/**
 * deserialise and ensure fields are re-interned when read back
 *
 * @param stream ObjectInputStream of objects
 *
 * @exception IOException
 * @exception ClassNotFoundException
 */
private void readObject ( ObjectInputStream stream )
throws IOException, ClassNotFoundException
{
   stream.defaultReadObject();
   // reintern all Strings in the object
   name = name.intern();
   extension = extension.intern();
}