关于CardLayout的问题

Question

在使用CardLayout时总是出错。谢谢！
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
class Frm extends JFrame{
JPanel curr;

JPanel jp1=new JPanel();
JPanel jp2=new JPanel();
JPanel jp3=new JPanel();
JPanel jp4=new JPanel();
JLabel lab1=new JLabel();
JLabel lab2=new JLabel();
JLabel lab3=new JLabel();
JButton but1=new JButton();
JButton but2=new JButton();

BorderLayout borderLayout=new BorderLayout();
CardLayout cardLayout=new CardLayout();

Frm(){
this.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
jIni();
}

private void jIni(){
curr=(JPanel)this.getContentPane();
this.setSize (300,300);
this.setLocation (300,300);
curr.setLayout(borderLayout);
curr.add(jp1,BorderLayout.CENTER);
curr.add(jp2,BorderLayout.SOUTH);
jp1.setBackground (new Color(255,0,0));
jp2.setBackground (new Color(0,255,0));
jp2.add (but1,BorderLayout.NORTH);
jp2.add (but2,BorderLayout.EAST);
but1.setText ("next");
but2.setText ("previous");
jp1.setLayout (cardLayout);
jp1.add (jp3,"j3");
jp1.add (jp4,"j4");

jp3.add (lab1);
jp4.add (lab2);

lab1.setText ("No.1");
lab2.setText ("No.2");

but1.addActionListener (new ActionListener(){
public void actionPerformed(ActionEvent e){
but1_Click(e);
}
});

but2.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
but2_Click(e);
}
});
}

private void but1_Click(ActionEvent e){
cardLayout.show(jp3,"j3");
}

private void but2_Click(ActionEvent e){
cardLayout.show(jp4,"j4");
}

public static void main(String[] args){
     Frm frm=new Frm();
    
     frm.show ();
    }
}

Answer 1

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
class Frm extends JFrame
{
    JPanel curr;    JPanel jp1 = new JPanel();
    JPanel jp2 = new JPanel();
    JPanel jp3 = new JPanel();
    JPanel jp4 = new JPanel();
    JLabel lab1 = new JLabel();
    JLabel lab2 = new JLabel();
    JLabel lab3 = new JLabel();
    JButton but1 = new JButton();
    JButton but2 = new JButton();    BorderLayout borderLayout = new BorderLayout();
    CardLayout cardLayout = new CardLayout();    Frm()
    {
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jIni();
    }    private void jIni()
    {
        curr = (JPanel)this.getContentPane();
        this.setSize(300, 300);
        this.setLocation(300, 300);
        
        curr.setLayout(borderLayout);
        curr.add(jp1, BorderLayout.CENTER);
        curr.add(jp2, BorderLayout.SOUTH);
        
        jp2.setBackground(new Color(0, 255, 0));
        jp2.add(but1, BorderLayout.NORTH);
        jp2.add(but2, BorderLayout.EAST);
        
        but1.setText("next");
        but2.setText("previous");
        
        lab1.setText("No.1");
        lab2.setText("No.2");
        
        jp3.add(lab1);
        jp4.add(lab2);
        
        jp1.setBackground(new Color(255, 0, 0));
        jp1.setLayout(cardLayout);
        jp1.add("j3", jp3);
        jp1.add("j4", jp4);        but1.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                but1_Click(e);
            }
        });        but2.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                but2_Click(e);
            }
        });
    }    private void but1_Click(ActionEvent e)
    {
       ( (CardLayout) jp1.getLayout()).show(jp1, "j4"); //card的父容器是jp1
    }    private void but2_Click(ActionEvent e)
    {
       ( (CardLayout) jp1.getLayout()).show(jp1, "j3");
    }    public static void main(String[] args)
    {
        Frm frm = new Frm();        frm.setVisible(true);
    }
}