JAVA SERVER/CLIENT PROBLEM

public class ServerThread extends Thread {
    Socket socket = null;
    int num;
    public ServerThread(Socket socket){
        this.socket = socket;
    }
    public void run(){
        try {
            switch(num){
                case 1:
                    DataInputStream dataInputStream= new DataInputStream(socket.getInputStream()); 
                    ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());
                    String id = dataInputStream.readUTF();
                    objectOutputStream.writeObject(login(id));
                    socket.close();
                    break;
                case 2:
                    ObjectInputStream objectInputStream= new ObjectInputStream(socket.getInputStream()); 
                    DataOutputStream dataOutputStream = new DataOutputStream(socket.getOutputStream());
                    register((Account) objectInputStream.readObject());
                    dataOutputStream.writeUTF("Account Added.");
                    socket.close();
                    break;
            }
        } catch (IOException ex) {
            ex.printStackTrace();
        } catch (ClassNotFoundException ex) {
            ex.printStackTrace();
        }
    }
    
    
    private Account login(String id){
        AccountDao accountDao = new AccountDaoImpl();
        Account login = accountDao.getAccount(id);
        return login;
    }
    
    private void register(Account account){
        AccountDao accountDao = new AccountDaoImpl();
        accountDao.addAccount(account);
    }

public class Client {
    
    private Account account;

    public Account getAccount() {
        return account;
    }

    public void setAccount(Account account) {
        this.account = account;
    }
    
    public Boolean connect(String studNo){
        Boolean flag= false;
        try{
            Socket socket = new Socket("localhost",7723);
            DataOutputStream dataOutputStream = new DataOutputStream(socket.getOutputStream());
            ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
            dataOutputStream.writeUTF(studNo);
            account = (Account)objectInputStream.readObject();
            System.out.println(account.toString());
            socket.close();
            
            if(account == null){
                flag = false;
            }else{
                flag = true;
            }
        }catch(Exception ex){
            System.out.println(ex.toString());
        }
        return flag;
    }
    
    public void register(Account regAcc){
        try{
            Socket socket = new Socket("localhost",7723);
            ObjectOutputStream objectOutputStream = new ObjectOutputStream(socket.getOutputStream());
            ObjectInputStream objectInputStream = new ObjectInputStream(socket.getInputStream());
            objectOutputStream.writeObject(regAcc);
            account = (Account)objectInputStream.readObject();
            System.out.println(account.toString());
            socket.close();
        }catch(Exception ex){
            System.out.println(ex.toString());
        }
    }