CardLayout card 错误:找不出符号.代码如下：

Question

在练习Java布局管理器CardLayout时ider报了
java: 找不到符号
符号: 变量 cardLayout
位置: 类 AWT.Cardlayout的错误，应该如何解决
代码如下：
package AWT;

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.ActiveEvent;

class Cardlayout extends Frame implements ActionListener{
Panel cardPanel=new Panel();
Panel controlpaPanel=new Panel();
Button nextbutton,preBtton;
Cardlayout cardlayout=new Cardlayout();
public Cardlayout(){
setSize(300,200);
setVisible(true);
this.addWindowListener(new WindowAdapter() {
@Override
public void windowClosing(WindowEvent e) {
Cardlayout.this.dispose();
}
});
cardPanel.setLayout(cardLayout);
cardlayout.add(new Label("第一个界面",Label.CENTER));
cardlayout.add(new Label("第二个界面",Label.CENTER));
cardlayout.add(new Label("第三个界面",Label.CENTER));
nextbutton=new Button("下一张卡片");
preBtton=new Button("上一张卡片");
nextbutton.addActionListener(this);
preBtton.addActionListener(this);
controlpaPanel.add(preBtton);
controlpaPanel.add(nextbutton);
this.add(cardPanel,BorderLayout.CENTER);
this.add(controlpaPanel,BorderLayout.SOUTH);
}

@Override
public void actionPerformed(ActionEvent e) {
    if(e.getSource()==nextbutton){
        cardlayout.next(cardPanel);
    }
    if(e.getSource()==preBtton){
        cardlayout.previous(cardPanel);
    }

}

private void next(Panel cardPanel) {
}

private void previous(Panel cardPanel) {
}

}
public class Example06 {
public static void main(String[] args) {

    Cardlayout cardlayout = new Cardlayout();
}

}

Answer 1

使用CardLayout卡片布局，会默认将放在容器内的卡片重叠，所以每次使用cardLayout.add(),无需再设置放在中心区域，只需将所有卡片放在panl容器中，再将panl设置放在Frame的中心区域即可。