Question

I have a JFrame on which I want to simulate a countdown (like a rocket launch). So I set up the frame by hiding various controls (setVisible(false)) and displaying a JLabel with the text (this is the text that is supposed to countdown: 3, 2, 1, Go).

The text on that JLabel starts out at “3”. My intent is to simply have the execution of the program wait for 1 second, then change the text to “2”, wait another second, change to “1”, etc.). At the end, I hide the JLabel and redisplay all of the controls and everything proceeds as normal.

What I am doing isn’t working. It seems to wait for the correct amount of time, and when it is done, my JFrame looks great and works as intended. But during the 4 seconds of the countdown method, all I see is a white JFrame. Not the 3, 2, 1 that I want.

Here is my code. Can anyone see what I am doing wrong? Thanks!

public void countdown() {
    long t0, t1;        

    myTest.hideTestButtons(true);
    myTest.repaint();

    t0 =  System.currentTimeMillis();
    do {
        t1 = System.currentTimeMillis();
    } while ( (t1 - t0) < 1000);

    myTest.TwoSeconds();
    myTest.repaint();
    t0 =  System.currentTimeMillis();
    do {
        t1 = System.currentTimeMillis();
    } while ( (t1 - t0) < 1000);


    myTest.OneSecond();
    myTest.repaint();
    t0 =  System.currentTimeMillis();
    do {
        t1 = System.currentTimeMillis();
    } while ( (t1 - t0) < 1000);


    myTest.Go();
    myTest.repaint();
    t0 =  System.currentTimeMillis();
    do {
        t1 = System.currentTimeMillis();
    } while ( (t1 - t0) < 1000);

    myTest.hideTestButtons(false);
    myTest.repaint();
}

public void TwoSeconds() {
    lblCountdown.setText("2");
}

public void OneSecond() {
    lblCountdown.setText("1");
}

public void Go() {
    lblCountdown.setText("Go!");
}

Answer 1

Use a Timer instead. An active wait is in most of the cases highly discouraged.
Here is the type of code you’ll need to integrate:

final Timer ti = new Timer(0, null);
ti.addActionListener(new ActionListener() {
    int countSeconds = 3;

    @Override
    public void actionPerformed(ActionEvent e) {
        if(countSeconds == 0) {
            lblCountdown.setText("Go");
            ti.stop();
        } else {
            lblCountdown.setText(""+countSeconds);
            countSeconds--;
        }
    }
});
ti.setDelay(1000);
ti.start();