It's hard to give you a reasonable answer without knowing how you made your buttons to trigger the ActionListener, and the logic you used in actionPerformed. Is that String sum = "+"; the only statement exist in your actionPerformed method? Also how did you assign your action listener(s) to the buttons?
The expression i<=numbers.length causes that exception. If you use <= operator, the length, which is 10, will also be assigned to i at the last iteration, which is beyond the scope of the array [0...9]. Replace <= with < and it should work fine. Is this the same problem you were trying to describe before in this topic?