Object Serialization with Inheritance in Java

Prerequisite: Serialization, Inheritance

Serialization is a mechanism of converting the state of an object into a byte stream. The byte array can be the class, version, and internal state of the object.

Deserialization is the reverse process where the byte stream is used to recreate the actual Java object in memory. This mechanism is used to persist the object.

There are some cases of Serialization with respect to inheritance:

Case 1: If the superclass is serializable, then subclass is automatically serializable

If the superclass is Serializable, then by default, every subclass is serializable. Hence, even though subclass doesn’t implement Serializable interface( and if its superclass implements Serializable), then we can serialize subclass object. 

Java

// Java program to demonstrate
// that if superclass is
// serializable then subclass
// is automatically serializable

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

// superclass A
// implementing Serializable interface
class A implements Serializable
{
    int i;

    // parameterized constructor
    public A(int i)
    {
        this.i = i;
    }

}

// subclass B
// B class doesn't implement Serializable
// interface.
class B extends A
{
    int j;

    // parameterized constructor
    public B(int i, int j)
    {
        super(i);
        this.j = j;
    }
}

// Driver class
public class Test
{
    public static void main(String[] args)
            throws Exception
    {
        B b1 = new B(10,20);

        System.out.println("i = " + b1.i);
        System.out.println("j = " + b1.j);

        /* Serializing B's(subclass) object */

        //Saving of object in a file
        FileOutputStream fos = new FileOutputStream("abc.ser");
        ObjectOutputStream oos = new ObjectOutputStream(fos);

        // Method for serialization of B's class object
        oos.writeObject(b1);

        // closing streams
        oos.close();
        fos.close();

        System.out.println("Object has been serialized");

        /* De-Serializing B's(subclass) object */

        // Reading the object from a file
        FileInputStream fis = new FileInputStream("abc.ser");
        ObjectInputStream ois = new ObjectInputStream(fis);

        // Method for de-serialization of B's class object
        B b2 = (B)ois.readObject();

        // closing streams
        ois.close();
        fis.close();

        System.out.println("Object has been deserialized");

        System.out.println("i = " + b2.i);
        System.out.println("j = " + b2.j);
    }
}

Output: 

i = 10
j = 20
Object has been serialized
Object has been deserialized
i = 10
j = 20

Case 2: If a superclass is not serializable, then subclass can still be serialized 

Even though the superclass doesn’t implement a Serializable interface, we can serialize subclass objects if the subclass itself implements a Serializable interface. So we can say that to serialize subclass objects, superclass need not be serializable. But what happens with the instances of superclass during serialization in this case. The following procedure explains this.

Case 2(a): What happens when a class is serializable, but its superclass is not?

Serialization: At the time of serialization, if any instance variable inherits from the non-serializable superclass, then JVM ignores the original value of that instance variable and saves the default value to the file.

De- Serialization: At the time of de-serialization, if any non-serializable superclass is present, then JVM will execute instance control flow in the superclass. To execute instance control flow in a class, JVM will always invoke the default(no-arg) constructor of that class. So every non-serializable superclass must necessarily contain a default constructor. Otherwise, we will get a runtime exception.

Java

// Java program to demonstrate
// the case if superclass need
// not to be serializable
// while serializing subclass

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

// superclass A
// A class doesn't implement Serializable
// interface.
class A {
    int i;

    // parameterized constructor
    public A(int i){
      this.i = i;
    }

    // default constructor
    // this constructor must be present
    // otherwise we will get runtime exception
    public A()
    {
        i = 50;
        System.out.println("A's class constructor called");
    }
}

// subclass B
// implementing Serializable interface
class B extends A implements Serializable {
    int j;

    // parameterized constructor
    public B(int i, int j)
    {
        super(i);
        this.j = j;
    }
}

// Driver class
public class Test {
    public static void main(String[] args) throws Exception
    {
        B b1 = new B(10, 20);

        System.out.println("i = " + b1.i);
        System.out.println("j = " + b1.j);

        // Serializing B's(subclass) object

        // Saving of object in a file
        FileOutputStream fos
            = new FileOutputStream("abc.ser");
        ObjectOutputStream oos
            = new ObjectOutputStream(fos);

        // Method for serialization of B's class object
        oos.writeObject(b1);

        // closing streams
        oos.close();
        fos.close();

        System.out.println("Object has been serialized");

        // De-Serializing B's(subclass) object

        // Reading the object from a file
        FileInputStream fis
            = new FileInputStream("abc.ser");
        ObjectInputStream ois = new ObjectInputStream(fis);

        // Method for de-serialization of B's class object
        B b2 = (B)ois.readObject();

        // closing streams
        ois.close();
        fis.close();

        System.out.println("Object has been deserialized");

        System.out.println("i = " + b2.i);
        System.out.println("j = " + b2.j);
    }
}

Output: 

i = 10
j = 20
Object has been serialized
A's class constructor called
Object has been deserialized
i = 50
j = 20

Case 3: If the superclass is serializable, but we don’t want the subclass to be serialized

There is no direct way to prevent sub-class from serialization in java. One possible way by which a programmer can achieve this is by implementing the writeObject() and readObject() methods in the subclass and needs to throw NotSerializableException from these methods. These methods are executed during serialization and de-serialization, respectively. By overriding these methods, we are just implementing our custom serialization.

Java

// Java program to demonstrate
// how to prevent
// subclass from serialization

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.NotSerializableException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

// superclass A
// implementing Serializable interface
class A implements Serializable
{
    int i;

    // parameterized constructor
    public A(int i)
    {
        this.i = i;
    }

}

// subclass B
// B class doesn't implement Serializable
// interface.
class B extends A
{
    int j;

    // parameterized constructor
    public B(int i,int j)
    {
        super(i);
        this.j = j;
    }

    // By implementing writeObject method,
    // we can prevent
    // subclass from serialization
    private void writeObject(ObjectOutputStream out) throws IOException
    {
        throw new NotSerializableException();
    }

    // By implementing readObject method,
    // we can prevent
    // subclass from de-serialization
    private void readObject(ObjectInputStream in) throws IOException
    {
        throw new NotSerializableException();
    }

}

// Driver class
public class Test
{
    public static void main(String[] args)
            throws Exception
    {
        B b1 = new B(10, 20);

        System.out.println("i = " + b1.i);
        System.out.println("j = " + b1.j);

        // Serializing B's(subclass) object

        //Saving of object in a file
        FileOutputStream fos = new FileOutputStream("abc.ser");
        ObjectOutputStream oos = new ObjectOutputStream(fos);

        // Method for serialization of B's class object
        oos.writeObject(b1);

        // closing streams
        oos.close();
        fos.close();

        System.out.println("Object has been serialized");

        // De-Serializing B's(subclass) object

        // Reading the object from a file
        FileInputStream fis = new FileInputStream("abc.ser");
        ObjectInputStream ois = new ObjectInputStream(fis);

        // Method for de-serialization of B's class object
        B b2 = (B)ois.readObject();

        // closing streams
        ois.close();
        fis.close();

        System.out.println("Object has been deserialized");

        System.out.println("i = " + b2.i);
        System.out.println("j = " + b2.j);
    }
}

Output: 

i = 10
j = 20
Exception in thread "main" java.io.NotSerializableException
    at B.writeObject(Test.java:44)

This article is contributed by Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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