用java代码实现部门表，用户表的对应关系，把用户放到对应的部门下面

实现如下所示： 

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * @author 王立朝
 * @date 2023/7/19
 * @description:
 */
public class Test2 {
    public static void main(String[] args) {
        // 创建用户列表和部门列表
        List<User> userList = createUserList();
        List<Department> departmentList = createDepartmentList();

        // 创建一个映射，将部门ID和部门对象关联起来
        Map<Integer, Department> departmentMap = createDepartmentMap(departmentList);

        // 将用户放到对应部门下
        for (User user : userList) {
            int departmentId = user.getDepartmentId();
            if (departmentMap.containsKey(departmentId)) {
                Department department = departmentMap.get(departmentId);
                department.addUser(user);
            }
        }

        // 打印部门及其所属用户
        for (Department department : departmentList) {
            System.out.println("部门名称：" + department.getName());
            System.out.println("所属用户：");
            for (User user : department.getUsers()) {
                System.out.println(user.getName());
            }
            System.out.println("-------------------");
        }
    }

    // 创建用户列表
    private static List<User> createUserList() {
        List<User> userList = new ArrayList<>();
        userList.add(new User("User1", 1));
        userList.add(new User("User2", 2));
        userList.add(new User("User3", 1));
        userList.add(new User("User4", 3));
        userList.add(new User("User5", 2));
        return userList;
    }

    // 创建部门列表
    private static List<Department> createDepartmentList() {
        List<Department> departmentList = new ArrayList<>();
        departmentList.add(new Department(1, "部门A"));
        departmentList.add(new Department(2, "部门B"));
        departmentList.add(new Department(3, "部门C"));
        return departmentList;
    }

    // 创建部门ID与部门对象的映射
    private static Map<Integer, Department> createDepartmentMap(List<Department> departmentList) {
        Map<Integer, Department> departmentMap = new HashMap<>();
        for (Department department : departmentList) {
            departmentMap.put(department.getId(), department);
        }
        return departmentMap;
    }
}

// 用户类
class User {
    private String name;
    private int departmentId;

    public User(String name, int departmentId) {
        this.name = name;
        this.departmentId = departmentId;
    }

    public String getName() {
        return name;
    }

    public int getDepartmentId() {
        return departmentId;
    }
}

// 部门类
class Department {
    private int id;
    private String name;
    private List<User> users;

    public Department(int id, String name) {
        this.id = id;
        this.name = name;
        this.users = new ArrayList<>();
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public List<User> getUsers() {
        return users;
    }

    public void addUser(User user) {
        users.add(user);
    }
}

效果如下： 

部门名称：部门A
所属用户：
User1
User3
-------------------
部门名称：部门B
所属用户：
User2
User5
-------------------
部门名称：部门C
所属用户：
User4
-------------------