Question

package com.n;

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

public class S implements Serializable {

    private static final long serialVersionUID = 1L;

    transient int i;

    public static void main(String[] args) throws Exception, IOException {

        ObjectOutputStream oos = 
             new ObjectOutputStream(new FileOutputStream("c:\\jav\\f.txt"));

        S obj1 = new S(10);
        oos.writeInt(obj1.i);
        oos.writeObject(obj1);

        ObjectInputStream ois = 
             new ObjectInputStream(new FileInputStream("c:\\jav\\f.txt"));

        System.out.println("Object contains >> " + ois.readObject());

        System.out.println("Transient variable written separately yields >> i =" 
                           + ois.readInt());
    }

    public S(int i) {
        this.i = i;
    }

    @Override
    public String toString() {
        return "i= " + i;
    }

}

The code above throws

 Exception in thread "main" java.io.OptionalDataException
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at com.n.S.main(S.java:27)

but when I interchange the output lines like

 System.out.println("Transient variable written separately yields >> i =" + ois.readInt());
 System.out.println("Object contains >> " + ois.readObject());

It runs fine. But why is it so? Do I have to write as well as read the serialized primitive values first and then read or write the object? And what is an OptionalDataException?

Answer 1

You need to read data from an ObjectInputStream in exactly the same order as they were written to the ObjectOutputStream.