初始程式碼

public Map<String,List<RgwstBean>> getMap(List<RgwstBean> lists){
		Map<String,List<RgwstBean>> map = new TreeMap<String,List<RgwstBean>>();
		if(lists==null) {
			return map;
		}
		for(RgwstBean rb :lists) {
			String newdate = rb.getDatetime();
			if(map.containsKey(newdate)) {
				map.get(newdate).add(rb);
			}else {
				List<RgwstBean> newlist2 = new ArrayList<RgwstBean>();
				newlist2.add(rb);
				map.put(newdate, newlist2);
			}
			
		}
		return map;
	}
 

lambda語法

public Map<String,List<RgwstBean>> getMap(List<RgwstBean> lists){
//groupingBy無排序
Map<String,List<RgwstBean>> map = lists.stream().collect(Collectors.groupingBy(RgwstBean::getDatetime));
Map<String,List<RgwstBean>> sortmap = new TreeMap<>();
//Map<String,List<RgwstBean>> sortmap = new TreeMap<>((o1,o2)->o2.compareTo(o1));//倒序
		map.entrySet()
		.stream()
		.forEach(x->sortmap.put(x.getKey(),x.getValue()));
		return sortmap;
}