Question

Only subclass has implemented Serializable interface.

import java.io.*;

public class NewClass1{

    private int i;
    NewClass1(){
    i=10;
    }
    int getVal() {
        return i;
    }
    void setVal(int i) {
        this.i=i;
    }
}

class MyClass extends NewClass1 implements Serializable{

    private String s;
    private NewClass1 n;

    MyClass(String s) {
        this.s = s;
        setVal(20);
    }

    public String toString() {
        return s + " " + getVal();
    }

    public static void main(String args[]) {
        MyClass m = new MyClass("Serial");
        try {
            ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("serial.txt"));
            oos.writeObject(m); //writing current state
            oos.flush();
            oos.close();
            System.out.print(m); // display current state object value
        } catch (IOException e) {
            System.out.print(e);
        }
        try {
            ObjectInputStream ois = new ObjectInputStream(new FileInputStream("serial.txt"));
            MyClass o = (MyClass) ois.readObject(); // reading saved object
            ois.close();
            System.out.print(o); // display saved object state
        } catch (Exception e) {
            System.out.print(e);
        }
    }
}

One thing, which I noticed here is, parent class is not serialized. Then, why didn’t it throw NotSerializableException indeed it is showing following

Output

Serial 20
Serial 10

Also, output differ from Serialization and De-serialization. I just only know, it is because of parent class has not implemented Serializable. But, If anyone explain me, what happens during object serialization and de-serialization. How it changes the value ? I’m not able to figure out, also I have used comment in my program. So, if I’m wrong at any point, please let me know.

Answer 1

according to the Serializable javadoc

During deserialization, the fields of non-serializable classes will be
initialized using the public or protected no-arg constructor of the
class. A no-arg constructor must be accessible to the subclass that is
serializable. The fields of serializable subclasses will be restored
from the stream.

also, serialization exception is only thrown if the class being serialized is not serializable. having non-serializable parents is fine (as long as they have a no-arg constructor). Object itself isnt Serializable, and everything extends it.
the quote above also explains why you get different values for the value field – the no-arg constructor for the parent class is set, which sets the value field to 10 – the field belongs to the (non-serializable) parent so its value isnt written to/read from the stream.