can anyone help me to put a countdown timer in a dialog box. i want to prompt the user when the time expires. tnx!

Use a javax.swing.Timer
When the timer expires it runs its actionPerformed method, where you can display a message or whatever you like.
See the API doc for details

Even though it is to late to post it now, here a simple countdown timer I created in my spare time. Feel free to review the code and tell me if there is anything wrong.

import java.awt.event.*;
import java.awt.*;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import javax.swing.*;
/**
 * Created by Xabush on 2/19/14.
 * Project: JavaProgramming @ 12:20 AM
 */
public class TimerClass extends JPanel implements Runnable{
    private int hours;
    private int minutes;
    private int seconds;
    private JLabel timerLabel;
    private Color originalColor;
    private Timer secTimer;
    private boolean started = false;
    private int initSec, initMin, initHr;
    private JButton startButton;
    private JButton stopButton;
    private JButton resetButton;
    private JButton addButton;
    private JButton newButton;




    public TimerClass(){
        setLayout(new BorderLayout());
        JPanel panel1 = new JPanel(new GridLayout(1,3,7,7));
        JPanel panel2 = new JPanel(new GridLayout(1,3,4,4));
        timerLabel = new JLabel("00 : 00 : 00");
        originalColor = timerLabel.getForeground();
        secTimer = new Timer(1000,new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if(started){
                    if(seconds == 0 && minutes == 0 && hours == 0){
                        timerLabel.setForeground(originalColor);
                        JOptionPane.showMessageDialog(TimerClass.this,"Time Up","Time's Up!",JOptionPane.INFORMATION_MESSAGE);
                        secTimer.stop();
                        addButton.setEnabled(true);
                    }
                    //Change the color when 5 seconds remain
                    if((seconds <= 5 && seconds > 0) && minutes == 0 && hours == 0){
                        timerLabel.setForeground(Color.RED);
                    }
                    if(seconds > 0){

                        seconds--;
                        timerLabel.setText(String.format("%02d : %02d : %02d",hours,minutes,seconds));
                    }
                    else{
                        if(minutes != 0 && hours == 0){
                            minutes--;
                            seconds = 60;
                            seconds--;
                        }
                        else if(minutes != 0 && hours != 0 ){
                            minutes--;
                            seconds = 60;
                            seconds--;
                        }
                        else if(minutes == 0 && hours != 0){
                            minutes = 60;
                            minutes--;
                            seconds = 60;
                            seconds--;
                            hours--;
                        }
                        timerLabel.setText(String.format("%02d : %02d : %02d",hours,minutes,seconds));
                    }
            }
            }
        });

        addButton = new JButton("Add");
        addButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                new TimerSpinner();

            }
        });
        //creates a new timer that runs in separate thread.
        newButton = new JButton("New...");
        newButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                ExecutorService executor = Executors.newSingleThreadExecutor();
                executor.execute(new TimerClass());
                executor.shutdown();
            }
        });
        panel1.add(timerLabel);
        panel1.add(addButton);
        panel1.add(newButton);
        startButton = new JButton("Start");
        stopButton = new JButton("Stop");
        resetButton = new JButton("Reset");
        startButton.setEnabled(false);
        stopButton.setEnabled(false);
        resetButton.setEnabled(false);
        panel2.add(startButton);
        panel2.add(stopButton);
        panel2.add(resetButton);
        startButton.addActionListener(new ButtonHandler());
        stopButton.addActionListener(new ButtonHandler());
        resetButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                timerLabel.setText(String.format("%02d : %02d : %02d",initHr,initMin,initSec));
                hours = initHr;
                minutes = initMin;
                seconds = initSec;
                secTimer.restart();

            }
        });

     add(panel1,BorderLayout.NORTH);
     add(panel2,BorderLayout.SOUTH);
    }


    class ButtonHandler implements ActionListener{
        @Override
        public void actionPerformed(ActionEvent e) {
            if(e.getSource().equals(startButton)){
                started = true;
                secTimer.start();
                addButton.setEnabled(false);

