我要在第二个面板的监听器中去调用第一个面板中的JRadioButton的isSelected()方法，请问该如何调用呢？下面是代码：

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

class DrawPanel1 extends JPanel{
public DrawPanel1(){
setLayout(new FlowLayout());
ButtonGroup bg=new ButtonGroup();
JRadioButton rb1=new JRadioButton("面积",true);
JRadioButton rb2=new JRadioButton("周长",false);
bg.add(rb1);
bg.add(rb2);
add(rb1);
add(rb2);
}
}

class DrawPanel2 extends JPanel{
public DrawPanel2(){
setLayout(new GridLayout(2,2));
JLabel lb1=new JLabel("半径");
JLabel lb2=new JLabel("计算结果");
JTextField txtA=new JTextField();
JButton btn=new JButton("计算");
add(lb1);
add(txtA);
add(btn);
add(lb2);

btn.addActionListener(new ActionListener(){

public void actionPerformed(ActionEvent ae){

double a,area,perimeter;
a=Double.parseDouble(txtA.getText());

if(在此处调用)
area=3.14*a*a;
else
perimeter=3.14*2*a;
}
});

}
}