scrollbar in JPanel

Scroll bar is not working when i am using the panel layout is null. If i change the panel layout to flow or grid its working. I have a great trouble in this please help me. Here my code.....

import java.awt.*;
import javax.swing.*;
class scroll
{
public static void main(String[] args)
{
Frame f = new Frame ("Demo");
JPanel p = new JPanel();
JButton b1 = new JButton("Button 1");
JButton b2 = new JButton("Button 2");
p.setLayout(null);
p.setBackground(Color.red);
b1.setBounds(100,500,150,20);
b2.setBounds(100,150,150,20);
p.add(b1);
p.add(b2);
JScrollPane s = new JScrollPane(p, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
f.add(s);
f.setSize(70,70);
f.setVisible(true);
}
}