List <manager>managerL=null;
            manager NEW=new manager(nID,nName,nPW);
            managerL.add(NEW);
            try{
                FileOutputStream fOut = new FileOutputStream("manager.ser");
                ObjectOutputStream out = new ObjectOutputStream(fOut);
                out.writeObject(managerL); //write NEW into manager.ser
                out.close();
                fOut.close();
                System.out.println("new user is able to login, try and see~");
            }catch(IOException ex){
                ex.printStackTrace();

            }