im developing a app in Android. in this project im using a List of Objects and i need to convert the list object in byte array. but there is a null pointer exception shown in logcat? here is my coading.

class MyContact
{
    String Name;
    Bitmap Image;
    String Number;

    public MyContact(String cName, String cNumber, Bitmap cImage) {

        Name    = cName;
        Number  = cNumber;
        Image   = cImage;
    }

    public String getName() {
        return Name;
    }

    public void setName(String name) {
        Name = name;
    }

    public Bitmap getImage() {
        return Image;
    }

    public void setImage(Bitmap image) {
        Image = image;
    }

    public String getNumber() {
        return Number;
    }

    public void setNumber(String number) {
        Number = number;
    }
}



List<MyContact> list = new ArrayList<MyContact>();

I added some values to the List object, using

MyContact contactObj = new MyContact(name, phoneNo, photo);
 list.add(contactObj);

bla bla bla ....

and i need store the entire list object into the Android Mobiles local file. i used ,

FileOutputStream fos = openFileOutput( fileName, Context.MODE_PRIVATE);
 fos.write(toByteArray(MyTrackList));   // null pointer error occured here
 fos.close();

here is a byte array conversion coding.

public static byte[] toByteArray (Object obj)
    {
      byte[] bytes = null;
      ByteArrayOutputStream bos = new ByteArrayOutputStream();
      try {
        ObjectOutputStream oos = new ObjectOutputStream(bos); 
        oos.writeObject(obj);
        oos.flush(); 
        oos.close(); 
        bos.close();
        bytes = bos.toByteArray ();
      }
      catch (IOException ex) {
        //TODO: Handle the exception
      }
      return bytes;
    }

but returns null pointer exception in the line of above i marked in comment how can i resolve this problem. any suggestion? thanks in advance....