为什么控件没显示，请高手指教

Question

package listener;

import java.awt.*;

import javax.swing.*;

public class ControCicle1 extends JFrame{
private JButton jbtEnlarge=new JButton("large");
private JButton jbtShrink=new JButton("shrink");
private CirclePanel canvas=new CirclePanel();

public ControCicle1(){
    JPanel panel =new JPanel();
    panel.add(jbtEnlarge);
    panel.add(jbtShrink);

    this.add(canvas, BorderLayout.CENTER);
    this.add(panel, BorderLayout.SOUTH);    

}

public static void main(String[] args){
    JFrame frame=new JFrame();
    frame.setSize(500, 500);
    frame.setTitle("ControCicle1");     
    frame.setLocationRelativeTo(null);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
}

}

可以运行，但是所有的控件都显示不出来，求高手指教