java Serializable interface has no function, why it affects "writeObject"/"readObject"

Question

I checked that the interface of

Serializable

Has no function definition, yet when I define

private void readObject(ObjectOutputStream oos){
    System.out.println("readObject!!");
}
private void writeObject(ObjectOutputStream oos){
    System.out.println("writeObject!!");
}

function in a class, they're called while object is being serialized.

This is odd to me, if the interface defines these 2 functions, then I should override them to make sure they're called.

But in Serializable, how does compiler generate code that if I define my own "writeObject"/"readObject", they're called while serialization?

I tried to append

@Override

annotation on top of both functions, compiler reports error.

So how it works at all, would you help to give some explanations?

Thanks a lot!

Answer 1

java.io.Serializable is a functional interface, so that means it doesn't define any methods in it. @Override annotation is put if only you really wanna make sure noone will try to modify your overridden method. The reason you got a compiler error on @Override is that there is no such method in Serializable, but instead you can find them in ObjectInputStream and ObjectOutputStream (which use as a low-level classes FileInputStream and FileOutputStream respectively). If you really wanna do Serialization on let's say, a list, you can do it like this:

package Chaper8.IO;

import java.io.*;

import java.util.*;

public class Serialization_Deserialization {

public static void main(String [] args){

    /*
     *  try-catch with resources, JVM makes sure to close the resources after you've finished using it
     * much easier than using finally and getting an exception for each resource closed
     * 
     */
    try(FileOutputStream out = new FileOutputStream("C:\\Users\\Andrei\\Desktop\\Exemple\\worker.txt");
        ObjectOutputStream oos = new ObjectOutputStream(out);

        FileInputStream in = new FileInputStream("C:\\Users\\Andrei\\Desktop\\Exemple\\worker.txt");
        ObjectInputStream ois = new ObjectInputStream(in);){

        //instances of the Worker class
        Worker w1 = new Worker("Worker1", 123456 , 2000.5);
        Worker w2 = new Worker("Worker2", 765436, 1500.15);
        Worker w3 = new Worker("Worker3", 364582, 1700.45);
        Worker w4 = new Worker("Worker4", 878234, 2100.34);
        ArrayList<Worker> list = new ArrayList<>();

        //just adding the persons in the list
        list.add(w1);
        list.add(w2);
        list.add(w3);
        list.add(w4);


        System.out.println("Doing serialization");
        oos.writeObject(list);

        System.out.println("Doing deserialization");
        ois.readObject();

    }catch(IOException | ClassNotFoundException e){
            e.printStackTrace();
    }
}
} 

Worker.java

/*
 *  Worker class, basic type with variables, constructor, and toString() overridden
 *  Here I have implemented Serializable for the reason that I need to make sure that
 *  I will serialize the object within the class
 *
 *  Note that I used transient for id. transient is a special keyword which makes sure 
 * that id will not be serialized, used for security reasons.
 * 
 *  serialVersionUID is another variable which is used during deserialization 
 * to verify that the sender and receiver of a serialized object have loaded 
 * classes for that object that are compatible with respect to serialization. 
 *  Throws InvalidClassException if the object has a different serialVersionUID
 * than that of the corresponding sender's class.
 *  
 */

import java.io.*;
class Worker implements Serializable{

private static final long serialVersionUID = 1L;
private String name;
private transient int id;
private double wage;

public Worker(String name, int id, double wage){
    this.name = name;
    this.id = id;
    this.wage = wage;
}
public String toString(){
    return "Person with name " +
    name + " and with id " +
    id + " has a salary of " + wage + "$";
    }
}