java 集合分组;list根据某一条件进行分组

「爱情、让人受尽委屈。」 2022-04-15 02:47 781阅读 0赞

集合中为对象，根据对象中的某一属性值分组

例子

1、实体类：构造方法、get/set方法、toString方法省略

public class User {
        private String id;
        
        private String name;
        
        private String age;
        
        .......
        .......
    }

2、测试类：

public static void main(String[] args){
    		List<User> list = new ArrayList<>();
    		User user1 = new User("1", "张三1", "10");
    		User user2 = new User("2", "张三2", "10");
    		User user3 = new User("2", "张三3", "10");
    		User user4 = new User("4", "张三4", "11");
    		User user5 = new User("5", "张三5", "11");
    		User user6 = new User("6", "张三6", "11");
    		User user7 = new User("7", "张三7", "12");
    		User user8 = new User("8", "张三8", "12");
    		User user9 = new User("9", "张三9", "12");
    		list.add(user1);
    		list.add(user2);
    		list.add(user3);
    		list.add(user4);
    		list.add(user5);
    		list.add(user6);
    		list.add(user7);
    		list.add(user8);
    		list.add(user9);
    		List<List<User>> groupList = new ArrayList<>();
    		list.stream().collect(Collectors.groupingBy(User::getAge,Collectors.toList()))
    	                .forEach((age,fooListByAge)->{
    	                	groupList.add(fooListByAge);
    	                });
    		
    		groupList.forEach((userList)->{
    			System.out.println(userList.size());
    			userList.forEach((user)->System.out.println(user));
    		});
    	}

结果为：

3  
User \[id=4, name=张三4, age=11\]  
User \[id=5, name=张三5, age=11\]  
User \[id=6, name=张三6, age=11\]  
3  
User \[id=7, name=张三7, age=12\]  
User \[id=8, name=张三8, age=12\]  
User \[id=9, name=张三9, age=12\]  
3  
User \[id=1, name=张三1, age=10\]  
User \[id=2, name=张三2, age=10\]  
User \[id=2, name=张三3, age=10\]

# 注意： #

list.stream().collect(Collectors.groupingBy(User::getAge,Collectors.toList()))  
                    .forEach((age,fooListByName)->\{  
                        groupList.add(fooListByName);  
                    \});

此句中的User::getAge

getAge 为对象中某属性的get方法