Develop a Java program that creates a GUI application for a stopwatch usingSwing. The stopwatch should have start, stop, and reset buttons, and displaythe elapsed time.

Sure, here is a simple implementation of a stopwatch using Java Swing:

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

public class Stopwatch extends JFrame {
    private long startTime = -1;
    private long duration = 0;

    private final JLabel timeLabel = new JLabel();
    private final JButton startButton = new JButton("Start");
    private final JButton stopButton = new JButton("Stop");
    private final JButton resetButton = new JButton("Reset");

    public Stopwatch() {
        super("Stopwatch");
        setSize(300, 200);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        timeLabel.setFont(timeLabel.getFont().deriveFont(40f));
        updateDisplay();

        startButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                startTime = System.currentTimeMillis();
                updateDisplay();
            }
        });

        stopButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                duration += System.currentTimeMillis() - startTime;
                startTime = -1;
                updateDisplay();
            }
        });

        resetButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                startTime = -1;
                duration = 0;
                updateDisplay();
            }
        });

        JPanel contentPane = (JPanel) getContentPane();
        contentPane.setLayout(new GridLayout(2, 2, 5, 5));
        contentPane.add(timeLabel);
        contentPane.add(startButton);
        contentPane.add(stopButton);
        contentPane.add(resetButton);
    }

    private void updateDisplay() {
        String display;
        if (startTime < 0) {
            // stopwatch is stopped
            display = formatTime(duration);
        } else {
            // stopwatch is running
            long now = System.currentTimeMillis();
            long clockTime = now - startTime + duration;
            display = formatTime(clockTime);
        }
        timeLabel.setText(display);
    }

    private String formatTime(long millis) {
        int seconds = (int) (millis / 1000);
        int minutes = seconds / 60;
        int hours = minutes / 60;
        return String.format("%02d:%02d:%02d", hours, minutes % 60, seconds % 60);
    }

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

This program creates a simple stopwatch with start, stop, and reset buttons. The elapsed time is displayed in the format "HH:MM:SS". The startButton starts the stopwatch, the stopButton stops the stopwatch, and the resetButton resets the stopwatch. The updateDisplay method updates the time displayed on the stopwatch. The formatTime method formats the time in milliseconds to the "HH:MM:SS" format.