问答题

本程序中，主窗口中有一个文本框和“开始”、“关闭”两个按钮，单击“开始”按钮后，“开始”按钮处于按下状态，文本框中的数开始从0计数到49，每增加1间隔50毫秒，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，如图所示。

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class exam_46
{
public static void main(String[] args)
{
JFrame frame=new CounterFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
}
}
class CounterFrame extends JFrame
{
public static final int WIDTH=250;
public static final int HEIGHT=150;
private TextField textField=new TextField(20);
public CounterFrame()
{
setSize(WIDTH, HEIGHT);
setTitle("exam_46");
Container contentPane=getContentPane();
JPanel textPanel=new JPanel();
textPanel.add(textField);
JPanel buttonPanel=new JPanel();
addButton(buttonPanel, "开始",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
addCounter();
}
}
);
addButton (buttonPanel, "关闭",
new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
______;
}
}
);
contentPane.add(textPanel, BorderLayout.NORTH);
contentPane.add(buttonPanel, BorderLayout.SOUTH);
}
public void addButton(Container c, String title, ActionListener
listener)
{
JButton button=new JButton(title);
c.add(button);
button, addActionListener(listener);
}
public void addCounter()
{
for (int i=0; i ＜ 50; i++)
{
try
{
textField.setText(Integer.toString i)) ;
______;
}
catch (InterruptedException e)
{}
}
}
}