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It's almost never a good idea to use a null layout. Use a real layout. What happens if the screen is resized or if you want to add/remove a component?

That error is telling me that the argument...

Why are you using a null layout in the first place? Just use a layout and let Swing do all the hard work for you.

What errors are you getting? What line are the errors on?

This code is pretty confusing. Where do you initialize ItemList (which really should be itemList, pay attention to standard naming...

That's definitely a lot of code for somebody else to wade through. Why not post an SSCCE instead? I'm not sure what your actual question is.