Java 8 method, inside the double collection, take out the field value of the inner collection

When I encounter Java 8 on the Internet, objects are manipulated from collections throughout. But there is no use of java8 to operate the contents of the collection in the collection. Now share one myself. From a List<List<User>> collection, take out a certain value of the List collection object inside for your reference. If you have a better way, you are welcome to suggest it.

Example: Find the class of primary school student 1 from a nested set. The following example is the correct answer

    public static void main(String[] args) {
        // 学生列表
        List<User> studentList = Arrays.asList(
                new User("小学生1","20","一班",new ArrayList<>()),
                new User("小学生2","30","二班",new ArrayList<>()),
                new User("小学生3","40","三班",new ArrayList<>())
        );

        // 员工列表
        List<User> employeeList = Arrays.asList(
                new User("员工1","20","信息中心",new ArrayList<>()),
                new User("员工2","30","科技中心",new ArrayList<>()),
                new User("员工3","40","运营中心",new ArrayList<>())
        );

        // 仆人列表
        List<User> servantList = Arrays.asList(
                new User("仆人1","20","西房",new ArrayList<>()),
                new User("仆人2","30","南房",new ArrayList<>()),
                new User("仆人3","40","北房",new ArrayList<>())
        );
        // 总的集合
        List<User> bigList = Arrays.asList(
                // 老师下面是学生
                new User("老师","20","三年级",studentList),
                // 老板下面是工人
                new User("老板","30","广发银行",employeeList),
                // 主人下面是仆人
                new User("主人","40","东房",servantList)
        );

      // 从bigList 集合中求出小学生1所在班级
      String className =   bigList.stream()
                .filter(user -> "老师".equals(user.getName()))
                .map(User::getUserList)
                .filter(list -> list != null && list.size()>0)
                .flatMap(list -> list.stream()
                        .filter(user -> "小学生1".equals(user.getName()))
                        .map(User::getAddress))
              .collect(Collectors.joining());

         System.out.println("小学生1所在班级："+className);



    }

Improve methods to increase robustness

 

User entity

package com.test;

import java.util.ArrayList;
import java.util.List;

public class User {
    private String name;
    private String age;
    private String address;
    private List<User> userList = new ArrayList<>();

    public List<User> getUserList() {
        return userList;
    }

    public void setUserList(List<User> userList) {
        this.userList = userList;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAge() {
        return age;
    }

    public void setAge(String age) {
        this.age = age;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public User(String name, String age, String address, List<User> userList) {
        this.name = name;
        this.age = age;
        this.address = address;
        this.userList = userList;
    }
}