JAVASwing通过按钮切换面板失败。

点击按钮之后，面板不会qie'h
public class ADD {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    JPanel b=new JPanel();
    Frame a=new Frame("..");
    ActionListener d=new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            // TODO Auto-generated method stub
            b.setOpaque(false);
            Jpanel B=new Jpanel();
            a.add(B);
        }
    };

    a.add(b);
    JButton c=new JButton("123");
    c.addActionListener(d);
    b.add(c);
    a.setVisible(true);
    a.setBounds(200, 200,200,200);
}

}

import java.awt.*;

import javax.swing.JButton;
import javax.swing.JPanel;
public class Jpanel extends JPanel{

@Override
public void setBackground(Color bg) {
    // TODO Auto-generated method stub
    super.setBackground(Color.blue);
}

}