List集合按某个属性或者字段进行分组操作代码

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List集合按某个属性或者字段进行分组

List分组 按照Student对象中的Institution（学院）属性进行分组统计

核心代码

Map> collect = stuList.stream().collect(Collectors.groupingBy(Student::getInstitution));

实现代码示例：

public static void main(String[] args) {
  List stuList=initStuList2();
  Map> collect = stuList.stream().collect(Collectors.groupingBy(Student::getInstitution));
  for(String key:collect.keySet()){
   System.out.println(key+":" +collect.get(key).size());
   System.out.println(collect.get(key));
  }
 }
 
 public static  List initStuList2(){
  List stuList=new ArrayList(1000);
  for(int i=0;i<10;i++){ 
   Student student = new Student();
   long stu=(long) ((Math.random()*9+10000)*1000000);
   String Idcard=String.valueOf(stu).substring(0, 9);
   String ids=UUID.randomUUID().toString().replaceAll("-","");
   student.setId(ids);
   student.setUsername("student"+i);
   student.setClasses("计算机"+i);
   student.setIdcard("362425199"+Idcard);
   String [] institution={"信息学院","文学院","音乐学院","体院","理学院","机电学院"};
   int ss=(int)(Math.random()*6);
   student.setInstitution(institution[ss]);
   student.setMobile("18179"+Idcard.substring(0, 6));
   student.setEmail(Idcard+"@qq.com");
   student.setQq(Idcard);
   student.setHomeaddress("广东省深圳市");
   student.setWeixin(Idcard);
   if(i%50==0){student.setSex("广东省深圳市");}
   else{
    String[] sexs={"男","女"};
    int ii=((int) Math.random());
    student.setSex(sexs[ii]); 
   }
   student.setCreateby("拿破仑");
   student.setCreatetime(new Date());
   stuList.add(student);
  }
  return stuList;
 } 

实现效果

按照学院分组，得到体院集合中6个对象，文学院2个对象，理学院1个对象，信息学院1个对象

List>分组统计 根据性别分类

核心代码：

Map>> gslist = agentList.stream().collect(Collectors.groupingBy(e -> e.get("sex").toString()));

实现代码示例

public static void main(String[] args) {
		List> agentList=init();
		HashMap hashMap =(HashMap) agentList.get(0);
		Map>> 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));
	    }
	}
 	
	/***
	 * 初始化联系信�?
	 * @return
	 */
	public static  List> init(){
		String insertID=UUID.randomUUID().toString().replaceAll("-","");
		List> concacts= new ArrayList>();
		long time1=System.currentTimeMillis();
		for(int i=0;i<10;i++){
			String id=UUID.randomUUID().toString().replaceAll("-","");
			Map map = new HashMap();
			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","人妖"};
			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;
	}	

实现效果

List分组的两种方式

java8之前List分组

假设有个student类，有id、name、score属性，list集合中存放所有学生信息，现在要根据学生姓名进行分组。

public Map> groupList(List students) {
 Map> map = new Hash<>();
 for (Student student : students) {
  List tmpList = map.get(student.getName());
  if (tmpList == null) {
   tmpList = new ArrayList<>();
   tmpList.add(student);
   map.put(student.getName(), tmpList);
  } else {
   tmpList.add(student);
  }
 }
 return map;
}

java8的List分组

public Map> groupList(List students) {
 Map> map = students.stream().collect(Collectors.groupingBy(Student::getName));
 return map;
}