code: private static synchronized Calendar buildCalendar() {
  
      if (calendarCache.isEmpty()) {
        return GregorianCalendar.getInstance();
      } else {
        Calendar calendar = calendarCache.firstElement();
        calendarCache.remove(0);
        return calendar;
      }
    }
docs: Gets a calendar using the default time zone and locale. The Calendar
  returned is based on the current time in the default time zone with the
  default locale.
  
  @return a calendar instance.