综合应用题下面是一个Applet程序，其功能是在绘图区域中通过鼠标的移动来绘制直线，

题目：

综合应用题下面是一个Applet程序，其功能是在绘图区域中通过鼠标的移动来绘制直线，并且有清除绘图区域按钮，用来清除已经绘制的图像。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。程序运行结果如下：

注意：不改动程序的结构，不得增行或删行。import java.awt.*;import java.applet.*;/* */public class ex1_3 extends Applet{ private Button btn; private boolean bDraw, bClear; private int upX, upY,downX, downY; public void init(){setLayout(null);bClear = false; bDraw = false;btn = new Button("clear");btn.reshape(250, 150, 70, 30);add(btn); } public void paint(Graphics g){if(bClear){g.clearRect(0, 0, getSize().width, getSize().height);bClear = false; }if(bDraw){g.drawLine(upY, upX, downY, downX);bDraw = false; } } public void update(Graphics g){ paint(g); } public boolean mouseDown(Event event, int x, int y){ downX = x; downY = y; return true; } public boolean mouseUp(Event event, int x, int y){ upX = x; upY = y; bDraw = false;repaint(); return true; }public boolean action(Event event, Object object){ if (event.target != clear){bClear = true;repaint(); }return true; }}ex1_3.html