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I'm not sure how to answer this question and would appreciate some advice:
In the actionPerformed(ActionEvent event) method in the class InputPanel, the first line of code is a decision statement determining if the event.getSource is a JButton. If the source is NOT a JButton, what else would it be?
Why is the decision statement mentioned in the previous question necessary?
If it's not a JButton, then it would be some other Swing component which supports ActionListeners.
And if that were the case, then casting it to a JButton in line 4 of the posted code would throw a ClassCastException. That would be a bad thing; doing nothing would be better, and that's what line 1 does for you.