Student类中groupCode属性存在一对多关系,如何用lambda给它处理一对多关系?

import com.google.gson.Gson;
import lombok.AllArgsConstructor;
import lombok.Data;

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Demo {
    @Data
    @AllArgsConstructor
    static class Student{
        private String name;
        private int age;
        private String groupCode;
    }

    public static void main(String[] args) {
        Student student0 = new Student("张三", 18, "1");
        Student student1 = new Student("李四", 18, "1,2");
        Student student2 = new Student("王五", 18, "1,2,3");
        Student student3 = new Student("老六", 18, "1,3");
        Student student4 = new Student("七八", 18, "2");
        Student student5 = new Student("老王", 18, "3");

        List<Student> list = Arrays.asList(student0, student1, student2, student3, student4, student5);

        Map<String, List<Student>> collect = list.parallelStream().collect(Collectors.groupingBy(Student::getGroupCode));

        System.out.println(new Gson().toJson(collect));
    }
}

定义一个Student类里面有姓名年龄和分组代码,但是一个学生可能在多个角色,又是升旗手又是班委,groupCode存在一对多关系.想把学生按map分类,但是同时拥有多个角色的学生在多个分类中也应该出现.比如李四groupCode"1,2",那他应该同时出现在分组1和分组2中.如何用lambda实现,或者其他简洁的方法实现分组.

package com.example.demo.demodownload.test;

import com.alibaba.fastjson.JSONObject;
import com.google.common.base.Splitter;
import com.google.common.collect.Lists;
import com.google.common.collect.Sets;
import lombok.AllArgsConstructor;
import lombok.Data;

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;


public class Demo {


    @Data
    @AllArgsConstructor
    static class Student {

        private String name;
        private int age;
        private String groupCode;




    }


    public static void main(String[] args) {


        Student student0 = new Student("张三", 18, "1");
        Student student1 = new Student("李四", 18, "1,2");
        Student student2 = new Student("王五", 18, "1,2,3");
        Student student3 = new Student("老六", 18, "1,3");
        Student student4 = new Student("七八", 18, "2");
        Student student5 = new Student("老王", 18, "3");
        List<Student> list = Arrays.asList(student0, student1, student2, student3, student4, student5);
        Set<String> groupCodeSet = Sets.newHashSet();
        list.forEach(dto -> groupCodeSet.addAll(Splitter.on(",").splitToList(dto.getGroupCode())));
        Map<String, List<Student>> resultMap = new HashMap<>(8);
        for (String st : groupCodeSet) {
            List<Student> collect = list.stream().filter(dto -> Splitter.on(",").splitToList(dto.getGroupCode()).contains(st)).collect(Collectors.toList());
            resultMap.put(st, collect);
        }
        System.out.println(JSONObject.toJSONString(resultMap));
        System.err.println(JSONObject.toJSONString(resultMap.get("3")));

    }


}

是不是你想要的