            }
            else if(e.getSource().equals(stopButton)){
                started = false;
                secTimer.stop();
                addButton.setEnabled(true);
            }
        }
    }
    class TimerSpinner extends JFrame{
        private GridBagLayout gbl;
        private GridBagConstraints gbc;
        public TimerSpinner(){
            super("Enter the Timer values");
            gbl = new GridBagLayout();
            gbc = new GridBagConstraints();

            Insets insets = new Insets(2,5,2,5);
            setLayout(gbl);
            JLabel message = new JLabel("Start counting from..");

            JLabel hoursLabel = new JLabel("Hours");
            final JSpinner hourSpinner = new JSpinner(new SpinnerNumberModel(0,0,null,1));

            JLabel minutesLabel = new JLabel("Minutes");
            final JSpinner minutesSpinner = new JSpinner(new SpinnerNumberModel(0,0,59,1));

            JLabel secondsLabel = new JLabel("Seconds");
            final JSpinner secondsSpinner = new JSpinner(new SpinnerNumberModel(0,0,59,1));

            JButton okButton = new JButton("OK");
            JButton cancelButton = new JButton("Cancel");

            //adding the components;
            gbc.fill = GridBagConstraints.BOTH;
            gbc.insets = insets;
            addComponents(message,0,0,4,1);

            addComponents(hoursLabel,1,0,2,1);
            addComponents(minutesLabel,1,2,1,1);
            addComponents(secondsLabel,1,3,1,1);

            gbc.fill = GridBagConstraints.HORIZONTAL;

            gbc.weightx = 2;
            addComponents(hourSpinner,2,0,2,1);
            gbc.weightx = 1;
            addComponents(minutesSpinner,2,2,1,1);
            addComponents(secondsSpinner,2,3,1,1);

            gbc.weightx = 0;
            addComponents(okButton,4,4,2,2);
            addComponents(cancelButton,4,6,2,2);

            setDefaultCloseOperation(DISPOSE_ON_CLOSE);
            setSize(350, 150);
            setResizable(false);
            setLocationRelativeTo(TimerClass.this);
            addButton.setEnabled(false);
            setVisible(true); 


            //adding action listeners to the buttons
            okButton.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    hours = (Integer) hourSpinner.getValue();
                    initHr = hours;
                    minutes = (Integer) minutesSpinner.getValue();
                    initMin = minutes;
                    seconds = (Integer) secondsSpinner.getValue();
                    initSec = seconds;
                    timerLabel.setText(String.format("%02d : %02d : %02d", hours, minutes, seconds));
                    timerLabel.setForeground(originalColor);
                    startButton.setEnabled(true);
                    stopButton.setEnabled(true);
                    resetButton.setEnabled(true);
                    addButton.setEnabled(true);
                    TimerSpinner.this.dispose();
                }
            });
            cancelButton.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    TimerSpinner.this.dispose();

                }
            });

        }

        public void addComponents(JComponent component, int row, int column, int width, int height){
            gbc.gridx = column;
            gbc.gridy = row;
            gbc.gridwidth = width;
            gbc.gridheight = height;
            gbl.setConstraints(component,gbc);
            add(component);
        }
    }
    public static void main(String[] args){
        JFrame mainFrame = new JFrame("Timer");
        TimerClass timerClass = new TimerClass();
        mainFrame.setSize(250,100);
        mainFrame.add(timerClass);
        mainFrame.setLocationRelativeTo(null);
        mainFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        mainFrame.setVisible(true);

    }
    public void run(){
            JFrame mainFrame = new JFrame("Timer");
            TimerClass timerClass = new TimerClass();
            mainFrame.setSize(250,100);
            mainFrame.add(timerClass);
            mainFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
            mainFrame.setVisible(true);
        }
    }
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