How can one customize the Serialization process? or What is the purpose of implementing the writeObject() and readObject() method?

When you want to store the transient variables state as a part of the serialized object at the time of serialization the class must implement the following methods –

private void wrtiteObject(ObjectOutputStream outStream)
{
//code to save the transient variables state as a part of serialized object
} private void readObject(ObjectInputStream inStream)
{
//code to read the transient variables state and assign it to the de-serialized object
}

e.g.

public class TestCustomizedSerialization implements Serializable{

    private static final long serialVersionUID =-22L;
    private String noOfSerVar;
    transient private int noOfTranVar;

    TestCustomizedSerialization(int noOfTranVar, String noOfSerVar) {
        this.noOfTranVar = noOfTranVar;
        this.noOfSerVar = noOfSerVar;
    }

    private void writeObject(ObjectOutputStream os) {

     try {
     os.defaultWriteObject();
     os.writeInt(noOfTranVar);
     } catch (Exception e) { e.printStackTrace(); }
     }

     private void readObject(ObjectInputStream is) {
     try {
     is.defaultReadObject();
     int noOfTransients = (is.readInt());
     } catch (Exception e) {
         e.printStackTrace(); }
     }

     public int getNoOfTranVar() {
        return noOfTranVar;
    }
   
}

The value of transient variable ‘noOfTranVar’ is saved as part of the serialized object manually by implementing writeObject() and restored by implementing readObject().
The normal serializable variables are saved and restored by calling defaultWriteObject() and defaultReadObject()respectively. These methods perform the normal serialization and de-sirialization process for the object to be saved or restored respectively.