下面是一个Applet程序，实现实心或者空心矩形的绘制。要求鼠标在Applet窗口中拖动实现矩形的绘制，可以选择填充或者空心，按钮用来清空窗口中的图像。请改正程序中的错误(有下划线的语句)，使程序能输出正确的结果。
注意：不改动程序的结构，不得增行或删行。
源程序代码文件清单如下：
import java.awt.*;
import java.applet.*;
/*
＜applet code="ex16_3.class" width=800 height=400＞
＜/applet＞
*/
public class ex16_3 extends Applet

private Button btnClear;
private CheckboxGroup cbg;
private Checkbox chk1, chk2;
private int upX, upY;
private int downX, downY;
private boolean bDraw, bClear, bStyle;
public void init()

bDraw = false;
bClear = false;
bStyle = false;
setLayout(null);
cbg = new CheckboxGroup();
chk1 = new Checkbox("Hollow", cbg, true);
chk2 = new Checkbox("Filled", cbg, false);
chk1.reshape(80,getSize().height-80,80,25);
chk2.reshape(160,getSize().height-80,80,25);
add(chk1);
add(chk2);
btnClear = new Button("ctear");
btnClear.reshape(240,getSize().height-80,80,25);
add(btnClear);

public void paint(Graphics g)

if (bDraw)

if(upX＞downX && upY＞downY)

if(bStyle)
g.fillRect(downX, downY, upX-downX, upY-downY);
else
g.drawRect(downX, downY, upX-downX, upY-downY);

else if(upX＞downX&&upY＜downY)

if(bStyle)
g.fillRect(downX, upY, upX-downX, downY-upY);
else
g.drawRect(downX, upY, upX - downX, downY - upY);

else if(upX＜downX && upY＞downY)

if(bStyle)
g.fillRect(upX, downY, downX-upX, upY-downY);
else
g.drawRect(upX, downY, downX-upX, upY-downY);

else if(upX＜downX && upY＜downY)

if(bStyle)
g.fillRect(upX, upY, downX-upX, downY-upY);
else
g.drawRect(upX, upY, downX-upX, downY-upY);

bDraw = false;

if (bClear)

g.clearRect(0,0, getSize().height, getSize().width);
bClear = false;


public void update(Graphics g)

paint(g);

public boolean mouseUp(Event e, int x, int y)

upX=x;
upY=y;
bDraw=true;
repaint();
return true;

public boolean mouseDown(Event e, int x, int y)

downX=x;
downY=y;
return true;

public boolean action(Event e, Object o)

if(e.target==btnClear)

bClear=true;
repaint();

if(chk1.getState()==true)

bStyle=true;

if(chk2.getState()==true)

bStyle=false;

return true;


ex9_3.html
＜HTML＞
＜HEAD＞
＜TITLE＞ex9_3＜/TITLE＞
＜/HEAD＞
＜BODY＞
＜applet code="ex9_3.class" width=800 height=400＞
＜/applet＞
＜/BODY＞
＜/HTML＞