为什么这个程序画了直线后,如果要想多画几条直线,怎样才能保留原先画的直线?
请指教!!
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

class pet extends Frame
{
    int x1,y1,x2,y2;
       public pet(String str)
       {
           super(str);
       } 
       public void put()
       {
           addWindowListener(new WindowAdapter()
          {
                public void windowClosing(WindowEvent e)
                {
                    System.exit(0);
                }
          });
          addMouseListener(new MouseAdapter()
          {
                public void mousePressed(MouseEvent e)
                {
                    x1=e.getX();
                    y1=e.getY();
                }
                public void mouseReleased(MouseEvent e)
                    {
                      x2=e.getX();
                      y2=e.getY();
                    }
          });
                 man m=new man();
                 add(m);
       }
      public static void main(String[] args)
      {
          pet p=new pet("nihao");
          p.setSize(600,400);
                 p.setLocation(180,180);
          p.put();
          p.setVisible(true);
      }
          class man extends JPanel
          { 
          public void paintComponent(Graphics g)
            {
                super.paintComponent(g);
             g.drawLine(x1,y1,x2,y2);
             repaint();
              }
          }
}