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＞