swing画图问题

Question

这是我编的小程序
package com.sinosoft.finance.paint;import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
/*为了达到画线的功能，我们分别implements MouseListener与MouseMotionListener.
*/
public class MenuPane extends JFrame implements MouseListener,MouseMotionListener{
     /**
 * 
 */
private static final long serialVersionUID = 1L;
int flag;//flag=1代表Mouse Moved,flag=2代表Mouse Dragged
     int x=0;
     int y=0;
     int startx,starty,endx,endy;//起始坐标与终点坐标
     public MenuPane(){
        Container contentPane=getContentPane();
        contentPane.addMouseListener(this);
        contentPane.addMouseMotionListener(this);
        setSize(300,300);
        show();
        addWindowListener(
          new WindowAdapter(){
             public void windowClosing(WindowEvent e){
                System.exit(0);
             }
              }
        );        
     } 
     /*由mousePressed(),mouseReleased()取得示拖曳的开始与结束坐标*/
     public void mousePressed(MouseEvent e){
        startx=e.getX();
        starty=e.getY();
     }
     public void mouseReleased(MouseEvent e){
        endx=e.getX();
        endy=e.getY();
     }
     public void mouseEntered(MouseEvent e){
     }     
     public void mouseExited(MouseEvent e){
     }
     public void mouseClicked(MouseEvent e){
     }
     /*mouseMoved(),mouseDragged()取得鼠标移动的每一个坐标，并调用repaint()方法*/ 
     public void mouseMoved(MouseEvent e){
       flag=1;
       x=e.getX();
       y=e.getY();
       repaint();
     }
     public void mouseDragged(MouseEvent e){
       flag=2;
       x=e.getX();
       y=e.getY();
       repaint();
     }
     public void update(Graphics g){
          g.setColor(this.getBackground());
          g.fillRect(0,0,getWidth(),getHeight());
          paint(g);
     }
     public void paint(Graphics g){
          g.setColor(Color.black);
          if (flag==1){
             g.drawString("鼠标坐标：("+x+","+y+")",10,50);
             g.drawLine(startx,starty,endx,endy);
          }
          if (flag==2){
             g.drawString("拖曳鼠标价坐标：("+x+","+y+")",10,50);
             g.drawLine(startx,starty,x,y);
          }
     }
     public static void main(String[] args){
          new MenuPane();
     }
}为什么在画图的时候 画出的线离鼠标位置有一段距离？？ 

Answer 1

应该是和鼠标的热点hotspot有关吧，至于什么是鼠标热点可参考http://topic.csdn.net/u/20080627/06/166d0be4-4ed2-417e-b026-b99b839ccb92.html

Answer 2

代码改一下，contentPane和JFrame的size是不同的，你差的那部分是frame的size减去contentPane的size的大小        /*Container contentPane=getContentPane();
        contentPane.addMouseListener(this);
        contentPane.addMouseMotionListener(this); */
        this.addMouseListener(this);
        this.addMouseMotionListener(this);