calendarSource = factory.call(param);
              if (calendarSource != null && !getCalendarSources().contains(calendarSource)) {
                  getCalendarSources().add(calendarSource);
              }
          }
      }
docs: Creates a new calendar source that will be added to the list of calendar
  sources of this date control. The method delegates the actual creation of
  the calendar source to a factory, which can be specified by calling
  {@link #setCalendarSourceFactory(Callback)}.
  
  @see #setCalendarSourceFactory(Callback)