how to store & retrieve arraylist of objects in file? [duplicate]

Possible Duplicate:
How to Serialize an ArrayLIst in java without getting errors?

I have two arraylists in two different classes. One is to store transaction objects temporarily & other for permanent. Im using a addAll method to copy objects from temporary arraylist to permanent. And then save objects from permanent arraylist to a file. When my program restarts, object in permanent arraylists are restored from the file. But Iam getting an exception. Whats wrong with my code ?

import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.util.ArrayList;

/** * * @author Haleemdeen */ public class FileImportExport {

private ArrayList<Stock> permntTransactions=new ArrayList<Stock>();


void cancatToPerrmntTransactions(ArrayList<Stock> arraylist1){
    permntTransactions.addAll(arraylist1);
}

ArrayList<Stock> displayPermntTransactions(){
    return permntTransactions;
}

    void exportToFile(){

    try{  // Catch errors in I/O if necessary.
    // Open a file to write to, named SavedObj.sav.
    FileOutputStream saveFile=new FileOutputStream("SaveObj.sav");

    // Create an ObjectOutputStream to put objects into save file.
    ObjectOutputStream save = new ObjectOutputStream(saveFile);

    // Now we do the save.
    save.writeObject(permntTransactions);

    // Close the file.
    save.close(); // This also closes saveFile.
    }
    catch(Exception exc){
    exc.printStackTrace(); // If there was an error, print the info.
    }
}

        void importFromFile(){

    try{
    // Open file to read from, named SavedObj.sav.
    FileInputStream saveFile = new FileInputStream("SaveObj.sav");

    // Create an ObjectInputStream to get objects from save file.
    ObjectInputStream save = new ObjectInputStream(saveFile);

    // Now we do the restore.
    // readObject() returns a generic Object, we cast those back
    // into their original class type.

    permntTransactions = (ArrayList<Stock>) save.readObject();

    // Close the file.
    save.close(); // This also closes saveFile.
    }
    catch(Exception exc){
    exc.printStackTrace(); // If there was an error, print the info.
    }

    // Print the values, to see that they've been recovered.
    System.out.println("\t\t" + permntTransactions);
    System.out.println();
} }

In main method :

FileImportExport file1=new FileImportExport();
file1.cancatToPerrmntTransactions(transactions);
file1.exportToFile();

This is what I get as exception:

java.io.NotSerializableException: m_media_cdstore.Stock at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156) at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326) at java.util.ArrayList.writeObject(ArrayList.java:570) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:945)