本题程序是一个Applet应用程序，功能是计算前n个自然数的和，程序中用进度条来表示

题目：

本题程序是一个Applet应用程序，功能是计算前n个自然数的和，程序中用进度条来表示计算的进程。页面中有两个文本框、两个按钮和一个进度条，在第一个文本框中输入要计算的自然数的个数，单击“开始”按钮则开始进行计算，进度条同步显示计算完成的情况。程序中存在若干错误，请找出并改正(注意：不得改动程序的结构，不得增行或删行)。
import javax.swing.*;
import javax.swing.border.*;
import java.awt.*;
import java.awt.event.*;
public class advance extends Japplet

Container container = null;
JButton startButton,stopButton;
JTextField inputTextField,outputTextField;
JProgressBar pBar = null;
Timer timer = null;
static int sum = 0;
static int counter = 0;
public void init()

container = this.getContentPane();
container.setLayout(new GridLayout(3,1));
Box hboxl = Box.createHorizontalBox();
container.add(hboxl);
hboxl.add(Box.createHorizontalGlue());
JLabel labell = new JLabel("前",JLabeI.LEFT);
labell.setFont(new Font("Dialog",Font.PLAIN,15));
hboxl.add(labell);
inputTextField = new JTextField("100",4);
hboxl.add(inputTextField);
JLabel label2 = new JLabel("个自然数和是",JLabel.LEFT);
label2.setFont(new Font("Dialog",Font.PLAIN,15));
hboxl.add(label2);
outputTextField = new JTextField(10);
hboxl.add(outputTextField);
hboxl.add(Box.createHorizontalGlue());
Box hbox2 = Box.createHorizontalBox();
container.add(hbox2);
startButton : new JButton("开始");
startButton.addActionListener(new ButtonListener());
hbox2.add(Box.createHorizontalGlue());
hbox2.add(startButton);
hbox2.add(Box.createHorizontalGlue());
stopButton = new JButton("结束");
stopButton.addActionListener(new ButtonListener() ;
hbox2.add(Box.createHorizontalGlue());
hbox2.add(stopButton);
hbox2.add(Box.createHorizontalGlue());
pBar = new JProgressBar();
pBar.setStringPainted(true);
Border border = BorderFactory.createLineBorder(Color.red,2);
pBar.setBorder(border);
pBar.setBackground(Color.white);
pBar.setForeground(Color.blue);
pBar.setMinimum(0);
pBar.setMaximum(Integer.parseInt(inputTextField.getText()));
container.add(pBar);
timer = new Timer(0,TimerListener());


class TimerListener implements ActionListener

public void actionPerformed(ActionEvent e)
if (Integer.parseInt (inputTextField.getText ()) ＞ 0)

counter ++;
sum = sum+counter;
pBar.setValue (counter);
outputTextField,setText (Integer.toString (sum));

else
outputTextField,setText ("0");

if (counter ＞= Integer.parseInt(inputTextField.getText ()))
timer,stop ();


class ButtonListener implements ActionListener

public void actionPerformed(ActionEvent e)

JButton button = e.getSource();
if (button.getText () == "开始")

outputTextField,setText ("");
if (inputTextField.getText() != " ")

pBar.setMaximum (Integer.parseInt (inputTextField.getText ()) );
sum = 0 ;
counter = 0;
timer.begin ();


else
if (button.getText () =="结束")

timer,stop ();
outputTextField,setText ("");
sum = 0;
counter = 0;
pBar.setValue(0);