添加了JButton但在运行时未显示

Question

I'm using a book to learn java. I'm have made a Swing window but when I try to make JButtons, they are not visible at runtime. I've copied the code exactly as it is, but it is still not visible. Here is the code:

import javax.swing.*;
class Buttons extends JFrame
{
    JPanel pnl = new JPanel();
    public Buttons()
    {
        super("Swing Window");
        setSize(500,200);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        add(pnl);
        setVisible(true);
        pnl.add(new JButton("Click Me"));
    }
    public static void main(String[] args)
    {
        Window gui = new Window();
    }
}

Answer 1

Change:

    setVisible(true);
    pnl.add(new JButton("Click Me"));

To:

    pnl.add(new JButton("Click Me"));
    pack(); // very important!
    setVisible(true); // should be last!

Answer 2

Change Window to Buttons and it works fine!!!

import javax.swing.*;    

class Buttons extends JFrame
{
JPanel pnl = new JPanel();
public Buttons()
 {
    super("Swing Window");
    setSize(500,200);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    add(pnl);
    setVisible(true);
    pnl.add(new JButton("Click Me"));
}
public static void main(String[] args)
{
    Buttons gui = new Buttons();
}

}