JScrollPanel,水平和垂直滚动不工作

public class MyScroll extends JFrame {

    public MyScroll() {
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();

        panel.setLayout(null);

        for (int i = 0; i < 10; i++) {
            JButton b = new JButton("Hello-" + i);
            b.setBounds(0, i * 50, 100, 45);
            panel.add(b);

            b.setLayout(null);
        }

        JScrollPane scrollPane = new JScrollPane(panel);
        scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        scrollPane.setBounds(50, 30, 100, 325);

        JPanel contentPane = new JPanel(null);
        contentPane.setPreferredSize(new Dimension(500, 400));
        contentPane.setBackground(Color.BLUE);
        contentPane.add(scrollPane);

        contentPane.setLayout(null);

        setContentPane(contentPane);
        pack();
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        setVisible(true);
    }
}

而且,它呈现出这样的效果:

JPanel .

有人能解释一下我做错了什么吗?

此代码基于以下代码: Scrolling a JPanel