code: public GoogleCalendar getPrimaryCalendar() {
          return (GoogleCalendar) getCalendars().stream()
                  .filter(calendar -> ((GoogleCalendar) calendar).isPrimary())
                  .findFirst()
                  .orElse(null);
      }
docs: Gets the calendar marked as primary calendar for the google account.
  
  @return The primary calendar, {@code null} if not loaded.