java 鼠标画线

Line.java文件-------驱动类
import javax.swing.JFrame;
public class Line
{
   public static void main(String[] arg)
   {
       JFrame frame=new JFrame("Draw lines");
       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       Linepanel pane=new Linepanel();
       frame.getContentPane().add(pane);
       frame.pack();
       frame.setVisible(true);
   }
}
 
Linepanel.java文件
 
 
 
 
import java.util.ArrayList;
import javax.swing.*;
import java.awt.*;
import java.awt.Point;
import java.awt.event.*;
public class Linepanel extends JPanel
{
       private Point point1=null,point2=null;
       private ArrayList<Point> pointlist;
       public Linepanel()
      {
            pointlist=new ArrayList<Point>();
            Linelistener listener=new Linelistener();
            addMouseListener(listener);
            addMouseMotionListener(listener);
            setBackground(Color.black);
            setPreferredSize(new Dimension(400,200));
       }
       public void paintComponent(Graphics page)
      {
            super.paintComponent(page);