Java获取Set中第一个值

Map<String, List<Cinema>> cinemaMap = _movieDateCinemaMap.get(movieId);
			if (cinemaMap != null && cinemaMap.size() > 0) {
				String firstDate = cinemaMap.keySet().iterator().next();
				if (firstDate.equals(DateUtil.getCurrDate_YYYYMMDD())) {
					System.out.println(firstDate);
				}
				returnMap = new LinkedHashMap<String, List<Cinema>>(cinemaMap);
			}