java 为什么点击按钮1下面的面板出现了按钮2，但是JTextArea消失的不彻底

Question

import java.awt.*;
import javax.swing.*;
import static java.awt.BorderLayout.*;
import javax.swing.ImageIcon.*;
import java.awt.Color;
import static java.awt.BorderLayout.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class ABCD
{
public void init()
{
JFrame jf=new JFrame();
final JPanel jp=new JPanel();
final JTextArea jta=new JTextArea(20,20);
JButton jb=new JButton("1");
JPanel jp1=new JPanel();
jp1.add(jb);
JPanel bgp=new JPanel ();
bgp.setPreferredSize(new Dimension(300, 500));
bgp.add(jta);
jp.add(bgp);
jf.setLayout(new BorderLayout());
jf.add(jp1,NORTH);
jf.add(jp,SOUTH);
jb.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
jp.removeAll();
JPanel jp3=new JPanel();
JButton jb1=new JButton("2");
jp3.add(jb1);
jp.add(jp3);
jp.validate();
}
});

    jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  
    jf.pack();
    jf.setVisible(true);
}
public static void main(String[] args)
{
    new ABCD().init();
}

}

Answer 1

http://zhidao.baidu.com/link?url=KaLgZ6fDFxte9kZIhCQ61u7am2Zqqhc9dILkhmWgEsLKbgnhO3lzk8AzKNCKUdajkdHN-ocnp57egkiQm6y7aa