倒数计时器不会启动

Question

我希望你能解决我的一个小问题。我的程序是一个倒数计时器程序，它是秒。所以我的问题是由于某种原因我无法使用从数据库中检索到的int值。所以在我的程序中，它将使用数据库中的int值填充一个JLabel，该值将是我的计时器的起始点，但倒数计时器将在我启动它时不会启动并且会给我一个错误

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at myproject2016.NLogin$event.actionPerformed(NLogin.java:202)

这是我的代码（请原谅我的编码，我是新手=）谢谢！）：

  package myproject2016;

import java.awt.EventQueue;

public class NLogin {

    JLabel promptLabel, timerLabel;
    int counter;
    JLabel tf;
    JButton button;
    Timer timer;
    private long startedAt;
    private JFrame frame;
    private JPanel contentPane;
    private JTextField txtUser;
    private JPasswordField txtPass;
    DataBase db = new DataBase();

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    NLogin window = new NLogin();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }


    public NLogin() {
        initialize();
    }


    private void initialize() {
        frame = new JFrame();
        frame.getContentPane().setBackground(new Color(0, 102, 102));
        frame.setBounds(100, 100, 450, 600);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().setLayout(null);

        JLabel lblNewLabel = new JLabel("Computer Laboratory Usage Login");
        lblNewLabel.setFont(new Font("Trajan Pro", Font.BOLD, 15));
        lblNewLabel.setBounds(66, 11, 320, 65);
        frame.getContentPane().add(lblNewLabel);

        JLabel lblUsername = new JLabel("Username:");
        lblUsername.setFont(new Font("Arial", Font.BOLD, 12));
        lblUsername.setBounds(107, 87, 62, 27);
        frame.getContentPane().add(lblUsername);

        JLabel lblPassword = new JLabel("Password:");
        lblPassword.setFont(new Font("Arial", Font.BOLD, 12));
        lblPassword.setBounds(107, 131, 62, 27);
        frame.getContentPane().add(lblPassword);

        txtUser = new JTextField();
        txtUser.setBounds(192, 91, 129, 20);
        frame.getContentPane().add(txtUser);
        txtUser.setColumns(10);

        final JPasswordField txtPass = new JPasswordField();
        txtPass.setColumns(10);
        txtPass.setEchoChar('*');
        txtPass.setBounds(192, 135, 129, 20);
        frame.getContentPane().add(txtPass);

        final JLabel timerLabel = new JLabel("",SwingConstants.CENTER);
        timerLabel.setForeground(Color.WHITE);
        timerLabel.setBackground(Color.BLACK);
        timerLabel.setFont(new Font("Tahoma", Font.BOLD, 25));
        timerLabel.setBounds(66, 402, 302, 111);
        frame.getContentPane().add(timerLabel);

            final JLabel tf = new JLabel("");
            tf.setFont(new Font("Tahoma", Font.BOLD, 12));
            tf.setBounds(231, 361, 78, 27);
            frame.getContentPane().add(tf);

        JButton btnNewButton = new JButton("LOGIN");
        btnNewButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {


                try {

                    String uName = txtUser.getText();
                    String pWord = txtPass.getText();

                    db.connect();
                    db.rs = db.st.executeQuery("select * from user where Username='"+uName+"' and Password='"+pWord+"'");
                    int count =0;
                    while(db.rs.next()){
                        count = count +1;
                        Login l = new Login();
                        JOptionPane.showMessageDialog(null,"Login successful");


                    }
                    if(count==1){


                         db.connect();
                         db.rs = db.st.executeQuery("select * from user where T_rem");
                        while(db.rs.next()){
                            tf.setText(String.valueOf(db.rs.getInt("T_rem")));

                        }


                    }
                    else{
                        JOptionPane.showMessageDialog(null,"User does not exist.");
                    }
                } catch (SQLException c) {

                    c.printStackTrace();
                }

            }
        });
        btnNewButton.setIcon(new ImageIcon("C:\\Users\\Carl\\Documents\\Icons\\check-icon.png"));
        btnNewButton.setFont(new Font("Stencil", Font.PLAIN, 18));
        btnNewButton.setBounds(66, 192, 123, 38);
        frame.getContentPane().add(btnNewButton);

        JLabel promptLabel = new JLabel("Time Remaining:");
        promptLabel.setFont(new Font("Tahoma", Font.BOLD, 15));
        promptLabel.setBounds(88, 361, 144, 27);
        frame.getContentPane().add(promptLabel);


        JButton btnLogout = new JButton("LOGOUT");
        btnLogout.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                 String sbutt = e.getActionCommand();

                 if(sbutt.equals("LOGOUT")) {

                        timer.stop();

                        Toolkit.getDefaultToolkit().beep();

                 }
            }
        });

        btnLogout.setFont(new Font("Stencil", Font.BOLD, 16));
        btnLogout.setBounds(257, 191, 123, 38);
        frame.getContentPane().add(btnLogout);


        JButton btnNewButton_1 = new JButton("Start");
        btnNewButton_1.setFont(new Font("Stencil", Font.PLAIN, 11));
        btnNewButton_1.setBounds(174, 300, 89, 23);
        frame.getContentPane().add(btnNewButton_1);

          event e =  new event();
          btnNewButton_1.addActionListener(e);

    }

         public void actionPerformed(ActionEvent e) {
                throw new UnsupportedOperationException("Not supported yet.");
            }
            public void run() {
                throw new UnsupportedOperationException("Not supported yet.");
            }
            public class event implements ActionListener {
                public void actionPerformed(ActionEvent e) {
                    int count = (int)(Double.parseDouble(tf.getText()));
                    timerLabel.setText("Time left: " + count);
                    TimeClass tc = new TimeClass(count);
                    timer = new Timer(1000, tc);
                    timer.start();
                }
            }

            public class TimeClass implements ActionListener {
                int counter;
                public TimeClass(int counter) {
                    this.counter= counter;
           }
                public void actionPerformed(ActionEvent tc) {
                    counter--;
                    if(counter >= 1) {
                        timerLabel.setText("Time left: " + counter);
                    }
                    else {
                        timer.stop();
                        timerLabel.setText("Done!");
                        Toolkit.getDefaultToolkit().beep();
                    }
                }
            }
        }