why I can't adding a window to a container,该怎么处理

why I can't adding a window to a container
源程序如下:

import   java.awt.*;
import   java.awt.event.*;
import   javax.swing.*;

public   class   ComCountry   extends   JFrame   implements   ItemListener
{
private   String[]   flagTitle   =   { "Canada ",   "China ",   "Denmark ",
        "France ",   "Germany ",   "India ",   "Norway ",   "United   Kingdom ",
        "United   States   of   America "};
private   ImageIcon   flagImage[]=   new   ImageIcon[9];
private   String   flagDescription[]=new   String[9];

private   JComboBox   countryNameComboBox=new   JComboBox();
private   DescriptionPanel   descriptionPanel=new   DescriptionPanel();

public   ComCountry()
{
        flagImage[0]   =   new   ImageIcon( "images/ca.gif ");
                flagImage[1]   =   new   ImageIcon( "images/china.gif ");
                flagImage[2]   =   new   ImageIcon( "images/denmark.gif ");
                flagImage[3]   =   new   ImageIcon( "images/fr.gif ");
                flagImage[4]   =   new   ImageIcon( "images/germany.gif ");
                flagImage[5]   =   new   ImageIcon( "images/india.gif ");
                flagImage[6]   =   new   ImageIcon( "images/norway.gif ");
                flagImage[7]   =   new   ImageIcon( "images/uk.gif ");
                flagImage[8]   =   new   ImageIcon( "images/us.gif ");

                //   Set   text   description
                flagDescription[0]   =   "The   Maple   Leaf   flag   \n\n "   +
                "The   Canadian   National   Flag   was   adopted   by   the   Canadian   "   +
                "Parliament   on   October   22,   1964   and   was   proclaimed   into   law   "   +
                "by   Her   Majesty   Queen   Elizabeth   II   (the   Queen   of   Canada)   on   "   +
                "February   15,   1965.   The   Canadian   Flag   (colloquially   known   "   +
                "as   The   Maple   Leaf   Flag)   is   a   red   flag   of   the   proportions   "   +
                "two   by   length   and   one   by   width,   containing   in   its   centre   a   "   +

                "white   square,   with   a   single   red   stylized   eleven-point   "   +
                "mapleleaf   centred   in   the   white   square. ";
                flagDescription[1]   =   "Description   for   China   ...   ";
                flagDescription[2]   =   "Description   for   Denmark   ...   ";
                flagDescription[3]   =   "Description   for   France   ...   ";
                flagDescription[4]   =   "Description   for   Germany   ...   ";
                flagDescription[5]   =   "Description   for   India   ...   ";
                flagDescription[6]   =   "Description   for   Norway   ...   ";
                flagDescription[7]   =   "Description   for   UK   ...   ";
                flagDescription[8]   =   "Description   for   US   ...   ";
               
JLabel   TitleLabel=new   JLabel( "QZM 's   first   program ");
JPanel   p1=new   JPanel();
p1.setLayout(new   BorderLayout());
p1.add(TitleLabel,BorderLayout.CENTER);
p1.add(countryNameComboBox,BorderLayout.WEST);

getContentPane().setLayout(new   BorderLayout());
getContentPane().add(p1,BorderLayout.NORTH);
getContentPane().add(descriptionPanel,BorderLayout.CENTER);

countryNameComboBox.addItemListener(this);
}

public   static   void   main(String   []   args)
{
//   DescriptionPanel   frame1=new   DescriptionPanel();
ComCountry   frame1=new   ComCountry();
frame1.setVisible(true);
frame1.pack();
frame1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

public   void   itemStateChanged(ItemEvent   e)
{
DisplayDescriptionPanel(countryNameComboBox.getSelectedIndex());
}

public   void   DisplayDescriptionPanel(int   index)
{
descriptionPanel.setImage(flagImage[index]);
descriptionPanel.setTitle(flagTitle[index]);
descriptionPanel.setText(flagDescription[index]);
}
}

class   DescriptionPanel   extends   JFrame
{
private   JTextArea   textArea1=new   JTextArea();
private   JLabel   countryImageLabel=new   JLabel();
private   JLabel   countryTitleLabel=new   JLabel();  
public   DescriptionPanel()
{
JPanel   p1=new   JPanel();

p1.setLayout(new   BorderLayout());
p1.add(countryImageLabel,BorderLayout.CENTER);
p1.add(countryImageLabel,BorderLayout.SOUTH);

JScrollPane   jScrollPane=new   JScrollPane();
jScrollPane.add(textArea1);
textArea1.setLineWrap(true);
textArea1.setWrapStyleWord(true);

setLayout(new   BorderLayout());
add(jScrollPane,BorderLayout.EAST);

add(p1,BorderLayout.CENTER);
}

public   void   setImage(ImageIcon   icon)
{
countryImageLabel.setIcon(icon);
Dimension   dimension=new   Dimension(icon.getIconWidth(),icon.getIconHeight());
countryImageLabel.setPreferredSize(dimension);  
}

public   void   setTitle(String   title)
{
countryTitleLabel.setText(title);
}

public   void   setText(String   text)
{
textArea1.setText(text);
}
}

编译   OK
运行出现如下错误。
Exception   in   thread   "main "   java.lang.IllegalArgumentException:   adding   a   window   to   a   container
                at   java.awt.Container.addImpl(Container.java:1022)
                at   java.awt.Container.add(Container.java:903)
                at   ComCountry. <init> (ComCountry.java:55)
                at   ComCountry.main(ComCountry.java:62)
本人系Java初学者，望高手指点。不甚感激。

[解决办法]
代码贴的太多，没看完。
[解决办法]
应该是这个的
class DescriptionPanel extends JFrame{}
这里的
private DescriptionPanel descriptionPanel=new DescriptionPanel();
生成的descriptionPanel 是一个窗口对象。
不能再向ComCountry中增加相同等级的对象了。
可以把class DescriptionPanel extends JFrame{}
改为一个轻量级的容器。比如说
改为class DescriptionPanel extends JPanel{}