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 jAv
A、ApplEt.*;
import jAv
A、Awt.*;
import jAv
A、Awt.EvEnt.*;
puBliC ClAss ADvAnCE ExtEnDsApplEt implEmEntsACtionlistEnEr
int n = 0;
imAgE im1,im2,showim;
Button Bn = nEwButton("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 ACtionpErFormEDACtionEvEnt 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);