java8 按对象属性值分组详解编程语言

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

public class HelloWorld { 
 
    public static void main(String[] args) { 
        Random random = new Random(); 
        List<User> list = new ArrayList<>(); 
        for(int i=1;i<=20;i++) { 
            String group = (random.nextInt(3) + 1) + "组";//1-3组随机 
            User u = new User(i, "用户-" + i, group); 
            list.add(u); 
        } 
 
        //按group属性分组 
        Map<String, List<User>> userMap = list.stream().collect(Collectors.groupingBy(User::getGroup)); 
 
        System.out.println("分组后：" + userMap); 
    } 
    private static class User{ 
        Integer id; 
        String name; 
        String group; 
 
        public User(Integer id, String name, String group) { 
            this.id = id; 
            this.name = name; 
            this.group = group; 
        } 
 
        public Integer getId() { 
            return id; 
        } 
 
        public void setId(Integer id) { 
            this.id = id; 
        } 
 
        public String getName() { 
            return name; 
        } 
 
        public void setName(String name) { 
            this.name = name; 
        } 
 
        public String getGroup() { 
            return group; 
        } 
 
        public void setGroup(String group) { 
            this.group = group; 
        } 
 
        @Override 
        public String toString() { 
            return "User{" + 
                    "id=" + id + 
                    ", name='" + name + '/'' + 
                    ", group='" + group + '/'' + 
                    '}'; 
        } 
    } 
 
}

{ 
    3 组 = [User { 
            id = 2, name = '用户-2', group = '3组' 
        }, User { 
            id = 5, name = '用户-5', group = '3组' 
        }, User { 
            id = 7, name = '用户-7', group = '3组' 
        }, User { 
            id = 8, name = '用户-8', group = '3组' 
        }, User { 
            id = 9, name = '用户-9', group = '3组' 
        }, User { 
            id = 12, name = '用户-12', group = '3组' 
        }, User { 
            id = 13, name = '用户-13', group = '3组' 
        }, User { 
            id = 15, name = '用户-15', group = '3组' 
        }, User { 
            id = 16, name = '用户-16', group = '3组' 
        }],  
    2 组 = [User { 
            id = 1, name = '用户-1', group = '2组' 
        }, User { 
            id = 17, name = '用户-17', group = '2组' 
        }, User { 
            id = 18, name = '用户-18', group = '2组' 
        }, User { 
            id = 20, name = '用户-20', group = '2组' 
        }],  
    1 组 = [User { 
            id = 3, name = '用户-3', group = '1组' 
        }, User { 
            id = 4, name = '用户-4', group = '1组' 
        }, User { 
            id = 6, name = '用户-6', group = '1组' 
        }, User { 
            id = 10, name = '用户-10', group = '1组' 
        }, User { 
            id = 11, name = '用户-11', group = '1组' 
        }, User { 
            id = 14, name = '用户-14', group = '1组' 
        }, User { 
            id = 19, name = '用户-19', group = '1组' 
        }] 
}