java 集合分组，集合根据某一条件分组，封装成map形式

清疚 2022-04-15 02:50 146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", "10");
    		User user2 = new User("2", "张三", "11");
    		User user3 = new User("2", "张三", "12");
    		User user4 = new User("4", "李四", "13");
    		User user5 = new User("5", "李四", "14");
    		User user6 = new User("6", "李四", "15");
    		User user7 = new User("7", "王五", "16");
    		User user8 = new User("8", "王五", "17");
    		User user9 = new User("9", "王五", "18");
    		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);
    		Map<String,List<User>> userGroupMap = list.stream().
    				collect(Collectors.groupingBy(User::getName));
    		Set<String> keySet = userGroupMap.keySet();
    		keySet.forEach((key)->{
    			System.out.println(key);
    			userGroupMap.get(key).forEach((user)->System.out.println(user));
    		});
    	}

结果为：

李四  
User \[id=4, name=李四, age=13\]  
User \[id=5, name=李四, age=14\]  
User \[id=6, name=李四, age=15\]  
张三  
User \[id=1, name=张三, age=10\]  
User \[id=2, name=张三, age=11\]  
User \[id=2, name=张三, age=12\]  
王五  
User \[id=7, name=王五, age=16\]  
User \[id=8, name=王五, age=17\]  
User \[id=9, name=王五, age=18\]

、

# 注意： #

Map<String,List<User>> userGroupMap = list.stream().  
                collect(Collectors.groupingBy(User::getName));

此句中的User::getName

getName 为对象中某属性的get方法