Hi,
I made the changes and I don't receiving error again but my JInternalFrame are not loading. I believe that my problem are in my JInternalFrame class but I can't see where are the problem.
Can you make a small, complete program that compiles and can be executed for testing? Do no post the whole program. Only post enough that will compile and execute to show the problem.
The code declares two JFrames, one at line 16 and on at line 33. The code then puts components in them separately and shows one of them.
Either do not extend JFrame on 16 or do not create a second JFrame on line 33.
You do have a JDesktopPane, but you never add it to the GUI. Your GUI is the JFrame which contains a JPanel containing one JButton; you can click on the button but doing that just does some stuff unrelated to the components you have in your GUI.