What to override inJComboBox

I need a class that extends JComboBox, but with slightly different
behaviour. I want to intercept mouse click on an item in the list, do
some processing, and sometimes prevent the item to be treated as
selected.
I think it could be done by overriding some JComboBox method but don't
know which one(s). I have tried with custom renderers, editors,
listmodels, but no success.
Can anyone give me a clue ?

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