综合应用题下面是一个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.*;/* */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);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; } }ex13_3.html
参考答案：g.clearRect(0,0,getSize().width,getSize().height)bStyle = falsebStyle = true
解析：本题主要考查Applet图形界面编程与鼠标事件相结合进行简单的图形绘制。解题关键是熟悉Applet中的事件机制，会利用鼠标事件进行简单的矩形绘制，通过mouseUp(), mouseDown()来获取鼠标移动的坐标，熟悉CheckboxGroup类Checkbox类的用法，通过Checkbox来控制矩形是否填充。本题中，第1个空，对象g的方法clearRect()的最后两个参数应该对调因为第3个参数是代表矩形的宽度，第四个是高度；第2个空，点击第1个Checkbox是不用填充，这里bStyle=false是表示用填充；第3个空，解释和第2个空的相似，正好相反。