一个画图程序,帮忙看看

[yzhxhwt]

import java.awt.*;
import java.awt.event.*;

class TestDrawLine extends Frame
{
TestDrawLine(String title)
{
super(title);
setSize(400,300);
MyPanel mp=new MyPanel();
add(mp);
}
public static void main(String args[])
{
TestDrawLine tf=new TestDrawLine("Test Draw Line");
tf.setVisible(true);
}
}

class MyPanel extends Panel implements MouseListener, MouseMotionListener
{
Point pS,pE;
boolean btnPressed;

MyPanel()
{
pS=new Point(0,0);
pE=new Point(0,0);
btnPressed=false;
addMouseListener(this);
addMouseMotionListener(this);
}

public void mouseClicked(MouseEvent e) {
}

public void mousePressed(MouseEvent e) {
if(e.getButton()==MouseEvent.BUTTON1)
{
btnPressed=true;
pS.x=e.getX();
pS.y=e.getY();
pE.x=e.getX();
pE.y=e.getY();
}
}

public void mouseReleased(MouseEvent e) {
btnPressed=false;
}

public void mouseEntered(MouseEvent e) {
}

public void mouseExited(MouseEvent e) {
}

public void mouseDragged(MouseEvent e) {
if(btnPressed)
{
pS.x=pE.x;
pS.y=pE.y;
pE.x=e.getX();
pE.y=e.getY();
repaint();
}
}

public void mouseMoved(MouseEvent e) {
}

public void update(Graphics g)
{
g.drawLine(pS.x,pS.y,pE.x,pE.y);
}
}

程序没任何问题:

这一段:
public void mouseDragged(MouseEvent e) {
if(btnPressed)
{
pS.x=pE.x;
pS.y=pE.y;
pE.x=e.getX();
pE.y=e.getY();
repaint();
当鼠标松开的时候,把pE.x赋给pS.x,就是把终点的坐标赋给起点的坐标,那不是把起点的坐标覆盖了吗?最后出来的只是一个点?这边我不太理解,请大家帮忙解释一下!

[EricHxy]

public void mousePressed(MouseEvent e)
在鼠标按下的时候，记下起点和终点

[EricHxy]

public void mouseDragged(MouseEvent e)
记下鼠标新的位置到pE
老的位置在pS

[EricHxy]

要记住老的位置是变化的，新的位置总是鼠标当前位置
这样，最终效果是轨迹

[yzhxhwt]

谢谢这一段
pS.x=pE.x;
pS.y=pE.y;
pE.x=e.getX();
pE.y=e.getY();
是记住从总点到起点的轨迹??

分别用注释写给我看看好吗?