对list中map中的key值进行排序

//对时间戳进行排序  获取最新的map
List<Map<String, Object>> dateList = new ArrayList<Map<String, Object>>();
            
            Map<String, Object> paramsValue = new HashMap<String, Object>();
            paramsValue.put("userId", user.getId());
            List<SafeConfess> safeConfessList = safeConfessService.querySafeConfessAndContent(paramsValue);
            if (safeConfessList.size()>0) {
                SafeConfess safeConfess = safeConfessList.get(0);
                Date date2 = DateUtils.parseDate(safeConfess.getCreateTime(), "yyyy-MM-dd HH:mm:ss");
                Map<String, Object> mapTemp = new HashMap<String, Object>();
                mapTemp.put("content", safeConfess.getSafeName());
                mapTemp.put("dateTime", date2.getTime());
                mapTemp.put("type", "1");
                dateList.add(mapTemp);
            }
            List<TrainEvaluation> resultList = trainEvaluationService.queryTrainEvaluation(paramsValue);
            if (resultList.size()>0) {
                TrainEvaluation trainEvaluation = resultList.get(0);
                Date date1 = DateUtils.parseDate(trainEvaluation.getCreateTime(), "yyyy-MM-dd HH:mm:ss");
                Map<String, Object> mapTemp = new HashMap<String, Object>();
                mapTemp.put("content", trainEvaluation.getTrainName());
                mapTemp.put("dateTime", date1.getTime());
                mapTemp.put("type", "2");
                dateList.add(mapTemp);
            }
            paramsValue.remove("userId");
            paramsValue.put("problemPersonId", user.getId());            
            List<Rectification> queryRectification = rectificationService.queryRectification(paramsValue);
            if (queryRectification.size()>0) {
                Rectification rectification = queryRectification.get(0);
                Date date3 = DateUtils.parseDate(rectification.getCreateTime(), "yyyy-MM-dd HH:mm:ss");
                Map<String, Object> mapTemp = new HashMap<String, Object>();
                mapTemp.put("content", rectification.getRectifyName());
                mapTemp.put("dateTime", date3.getTime());
                mapTemp.put("type", "3");
                dateList.add(mapTemp);
            }
            for (Map<String, Object> map1 : dateList) {
                System.out.println(map1.get("dateTime"));
            }
            //按时间升序排列
            Collections.sort(dateList, new Comparator<Map<String, Object>>() {
                public int compare(Map<String, Object> o1, Map<String, Object> o2) {
                    BigInteger date1 = new BigInteger(o1.get("dateTime").toString()) ;
                    BigInteger date2 = new BigInteger(o2.get("dateTime").toString()) ; 
                    return date1.compareTo(date2);
                }
            });
            //排序后 
            System.out.println("-------------------");
            for (Map<String, Object> map1 : dateList) {
                System.out.println(map1.get("dateTime"));
            }