2.13

进项一个简单的增删改查代码演示：

public class User {
    private int id;
    private String name;
    private int age;

    // 省略构造函数和getter、setter方法
}

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

public class UserDao {
    private List<User> users = new ArrayList<>();

    // 创建用户
    public void addUser(User user) {
        users.add(user);
    }

    // 根据id查询用户
    public User getUserById(int id) {
        for (User user : users) {
            if (user.getId() == id) {
                return user;
            }
        }
        return null;
    }

    // 更新用户信息
    public void updateUser(User newUser) {
        for (User user : users) {
            if (user.getId() == newUser.getId()) {
                user.setName(newUser.getName());
                user.setAge(newUser.getAge());
                break;
            }
        }
    }

    // 删除用户
    public void deleteUser(int id) {
        users.removeIf(user -> user.getId() == id);
    }
}

public class Main {
    public static void main(String[] args) {
        UserDao userDao = new UserDao();

        // 添加用户
        User user1 = new User(1, "Alice", 25);
        userDao.addUser(user1);

        // 查询用户
        User user = userDao.getUserById(1);
        System.out.println("查询到用户：" + user.getName() + ", " + user.getAge());

        // 修改用户信息
        user.setAge(26);
        userDao.updateUser(user);

        // 再次查询用户
        User updatedUser = userDao.getUserById(1);
        System.out.println("更新后的用户信息：" + updatedUser.getName() + ", " + updatedUser.getAge());

        // 删除用户
        userDao.deleteUser(1);

        // 查询已删除用户
        User deletedUser = userDao.getUserById(1);
        if (deletedUser == null) {
            System.out.println("用户已删除");
        }
    }
}