Hauptprogramm

class Shop {
  public static void main (String [] argv) {
    Clerk cl = new Clerk ();
    Customer [] customer = new Customer [5];
    for (int i=0; i<customer.length; i++) {
      customer[i] = 
          new Customer ("Cust" + i, cl, 5);
      new Thread (customer [i]).start ();
    }
  }
}

fehlt natürlich noch der Server (Verkäufer)!