EmployeeGUI 继承自 JFrame，但是在你的 main 方法中，你正在创建一个新的 JFrame 并尝试向其中添加 EmployeeGUI 的实例.

EmployeeGUI extends from JFrame, but in your main method, you are creating a new JFrame and are trying to add an instance of EmployeeGUI to it.

更改 EmployeeGUI 使其从 JPanel 扩展而不是

Change EmployeeGUI so it extends from JPanel instead