Java Programming

Using code below, I receive a "cannot resolve symbol
symbol : method pack ()" error
Can someone help ?
Thanks.

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

public class test extends JPanel {
    private JButton b1, b2;

    public test() {
        b1 = new JButton ("Button 1");
        b2 = new JButton ("Button 2");

        Dimension d = Toolkit.getDefaultToolkit ().getScreenSize ();
        pack ();
        setLocation (d.width / 2 - getWidth () / 2, d.height / 2 - getHeight () / 2);

        setPreferredSize (new Dimension (496, 371));
        setLayout (null);

        add (b1);
        add (b2);

        b1.setBounds (170, 145, 100, 20);
        b2.setBounds (310, 210, 100, 20);
    }

 public static void main (String[] args) {
        JFrame frame = new JFrame ("test");
        frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add (new test());
        frame.pack();
        frame.setVisible (true);
    }
}