list.stream()

Map<String,List<AlarmStatisticalDO>> map = alarmStatisticalDOList.stream().collect(Collectors.groupingBy(a -> a.getAlarmDate()));
for (String alarmDateStr:map.keySet()) {
    List<LibAlarm> libAlarmList = new ArrayList<>();
    List<AlarmStatisticalDO> alarmStatisticalDOS = map.get(alarmDateStr);
　　 ....
}

List<LibAlarm> libAlarms = libAlarmList.stream().sorted(Comparator.comparing(LibAlarm::getAlarmCount).reversed()).collect(Collectors.toList());//倒序