如何实现JFrame按钮来开始计时器？

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

public class TimerExample {
    private JFrame frame;
    private JButton startButton;
    private JLabel timerLabel;
    private Timer timer;
    private int seconds = 0;

    public TimerExample() {
        frame = new JFrame("Timer Example");
        frame.setSize(300, 200);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(null);

        startButton = new JButton("Start Timer");
        startButton.setBounds(100, 50, 100, 30);
        frame.add(startButton);

        timerLabel = new JLabel("0 seconds");
        timerLabel.setBounds(100, 100, 100, 30);
        frame.add(timerLabel);

        startButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                startTimer();
            }
        });

        frame.setVisible(true);
    }

    private void startTimer() {
        if (timer == null || !timer.isRunning()) {
            timer = new Timer(1000, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    seconds++;
                    timerLabel.setText(seconds + " seconds");
                }
            });
            timer.start();
        }
    }

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