看不到TextField和Button为什么啊。
package defaults;

import java.awt.Color;
import java.awt.Container;
import java.awt.TextField;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Counter {
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("Counter");
Container contentPane = getContentPane();
JPanel textPanel = new JPanel();
textField.setBackground(new Color(10,10,10));
textPanel.add(textField);

JPanel buttonPanel = new JPanel();
addButton(buttonPanel,"start",new ActionListener(){
public void actionPerformed(ActionEvent evt){
addCounter();
}
});
}
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));
Thread.sleep(50);
}catch(InterruptedException e){

}
}
}
}

[解决办法]
你没有textPanel，buttonPanel加入到容器中，而且给容器添加布局器、
[解决办法]
楼上正解：

Java code

import java.awt.Color;import java.awt.Container;import java.awt.TextField;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.awt.BorderLayout;//引入布局管理器。import javax.swing.JButton;import javax.swing.JFrame;import javax.swing.JPanel;public class Counter{    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("Counter");        Container contentPane = getContentPane();        JPanel textPanel = new JPanel();        textField.setBackground(new Color(255,255,255));//换成白底。        textPanel.add(textField);        contentPane.add(textPanel,BorderLayout.NORTH);//添加 textPanel.                  JPanel buttonPanel = new JPanel();        addButton(buttonPanel,"start",new ActionListener()                {                    public void actionPerformed(ActionEvent evt)                    {                        addCounter();                    }                });        contentPane.add(buttonPanel,BorderLayout.SOUTH);//添加buttoPanel.    }        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));                Thread.sleep(50);            }            catch(InterruptedException e)            {                            }        }    }}