Java计时器不增加

Question

I tried creating a timer in Java that displays in a JFrame window but I'm having no luck!

The timer always remains at 0. Would appreciate if anyone can see where I am going wrong :(

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

class Globals 
{
    public static int seconds = 0;
}

class main
{
    public static void main(String Args[])
    {

        JFrame testing = new JFrame();
        testing.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        testing.setSize(1000,1000);
        testing.setVisible(true);

        Countdown timer = new Countdown();
        Timer countdown = new Timer(5000, timer);
        countdown.start();

        JLabel firstline = new JLabel();
        firstline.setText("Time is:" + Globals.seconds);
        testing.add(firstline);

    }
}

class Countdown implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {
    Globals.seconds++;

    }
}

Answer 1

You set the JLabel's text here:

JLabel firstline = new JLabel();
firstline.setText("Time is:" + Globals.seconds);

But once set that's it, you never change it.

All the timer does is change the state of a variable. It does nothing to the GUI, such as tell the JLabel to use the new value. Please understand that a JLabel is not going to magically track any variable used to create its text String. All it knows is that it has been set with an immutable String, and that's it.

Your Timer should instead call setText(...) on the JLabel to update its text, or better, use a reference to the class holding the JLabel, and call a public method of that class to change the JLabel's state.

For example:

import java.awt.Dimension;
import java.awt.event.*;
import javax.swing.*;

public class TimerTest {
   private static final int TIMER_DELAY = 20;

   private static void createAndShowGui() {
      MyGui myGui = new MyGui();
      TimerListener timerListener = new TimerListener(myGui, 2 * 60 * 1000);
      new Timer(TIMER_DELAY, timerListener).start();
      JFrame frame = new JFrame("TimerTest");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(myGui.getMainComponent());
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

class MyGui {
   private static final int PREF_W = 200;
   private static final int PREF_H = 80;
   private JPanel mainPanel = new JPanel() {
      public Dimension getPreferredSize() {
         return MyGui.this.getPreferredSize();
      };
   };
   private JLabel statusLabel = new JLabel();

   public MyGui() {
      mainPanel.add(statusLabel);
   }

   public Dimension getPreferredSize() {
      return new Dimension(PREF_W, PREF_H);
   }

   public JComponent getMainComponent() {
      return mainPanel;
   }

   public void setStatus(String text) {
      statusLabel.setText(text);
   }
}

class TimerListener implements ActionListener {
   private static final String FORMAT = "Count Down: %02d:%02d:%03d";
   private static final int MS_PER_SEC = 1000;
   private static final int SEC_PER_MIN = 60;
   private MyGui myGui;
   private long countDownTime;
   private long startTime;

   public TimerListener(MyGui myGui, long countDownTime) {
      this.myGui = myGui;
      this.countDownTime = countDownTime;
      startTime = System.currentTimeMillis();
   }

   @Override
   public void actionPerformed(ActionEvent evt) {
      long currentTime = System.currentTimeMillis();
      long deltaTime = countDownTime - (currentTime - startTime);
      if (deltaTime < 0) {
         ((Timer) evt.getSource()).stop();
         deltaTime = 0;
      }
      int minutes = (int) (deltaTime / (MS_PER_SEC * SEC_PER_MIN));
      deltaTime = deltaTime % (MS_PER_SEC * SEC_PER_MIN);
      int seconds = (int) (deltaTime / MS_PER_SEC);
      deltaTime = deltaTime % MS_PER_SEC;
      int msecs = (int) deltaTime;
      String text = String.format(FORMAT, minutes, seconds, msecs);
      myGui.setStatus(text);
   }
}

Answer 2

The timer starts, waiting for 5 seconds.

During this 5 seconds, it runs this code:

JLabel firstline = new JLabel();
firstline.setText("Time is:" + Globals.seconds);
testing.add(firstline);

Then it increments your value.

Since the above code isn't in a loop or anything, it never runs it again.

Perhaps try putting the JLabel update in the timer itself:

class Countdown implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {
        Globals.seconds++;
        main.firstline.setText("Time is:" + Globals.seconds);
    }
}

Obviously you need to have access to the JLabel here, I'll leave figuring that out to you.