import java.awt.*;
import javax.swing.*;

public class Guruguru extends JPanel implements Runnable {
    private int r = 50;
    private volatile int x = 110, y = 70 ;
    private double theta = 0; // 角度
    private volatile Thread thread = null;

    public Guruguru() {
        setPreferredSize(new Dimension(200, 180));
        JButton startBtn = new JButton("start");
        startBtn.addActionListener(e -> startThread());
        JButton stopBtn = new JButton("stop");
        stopBtn.addActionListener(e -> stopThread());
        setLayout(new FlowLayout());
        add(startBtn); add(stopBtn);
        startThread();
    }
    
    private void startThread() {
        if (thread == null) {
            thread = new Thread(this);
            thread.start();
        }
    }
    
    private void stopThread() {
        thread = null;
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g); // スーパークラスの paintComponent を呼び出す
        // 全体を背景色で塗りつぶす。
        g.drawString("Hello, World!", x, y);
    }

    public void run() {
        Thread thisThread = Thread.currentThread();
        for (; thread == thisThread; theta += 0.02) {
            x = 60 + (int)(r * Math.cos(theta)); y = 100 - (int)(r * Math.sin(theta));
            repaint();  // paintComponent を間接的に呼出す
            try {
                Thread.sleep(30); // 30 ミリ秒お休み
            } catch (InterruptedException e) {}
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JFrame frame = new JFrame("ぐるぐる!");
            frame.add(new Guruguru());
            frame.pack();
            frame.setVisible(true);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        });
    }
}