why wrote:
如果只是一句公式的感谢, 可免矣, 無謂浪費資源 (包括敝人打開帖子看的時間).
敝人希望見到的是淸晰回應: 我們的回覆是否幫助解決閣下的問題, 簡述閣下如何實際解決問題, 閣下有甚麼體會, 諸如此類.

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

public class Drawing extends Frame 
{ 
MenuBar menuBar=new MenuBar();
Menu choose=new Menu("选择图形");
MenuItem chooseLine=new MenuItem("直线"); 
MenuItem chooseRect=new MenuItem("矩形");

Point startPoint=new Point(0,0);
Point endPoint=new Point(0,0);

boolean isDrawing=false;
boolean drawLine=true;
boolean drawRect=false;

Graphics g=this.getGraphics();// 

Drawing()
{ 
choose.add(chooseLine);
choose.add(chooseRect); 
menuBar.add(choose);
this.setMenuBar(menuBar);

chooseLine.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){ 
System.out.println("line been choosed"); 
drawLine=true; drawRect=false; 
} 
}); 

chooseRect.addActionListener(new ActionListener(){ 
public void actionPerformed(ActionEvent e){ 
System.out.println("line been choosed"); 
drawLine=false; drawRect=true; 
} 
}); 

this.setBounds(0,0,400,400); 
this.addWindowListener(new WindowAdapter(){  
public void windowClosing(WindowEvent e) { 
System.out.println("main window colsed!"); 
System.exit(0);} }); 

this.addMouseListener(new MouseAdapter()
{ 
public void mousePressed(MouseEvent e){
isDrawing=true; 
startPoint=e.getPoint();
} 
public void mouseReleased(MouseEvent e){ 
isDrawing=false; 
endPoint=e.getPoint(); 
paint(g); 
System.out.println("released!"+e.getX()+"and"+e.getY());} 
}); 

this.addMouseMotionListener(new MouseMotionAdapter() { 
public void mouseDragged(MouseEvent e){ 
repaint(); 
endPoint=e.getPoint(); 
paint(g); 

System.out.println("move!"+e.getX()+"and"+e.getY()); 
} 
}); 
} 
public void paint(Graphics g)// 
{ 
if(drawLine) 
g.drawLine(startPoint.x,startPoint.y,endPoint.x,endPoint.y); 

else if(drawRect) 
{ 
if((endPoint.x-startPoint.x)>0){ 
if((endPoint.y-startPoint.y)>0) 
g.drawRect(startPoint.x,startPoint.y, 
(endPoint.x-startPoint.x),(endPoint.y-startPoint.y)); 
if((endPoint.y-startPoint.y)<=0) 
g.drawRect(startPoint.x,endPoint.y, 
(endPoint.x-startPoint.x),(startPoint.y-endPoint.y)); 
} 
if((endPoint.x-startPoint.x)<=0){ 
if((endPoint.y-startPoint.y)>0)  
g.drawRect(endPoint.x,startPoint.y, 
startPoint.x-endPoint.x,endPoint.y-startPoint.y); 
if((endPoint.y-startPoint.y)<=0)
g.drawRect(endPoint.x,endPoint.y, 
(startPoint.x-endPoint.x),(startPoint.y-endPoint.y)); 
} 
} 
} 
public static void main(String argv[]) 
{ 
Drawing app=new Drawing(); 
app.show(); 
} 

}