请完成下列Java程序:生成一个窗口,包含一个长度为100的横向滚动条,实现对这个滚动条状态的监视,计算出滚动条的位置在整个长度的百分比,并通过画布将该值输出在滚动条上方的区域中。 注意:请勿改动main()主方法和其他已有语句内容,仅在下划线处填入适当的语句。 程序运行结果如下: import java.awt.*; import java.awt.event.*; public class ex19_2 extends Frame. implements AdjustmentListener { private Scrollbar sb; private msgWnd mw; private int val; public static void main(String[] arg) { ex19_2 obj19_2=new ex19_2(); } public ex19_2() { setTitle('ex19_2'); addWindowListener((WindowListener)new winListener()); mw = new msgWnd(); add('Center',mw); sb = new Scrollbar(Scrollbar. HORIZONTAL); sb.setVisibleAmount(10); sb.setBlockIncrement(6); sb.setUnitIncrement(5); sb.addAdjustmentListener(this); add('South',sb); pack(); show(); } public void adjustmentValueChanged(AdjustmentEvent ae) { val = ae.getValue(); _______________; } class msgWnd extends Canvas { msgWnd() { setSize(450,20); } public void paint(Graphics g) ( g.drawString(' val=' +____________________+'%',10,15); } } class winListener extends WindowAdapter { public void windowClosing(WindowEvent we) { System.exit(0); } } }