Как сериализовать объект java

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Map;

public class App {
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        class Student implements Serializable {
            String name;
            int age;
            Map<String, String> tel;

            public Student(String name, int age, Map<String, String> tel) {
                this.name = name;
                this.age = age;
                this.tel = tel;
            }

            @Override
            public String toString() {
                return "Student{" +
                        "name='" + name + '\'' +
                        ", age=" + age +
                        ", tel=" + tel +
                        '}';
            }
        }
        Student student = new Student("Ivan", 25, Map.of("mob", "123-45-67"));

        FileOutputStream fileOutputStream = new FileOutputStream("file.txt");
        ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);
        objectOutputStream.writeObject(student);
        objectOutputStream.close();

        // Десериализуем объект и выведем его для проверки
        FileInputStream fileInputStream = new FileInputStream("file.txt");
        ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);
        Student student1 = (Student) objectInputStream.readObject();
        System.out.println(student1); // => Student{name='Ivan', age=25, tel={mob=123-45-67}}
        objectInputStream.close();
    }
}