List＜T＞集合转Map＜String, List＜T＞＞

 没描述，直接看结果吧。

package com.anchnet.sync;
 
import cn.hutool.json.JSONUtil;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.extern.slf4j.Slf4j;
 
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
 
/**
 * @author H.Yang
 * @date 2021/11/19
 */
@Slf4j
public class AppDemo {
 
    public static void main(String[] args) {
        List list = new LinkedList<>();
 
        list.add(new Student("A1", "男", 10001));
        list.add(new Student("A2", "女", 10002));
        list.add(new Student("A3", "男", 10002));
        list.add(new Student("A4", "女", 10001));
        list.add(new Student("A5", "男", 10003));
 
        // 核心代码
        Map> map = list.stream().collect(Collectors.groupingBy(item -> item.getTeacherId(), Collectors.mapping(item -> item, Collectors.toList())));
 
        System.out.println(JSONUtil.toJsonStr(map));
    }
 
}
 
@Data
@NoArgsConstructor
@AllArgsConstructor
class Student {
    private String name;
    private String sex;
    private Integer teacherId;
}

结果 

{
    "10002": [
        {
            "sex": "女",
            "teacherId": 10002,
            "name": "A2"
        },
        {
            "sex": "男",
            "teacherId": 10002,
            "name": "A3"
        }
    ],
    "10001": [
        {
            "sex": "男",
            "teacherId": 10001,
            "name": "A1"
        },
        {
            "sex": "女",
            "teacherId": 10001,
            "name": "A4"
        }
    ],
    "10003": [
        {
            "sex": "男",
            "teacherId": 10003,
            "name": "A5"
        }
    ]
}