JButton已添加但未在运行时显示

Question

我正在用一本书来学习java.我已经创建了一个Swing窗口,但是当我尝试创建JButton时,它们在运行时不可见.我完全按原样复制了代码,但它仍然不可见.这是代码:

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

更改:

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

至:

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