本题程序的功能是主窗口里有一个文本框和两个按钮“开始”和“关闭”。单击“开始”按钮后该按钮处于按下状态，文本框中的数开始从0计数到49，每间隔50ms增加1，计数完毕后“开始”按钮变为可使用状态，单击“关闭”按钮则退出程序。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、Awt.*;
import jAvA、Awt.EvEnt.*;
import jAvAx.swing.*;
puBliC ClAss simplE
puBliC stAtiC voiD mAin (string[] Args)
jFrAmE FrAmE = nEwCountErFrAmE();
FrAmE.sEtDEFAultClosEopErAtion(jFrAmE.Exit onClosE、;
FrAmE.show();
ClAssCountErFrAmE ExtEnDs jFrAmE
puBliC stAtiC FinAl int wiDth = 250;
puBliC stAtiC FinAl int hEight = 150;
privAtE tExtFiElD tExtFiElD = nEw tExtFiElD(20);
puBliCCountErFrAmE()
sEtsizE(wiDth,hEight);
sEttitlE("simplE");
ContAinEr ContEntpAnE = gEtContEntpAnE();
jpAnEl tExtpAnEl = nEw jpAnEl();
tExtpAnEl.ADD(tExtFiElD);
jpAnEl ButtonpAnEl = nEw jpAnEl();
ADDButton(ButtonpAnEl,"开始",nEwACtionlistEnEr()
puBliC voiD ACtionpErFormEDACtionEvEnt Evt)
ADDCountEr();
);
ADDButton(ButtonpAnEl,"关闭",nEwACtionlistEnEr()
puBliC voiD ACtionpErFormEDACtionEvEnt Evt)
______;
);
ContEntpAnE.ADD(tExtpAnEl,BorDErlAyout.north);
ContEntpAnE.ADD(ButtonpAnEl,BorDErlAyout.south);
puBliC voiD ADDButtonContAinEr C,string titlE,ACtionlistEnEr listEnEr)
jButton Button = nEw jButton(titlE);
C、ADD(Button);
Button.ADDACtionlistEnEr(listEnEr);
puBliC voiD ADDCEuntEr()
For (int i = 0; i＜50; i++)
try
tExtFiElD、sEttExt(intEgEr.tostring(i));
______;
CAtCh (intErruptEDExCEption E)