java8 stream groupingBy分组构造map数据字典

按照id分组构造 Map<String, SysDepart>

		Map<String, SysDepart> sysDepartMap = 
		departList.stream().collect(
		Collectors.groupingBy(SysDepart::getId,
		Collectors.collectingAndThen(
		Collectors.toList(), 
		sysDeparts -> sysDeparts.get(0))));

按照parentId分组构造 Map<String, List>

		Map<String, List<SysDepart>> sysDepartListMap = 
		departList.stream().collect(
		Collectors.groupingBy(SysDepart::getParentId));

按照id和日期一起分组（以“##”隔开）构造 Map<String, List>

Map<String,List<SysDepart>> sysDepartListMap = 
departList.stream().collect(Collectors.groupingBy(
sysDepart ->
sysDepart.getId()+"##"+sysDepart.getCreateTime()));