Layout为空时ScrollBar不工作

当布局设置为空时,滚动条不会出现。如何使滚动条进入面板?还有其他方法可以实现吗?我想在 scrollPane

import java.awt.Dimension;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;

public class ScrollBarExample {

    public static void main(String... args) {
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();
        panel.setLayout(null);
        for (int i = 0; i < 10; i++) {
            JButton jbutton = new JButton("Hello-" + i);
            jbutton.setBounds(i * 100, i*30, 100, 40);
            panel.add(jbutton);
        }
        JScrollPane scrollPane = new JScrollPane(panel);
        scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
        scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED);
        scrollPane.setBounds(50, 10, 320, 100);
        JPanel contentPane = new JPanel(null);
        contentPane.setPreferredSize(new Dimension(500, 400));
        contentPane.add(scrollPane);
        frame.setContentPane(contentPane);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.setVisible(true);
    }
}