如何实现JFrame按钮来开始计时器？

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

public class TimerApp extends JFrame {
    private Timer timer;
    private int seconds;

    public TimerApp() {
        setTitle("计时器");
        setSize(300, 200);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        setLayout(new FlowLayout());

        JButton startButton = new JButton("开始");
        startButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if (startButton.getText().equals("开始")) {
                    startTimer();
                    startButton.setText("停止");
                } else {
                    stopTimer();
                    startButton.setText("开始");
                }
            }
        });

        add(startButton);
    }

    private void startTimer() {
        seconds = 0;
        timer = new Timer(1000, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                seconds++;
                System.out.println("已经计时：" + seconds + "秒");
            }
        });
        timer.start();
    }

    private void stopTimer() {
        timer.stop();
        System.out.println("计时结束");
    }

    public static void main(String[] args) {
        TimerApp timerApp = new TimerApp();
        timerApp.setVisible(true);
    }
}