[java绘图问题]为什么在Canvas上，无法显示图形讷？

[Forrest-Lyu]

package com.lun.part6;

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

public class RoseJFrame extends JFrame implements ActionListener {

	public RoseCanvas canvas;
	public JButton jbtn;
	public JPanel jp;

	public RoseJFrame() {
		super("四叶玫瑰线");
		Dimension dim = this.getToolkit().getScreenSize();
		this.setBounds(dim.width / 4, dim.height / 4, dim.width / 2, dim.height / 2);
		// this.setSize(new Dimension(300, 300));
		// this.setLocationRelativeTo(null);
		this.setDefaultCloseOperation(EXIT_ON_CLOSE);

		jbtn = new JButton("选择颜色");
		jp = new JPanel();
		canvas = new RoseCanvas(Color.blue);

		this.getContentPane().add(jbtn, BorderLayout.NORTH);
		jp.add(canvas);
		this.getContentPane().add(jp, BorderLayout.CENTER);
		jp.setOpaque(true);
		jbtn.addActionListener(this);

		this.setVisible(true);

	}

	class RoseCanvas extends Canvas {
		private Color color;

		public RoseCanvas(Color c) {
			this.setColor(c);
		}

		public void setColor(Color c) {
			this.color = c;
		}

		public void paint(Graphics g) {
			int x = this.getWidth() / 2;
			int y = this.getHeight() / 2;
			g.setColor(this.color);
			g.drawLine(x, 0, x, y * 2);
			g.drawLine(0, y, x * 2, y);
			for (int j = 40; j < 200; j += 20) {
				for (int i = 0; i < 1023; ++i) {
					double angle = i * Math.PI / 512;
					double radius = j * Math.sin(2 * angle);
					int x1 = (int) Math.round(radius * Math.cos(angle) * 2);
					int y1 = (int) Math.round(radius * Math.sin(angle));
					g.fillOval(x1 + x, y1 + y, 2, 2);
				}
			}
		}
	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		new RoseJFrame();
	}

	@Override
	public void actionPerformed(ActionEvent e) {
		// TODO Auto-generated method stub
		Color c = JColorChooser.showDialog(this, "选择颜色", Color.blue);
		this.canvas.setColor(c);
		this.canvas.repaint();
	}

}

我不知道，为什么显示不出自己在RoseCanvas中绘画出的图形？