Java Error: IllegalArgumentException: adding a window to a container

Question

I keep receiving the error:

Exception in thread \"main\" java.lang.IllegalArgumentException: adding a window to a container
  at java.awt.Container.checkNotA         


        

Answer 1

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.

Change EmployeeGUI so it extends from JPanel instead

Answer 2

Here:

frame.getContentPane().add(app, BorderLayout.CENTER);

you're trying to add a JFrame to a JFrame which makes no sense.

Why not instead just try to display app rather than add it to a JFrame. Or even better, not have EmployeeGUI extend JFrame.

Answer 3

You can't add a JFrame to another JFrame. But you can add a JPanel to a JFrame, in other words change EmployeeGUI to make it extends JPanel.