您的Main类扩展了JPanel，具有paintComponent方法-但是您从未将Main的实例添加到GUI ，而是在GUI上添加一个普通的JPanel，drawBox，当然，paintComponent将永远不会被调用.

Your Main class extends JPanel, has a paintComponent method -- but you never add an instance of Main to the GUI, instead you add a plain-vanilla JPanel, drawBox to the GUI, and so of course paintComponent will never be called.

解决方案:将Main或this添加到GUI，而不是普通的JPanel.

Solution: add Main or this to the GUI, not a plain JPanel.