Map按照parentId分组构造 MapsysDepartMap = departList.stream().collect( Collectors.groupingBy(SysDepart::getId, Collectors.collectingAndThen( Collectors.toList(), sysDeparts -> sysDeparts.get(0))));
Map按照id和日期一起分组(以“##”隔开)构造 Map> sysDepartListMap = departList.stream().collect( Collectors.groupingBy(SysDepart::getParentId));
Map> sysDepartListMap = departList.stream().collect(Collectors.groupingBy( sysDepart -> sysDepart.getId()+"##"+sysDepart.getCreateTime()));



