一个关于JFrame和JPanel的问题

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;public class Calculator
{
public static void main(String[] args)
{
JFrame frame=new JFrame("Calculator");
        frame.addWindowListener(new WindowAdapter()
        {
            public void windowClosing(WindowEvent e)
            {
                System.exit(0);
         }
        }); Container c=frame.getContentPane();
JPanel pane1=new JPanel();
pane1.setLayout(new GridLayout(4,6,3,3));
pane1.add(new JButton("sin"));
pane1.add(new JButton("MC"));
pane1.add(new JButton("0"));
pane1.add(new JButton("1"));
pane1.add(new JButton("2"));
pane1.add(new JButton("+"));
pane1.add(new JButton("cos"));
pane1.add(new JButton("MR"));
pane1.add(new JButton("3"));
pane1.add(new JButton("4"));
pane1.add(new JButton("5"));
pane1.add(new JButton("-"));
pane1.add(new JButton("tan"));
pane1.add(new JButton("MS"));
pane1.add(new JButton("6"));
pane1.add(new JButton("7"));
pane1.add(new JButton("8"));
pane1.add(new JButton("*"));
pane1.add(new JButton("+/-"));
pane1.add(new JButton("M+"));
pane1.add(new JButton("9"));
pane1.add(new JButton("."));
pane1.add(new JButton("="));
pane1.add(new JButton("/")); JPanel pane2=new JPanel();
pane2.setLayout(null); JTextField tf1=new JTextField();
tf1.setBounds(0,0,30,20);
tf1.setEditable(false);
tf1.setBackground(Color.blue);
pane2.add(tf1); c.add(BorderLayout.CENTER,pane1);
c.add(BorderLayout.NORTH,pane2);
frame.setSize(350,200);
frame.setVisible(true);
}
}
为什么pane2显示不出来呢？