您的班级 DrawPanel 仅限于 main 方法，对你的构造函数不可见。

Your class DrawPanel is confined to the scope of your main method and is not visible to your constructor.

你需要移动 DrawPanel 退出主方法，然后将其添加到 JFrame ：

You need to move DrawPanel out of your main method, then add it to your JFrame:

frame.add(panel);

此外，***调用 frame.setVisible（true）添加完所有组件后。

Also, better to call frame.setVisible(true) after all components have been added.