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Oh and the exact error message

We don't read minds. Post your code. If it is too long then create a SSCCE.

As I said you are comparing the button with the String. Perhaps you should get the String that is displayed on the button and compare that instead.

Object source = (JButton)evt.getSource();
if (source.equals("8897")) {

You are comparing the button that caused the event to the String. Obviously this will never be true.