问答题 本题程序是一个Applet应用程序，功能是更改显示的图片。页面中有一个按钮“改变图形”，单击该按钮，面板中显示的图片改变；继续单击，图片继续改变。请将下述程序补充完整(注意：不得改动程序的结构，不得增行或删行)。
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class advance extends Applet implements ActionListener
{
int n = 0;
Image im1,im2,showim;
Button bn = new Button("改变图形");
public void init()
{
add (bn);
______;
im1 = getImage(getCodeBase(),"advance_1.jpg");
im2 = getImage(getCodeBase(),"advance_2.jpg");
bn.addActionListener ( this );
showim = im2;
}
public void actionPerformed(ActionEvent e)
{
______;
if (e.getActionCommand ()== "改变" )
{
if(n%2 == 0)
{
showim = im1;
n++;
}
else
{
showim = im2;
n++;
}
______;
}
}
public void paint (Graphics g)
{
g.drawImage (showim,0,0,this);
}