import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
public class Calc extends JFrame
{
  public Calc()
  {
    this.setDefaultCloseOperation(3);
    setLayout(new BorderLayout());
    Container contentPane=getContentPane();
    NorthPanel np=new NorthPanel();
    contentPane.add(np,BorderLayout.NORTH);
    CenterPanel cp=new CenterPanel();
    contentPane.add(cp,BorderLayout.CENTER);
  }  
  public static void main(String[] args)
    {
      Calc calc=new Calc();
      calc.setSize(300,200);
      calc.setVisible(true);
    }
}

class NorthPanel extends JPanel
{
  JRadioButton btn1,btn2;
  public NorthPanel()
  {
    btn1=new JRadioButton("面积");
    btn2=new JRadioButton("周长");
    ButtonGroup bg=new ButtonGroup();
    bg.add(btn1);
    bg.add(btn2);
    add(btn1);
    add(btn2);
  }
  
}

class CenterPanel extends JPanel
{
  JLabel lbl1,lbl2;
  JTextField jf;
  JButton btn;
  NorthPanel np=new NorthPanel();
  public CenterPanel()
  {
    setLayout(new GridLayout(2,2));
    lbl1=new JLabel("半径");
    add(lbl1);
    //lbl2=new JLabel();
    jf=new JTextField(4);
    String s=jf.getText();
    
//    try 
//    {
      final double d=Double.parseDouble(s);
//      }
//      catch (Exception ex) 
//      {
//        ex.printStackTrace();
//      }
    jf.addKeyListener(new KeyAdapter()
    {
      public void keyPressed(KeyEvent e)
      {
        if(jf.getText().length()>3)
        {
          JOptionPane.showMessageDialog(null,"输入的位数不能大于4位数","输入错误",2);
          jf.setText("");
        }
        
      }
      public void keyReleased(KeyEvent e)
      {
        if(!((e.getKeyChar()>='1') && (e.getKeyChar()<='9')))
        {
          JOptionPane.showMessageDialog(null,"只能输入数字","输入错误",2);
          jf.setText("");
        }
      }
      });
    add(jf);
    lbl2=new JLabel("这边显示计算结果");
    btn=new JButton("计算");
    btn.addActionListener(new ActionListener()
    {
      public void actionPerformed(ActionEvent e)
      {
        if(np.btn1.isSelected())
        {
          try
          {
            lbl2.setText("结果为:"+3.14*3.14*d);
          }
          catch(NumberFormatException a)
          {
            a.printStackTrace();  
          }
          
        }
        else if(np.btn2.isSelected())
        {
          try 
          {
            lbl2.setText("结果为"+3.14*2.0*d);
            }
            catch (NumberFormatException ex) 
            {
              ex.printStackTrace();
            }
          
        }
      }
      });
    add(btn);
    add(lbl2);
  }
}