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

public class Guruguru extends JPanel implements Runnable {
	private int r = 50, 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);
		});
	}
}