Writing to a file using Serialization (using writeObject method) in Java

Question

In the following code i am trying to write a serialized object to a sequential access file. This code uses another class "Account" contained in a package which implements "Serializable" interface and contains 4 instance variables viz. acc_no, first, last and balance for maintaining record of an individual along with his first name, last name, account no and balance. Now the problem i am facing is that when i add 2 or more records i am able to write only the first record two or more times and the second record and successive is not saved i.e for the following entry when the code is executed :

Enter in the following order :
Account number, first name, last name, balance : 100 russel crowe 1000000
200 tom hanks 3000000
300 will smith 4000000

When i read the above data from the file by deserializing (code not mentioned here) it following output is observed :

100 russel crowe 1000000.0
100 russel crowe 1000000.0
100 russel crowe 1000000.0

However, when i create a new Account object for every record entry i get the expected output.

So, my question is why is that "record" object with "writeObject(record)" method retains only the first entry? And why is it always necessary to create a new Account object to obtain expected output? Sorry, my doubt has gone a bit lengthy. Hope i have successfully conveyed what i wanted to ask.

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.util.NoSuchElementException;
import java.util.Scanner;

import packtest.account.Account;
public class SerizbleTest {

    private static ObjectOutputStream obj;
    public static void main(String[] args) throws IOException {
        // TODO Auto-generated method stub
        Scanner in = new Scanner(System.in);
        Account record = new Account();
        System.out.printf("Enter in the following order :%nAccount number, first name, last name, balance : ");
        try {
            obj = new ObjectOutputStream(new FileOutputStream("sertest1.ser"));
            while (in.hasNext()) {
                //record = new Account();
                record.setAccount(in.nextInt());
                record.setFirst(in.next());
                record.setLast(in.next());
                record.setBalance(in.nextDouble());
                //System.out.println(record.getAccount());
                obj.writeObject(record);
            }
        }
        catch (NoSuchElementException e) {
            System.out.println("Invalid input entered !!!");
            in.nextLine(); //discarding the input
        }
        catch (FileNotFoundException e) {
            System.out.println("File Does not Exist");
            in.close();
            //obj.close();
            System.exit(1);
        }
        catch (IOException e) {
            System.out.println("IOException occurred !!!");
        }
    }

}

tddmonkey · Accepted Answer · 2014-12-26 20:41:55Z

1

Java Serialization is designed for serializing graphs of objects. With each object it records an id, and if you write the same object twice, it just records that you wrote that object again. This is why they are all the same as the first object, as a given object is only written once.

Two solutions;

write a new object each time.
call obj.reset(); between writes or use obj.writeUnshared(record); which writes an "unshared" object to the ObjectOutputStream. This method is identical to writeObject, except that it always writes the given object as a new, unique object in the stream (as opposed to a back-reference pointing to a previously serialized instance).

edited Dec 26, 2014 at 20:41

tddmonkey

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answered Dec 26, 2014 at 20:34

Peter Lawrey

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