刚学GUI.实验中遇到问题.

[sy9988111]

做了个鼠标画线的实验,可是上面的菜单栏显示不出来,为什么
代码如下:
package event;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Try extends JPanel
{
private int x=-1,y=-1;


public Try()
{
this.setLayout(new BorderLayout(5,5));
JPanel p1 = new JPanel(new FlowLayout());
JPanel p2 = new JPanel(new FlowLayout());


p1.addMouseMotionListener(
new MouseMotionAdapter()
{
public void mouseDragged(MouseEvent event)
{
x=event.getX();
y=event.getY();
Graphics g=getGraphics();
paint(g);
}
}

);

this.add(p1,"Center");
JMenu m1 = new JMenu("编辑");
JMenu m2 = new JMenu("查看");
JMenu m3 = new JMenu("帮助");

JMenuBar bar = new JMenuBar();
bar.add(m1);
bar.add(m2);
bar.add(m3);
p2.add(bar);
this.add(p2,"North");
}

public static void main(String args[])
{
Try a = new Try();
JFrame application = new JFrame();

application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
application.add(a);
application.setSize(400,300);
application.setVisible(true);


}
public void paint(Graphics g)
{
g.fillOval(x,y+40,5,5);

}


}

[a_nuo]

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

public class Try extends JPanel
{
    private int x=-1,y=-1;
    

    public Try() 
    {
        this.setLayout(new BorderLayout(5,5));
        JPanel p1 = new JPanel(new FlowLayout());
        JPanel p2 = new JPanel(new FlowLayout());
        
        
        p1.addMouseMotionListener(
                new MouseMotionAdapter()
        {
            public void mouseDragged(MouseEvent event)
            {
                x=event.getX();
                y=event.getY();
                Graphics g=getGraphics(); 
                paint(g); 
            }
        }

        );
        
        this.add(p1,"Center");
        JMenu m1 = new JMenu("编辑");
        JMenu m2 = new JMenu("查看");
        JMenu m3 = new JMenu("帮助");
        
        JMenuBar bar = new JMenuBar();
        bar.add(m1);
        bar.add(m2);
        bar.add(m3);
        p2.add(bar);
        this.add(p2,"North");
    }

    public static void main(String args[])
    {
        Try a = new Try();
        JFrame application = new JFrame();    
        
        application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        application.add(a);
        application.setSize(400,300);
        application.setVisible(true);
        
          
    }
    public void paint(Graphics g)
    {
    	super.paint(g);//增加此句
        g.fillOval(x,y+40,5,5);
             
    }
        
    
} 

[a_nuo]

需要调用父类的paint方法
你最好找一本好的书看看Swing的编程