list分组--根据某一个字段分组

List分组-对象List分组-MapList分组List中的对象元素以某个属性分组。例如，用id分组，将id放在一起//List 以ID分组 Map<Integer,List<Apple>>Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId)); System.err.println("groupBy:"+groupBy);{1=[Apple{id=1, name='苹果1', money=3.25, num=10}, Apple{id=1, name='苹果2', money=1.35, num=20}], 2=[Apple{id=2, name='香蕉', money=2.89, num=30}], 3=[Apple{id=3, name='荔枝', money=9.99, num=40}}List中的Map元素，以某种属性分组public static void main(String[] args) {        List<Map<String,Object>> agentList=init();        HashMap<String, Object> hashMap =(HashMap<String, Object>) agentList.get(0);        Map<String, List<Map<String, Object>>> gslist = agentList.stream().collect(Collectors.groupingBy(e -> e.get("sex").toString()));        for(String key:gslist.keySet()){            System.out.println(key+" : "+gslist.get(key).size());            System.out.println(gslist.get(key));        }    }                     /***     * 联系人的初始化     * @return     */    public static  List<Map<String,Object>> init(){        String insertID=UUID.randomUUID().toString().replaceAll"""";        List<Map<String,Object>> concacts= new ArrayList<Map<String,Object>>();        long time1=System.currentTimeMillis();        for(int i=0;i<10;i++){            String id=UUID.randomUUID().toString().replaceAll("-","");            Map<String, Object> map = new HashMap<String,Object>();            map.put("id", id);            map.put("name", "张三");            map.put("identity_no", "36242519961225"+(int)(Math.random()*10000+1000);            map.put("telphone","1852562"+(int)(Math.random()*10000+1000);            map.put("address"江西吉安");            map.put("levels", "VIP");            map.put("source", 0);            map.put("flight_no", "ZH9101");            map.put("planned_takeofftime", "1220");            map.put("estimated_takeofftime", "1425");            map.put("flight_change_type", 1);            map.put("flight_change_reason", “军事活动”);            map.put("flightdate","2019-05-01");            map.put("en_name", "ZHANG/SAN");            map.put("traveller_idx", (int)(Math.random()*1000+100);            String [] sexs={“男”，“女”，“同性恋”;            int kk=(int) (Math.random()*4);            map.put("sex", sexs[kk]);            map.put("phone_num","1302880"+(int)(Math.random()*10000+1000);            map.put("originating", "SZX");            map.put("terminus", "BKK");            map.put("ticketid", (int)(Math.random()*10000+1000);            map.put("mainspace", "J");            map.put("sonspace", "C");            map.put("message_info", "4");            map.put("extension", "1892562"+(int)(Math.random()*10000+1000);            map.put("officeid", (int)(Math.random()*10000+1000);            map.put("pnrics", (int)(Math.random()*10000+1000);            map.put("traveller_safe", "2019-02-23 ZH9007");            map.put("phone_inform", 1);            concacts.add(map);        }        long time2=System.currentTimeMillis();        //System.out.println(初始化数据花�?“+(time2-time1)/1000;        return concacts;    }    过滤filter从集合中过滤出符合条件的元素：//过滤出符合条件的数据list<Apple> filterList = appleList.stream().filter(a -> a.getName().equals(香蕉).collect(Collectors.toList()); System.err.println("filterList:"+filterList);[Apple{id=2, name='香蕉', money=2.89, num=30}按属性求和集合中的数据：//计算 BigDecimal总金额 totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);System.err.println("totalMoney:"+totalMoney);  //totalMoney:17.48根据某一属性排序Listtt<Map<String, Object>> executorList = getExecutorList();executorList.sort(Comparator.comparingInt(x -> Integer.parseInt(x.get("sortnumber").toString())));