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，有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。

程序运行结果如下：

import java.awt.*;

import java.applet.*;

/*

＜applet code="ex13_3.class" width=800 height=400 ＞

＜/applet＞

*/

public class ex13_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("clear");

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);

b