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What exactly do you think FlowLayout.CENTER is doing?

Recommended reading: http://docs.oracle.com/javase/tutorial/uiswing/layout/grid.html

Your real code might be doing more than this, but note that the code you posted doesn't really need the ArrayList at all.

Ah, I see. you've imported java.awt.List, which is an AWT component. It is indeed not generic, hence the error.

You're looking for java.util.List, which is an interface. It *is* generic.
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This code doesn't make a lot of sense. Your buttons variable is a List, so why are you using the array index operator [i] on it?

Can you show us an MCVE (note: NOT your whole project!) that...