public void guardar (Jugador j) throws IOException {
		ganadores.add(j);
		File f= new File("jugador7.dat");
		 if( f.exists()){
			MiObjectOutputStream salida= new MiObjectOutputStream(new FileOutputStream(f));
			salida.writeObject(ganadores);
			salida.close();
		} else {
		ObjectOutputStream salida= new ObjectOutputStream(new FileOutputStream("jugador9.dat"));
		salida.writeObject(ganadores);
		salida.close();
 
		 }
 
 
 
y asi los leo:
 
public String recuperar() throws FileNotFoundException, IOException, EOFException, ClassNotFoundException, StreamCorruptedException {
		String ganadoress="";
		//ObjectInputStream entrada=null;
		System.out.println("antes");
		int i=0;
 
		try {
		ObjectInputStream entrada = new ObjectInputStream(new FileInputStream("jugador9.dat"));
 
		System.out.println("Entro");
	     ganadores = (ArrayList<Jugador>) entrada.readObject();
	     entrada.close();
	   try {
	    	while (true) {
	    		System.out.println(ganadores.get(i).getNombre());
	    		i++;
	    		ganadores = (ArrayList<Jugador>) entrada.readObject();
 
	    	}
 
 
			return ganadoress;
 
	}
 
 
Clase miObjectOutputStream:
 
public class MiObjectOutputStream extends ObjectOutputStream{
 
	@Override
	protected void writeStreamHeader() throws IOException  {
		//nada
	}
 
	public MiObjectOutputStream() throws IOException {
		super();
	}
 
	public MiObjectOutputStream (OutputStream fileOutputStream) throws IOException  {
		// TODO Auto-generated constructor stub
		super(fileOutputStream);
	}