Java Swing Timer Countdown

I have to make a countdown program which also shows the tenths of a second; for example from a 10.0 second countdown, it should display 9.9s, 9.8s, ... 0.0s

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         

        timer.start();
        timer2.start();

}                                        


Double timeLeft=5000; //5 seconds
Timer timer=new Timer(1,countDown);
Timer timer2=new Timer(1000,countDown2);
ActionListener countDown=new ActionListener()
{
    public void actionPerformed(ActionEvent e)
    {
        timeLeft--;
        SimpleDateFormat df=new SimpleDateFormat("mm:ss:S");
        jLabel1.setText(df.format(timeLeft));
        if(timeLeft<=0)
        {
            timer.stop();
        }
    }
};

what happens is it's taking more than 5 seconds to finish the 5 seconds.

I compared the code above with another Timer

int timeLeft2=5;

ActionListener countDown2=new ActionListener()
{
    public void actionPerformed(ActionEvent e)
    {
        timeLeft2--;

        jLabel2.setText(String.valueOf(timeLeft2));
        if(timeLeft2<=0)
        {
            time2.stop();
        }                  
    }
};

is it natural that they don't get the same?

The time between ticks (the time when actionPerfomed is called) is variable, it only guarantees that it will be at least n milliseconds

Instead of relying on some counter, which could become unreliable over time, you should try and calculate the difference between the time the Timer was started and the current time, for example...

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.text.SimpleDateFormat;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class CountDown {

    public static void main(String[] args) {
        new CountDown();
    }

    public CountDown() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private Timer timer;
        private long startTime = -1;
        private long duration = 5000;

        private JLabel label;

        public TestPane() {
            setLayout(new GridBagLayout());
            timer = new Timer(10, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    if (startTime < 0) {
                        startTime = System.currentTimeMillis();
                    }
                    long now = System.currentTimeMillis();
                    long clockTime = now - startTime;
                    if (clockTime >= duration) {
                        clockTime = duration;
                        timer.stop();
                    }
                    SimpleDateFormat df = new SimpleDateFormat("mm:ss:SSS");
                    label.setText(df.format(duration - clockTime));
                }
            });
            timer.setInitialDelay(0);
            addMouseListener(new MouseAdapter() {
                @Override
                public void mouseClicked(MouseEvent e) {
                    if (!timer.isRunning()) {
                        startTime = -1;
                        timer.start();
                    }
                }
            });
            label = new JLabel("...");
            add(label);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

    }

}

Updating the label probably takes more that 1ms, which is why it can't keep up. If you only need to display tenths of a second, simply have your timer update less often.

ActionListener countDown=new ActionListener()
{
    public void actionPerformed(ActionEvent e)
    {
        timeLeft -= 100;
        SimpleDateFormat df=new SimpleDateFormat("mm:ss:S");
        jLabel1.setText(df.format(timeLeft));
        if(timeLeft<=0)
        {
            timer.stop();
        }
    }
};
Timer timer=new Timer(100, countdown);