Serializing problems. Help appreciated.

I am modelling the universe for a Java game. I have it working with JBDC and
Access tables but I want to make it far more simple (and make it harder for
people to hack the tables and cheat)
I decided to use serialization to hande the Unvirse Object but all I get is
a 38 byte file.

The SaveGame class eventually will handle all the loading and saving of the
game using serialization.

Can anybody tell me where I am going wrong? I have checked and the Universe
object is full of data but it does not serialize to a file.

Many thanks

Andy

Further details below:-

The Universe object comprises an arraylist of Galaxy objects.
Galaxy comprises a String name and an arraylist of SolarSystem objects
SolarSystem Objects comprise a String name and an arraylist of
PlanetaryBodies objects
PlanetaryBodies objects comprise a collection of primitives, Strings etc

All objects have been declared similar to "public class Galaxy implements
Serializable{"

The SaveGame Class is below:-

import java.io.ObjectOutputStream;
import java.io.FileOutputStream;

import java.io.IOException;

public class SaveGame{

public SaveGame(){

String filename="SaveGame1.gam";

Universe uni;

FileOutputStream fos = null;

ObjectOutputStream out = null;

try

{

uni=new Universe();

fos = new FileOutputStream(filename);

out = new ObjectOutputStream(fos);

out.writeObject(uni);

out.close();

}

catch(IOException ex)

{

ex.printStackTrace();

}

}

}