java 为什么点击首页按钮在旁边的面板上面不显示东西

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 SwingDemo6
{
public void empty()
{
JFrame jf=new JFrame();
JButton jb=new JButton("首页");
JButton jb1=new JButton("搜索");
JButton jb2=new JButton("提问");
JButton jb3=new JButton("我的");
JButton jb4=new JButton("圈子");
jb.setPreferredSize(new Dimension(95,30));
jb.setBackground(Color.white);
jb.setFont(new Font("楷体",1,15));
jb1.setPreferredSize(new Dimension(95,30));
jb1.setBackground(Color.white);
jb1.setFont(new Font("楷体",1,15));
jb2.setPreferredSize(new Dimension(95,30));
jb2.setBackground(Color.white);
jb2.setFont(new Font("楷体",1,15));
jb3.setPreferredSize(new Dimension(95,30));
jb3.setBackground(Color.white);
jb3.setFont(new Font("楷体",1,15));
jb4.setPreferredSize(new Dimension(95,30));
jb4.setBackground(Color.white);
jb4.setFont(new Font("楷体",1,15));
final JPanel jp=new JPanel();
final JPanel jp1=new JPanel();
final JPanel jp2=new JPanel();
JPanel jp3=new JPanel();
final JButton jb5=new JButton("测试");
jp.add(jb);
jp.add(jb1);
jp.add(jb2);
jp.add(jb3);
jp.add(jb4);
jp.setPreferredSize(new Dimension(100, 500));
jp1.setPreferredSize(new Dimension(400, 500));
jp2.setPreferredSize(new Dimension(400, 500));
jf.setLayout(new BorderLayout());
jf.add(jp,WEST);
jf.add(jp1,CENTER);
jf.add(jp2,EAST);
jb.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{

            jp1.setBackground(Color.white);
            jp1.add(jb5);
        }
    });
    jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  
    jf.pack();
    jf.setVisible(true);
}
public static void main(String[] args)
{
    new SwingDemo6().empty();
}

}

Answer 1

试下让frame validate（）