Why does Exception pop up?

import java.io.*; public class Hotel implements Serializable { private Room room = new Room(); public static void main(String[] args) { Hotel h = new Hotel(); try { FileOutputStream fos = new FileOutputStream("Hotel.dat"); ObjectOutputStream oos = new ObjectOutputStream(fos); oos.writeObject(h); oos.close(); } catch(Exception ex) { ex.printStackTrace(); } } } class Room { }