list集合分组：根据对象某一属性分组

private Map<String, List<CreditContractSignSerial>> listGroup2MapTwo(List<CreditContractSignSerial> list) {
   Map<String, List<CreditContractSignSerial>> userSignSerialMap = Maps.newConcurrentMap();
   if (CollectionUtils.isNotEmpty(list)) {
      // 分组
      for (CreditContractSignSerial vo : list) {
         String transDate = new SimpleDateFormat("yyyyMMdd").format(vo.getCreateTime());

         List<CreditContractSignSerial> tempList = userSignSerialMap.get(transDate);
         /*如果取不到数据,那么直接new一个空的ArrayList**/
         if (tempList == null) {
            tempList = Lists.newArrayList();
            tempList.add(vo);
            userSignSerialMap.put(transDate, tempList);
         }
         else {
            /*某个vo之前已经存放过了,则直接追加数据到原来的List里**/
            tempList.add(vo);
         }
      }
   }
   return userSignSerialMap;
}