Change of JPanel detection

[kilroyFR]

Hey there.

I am currently developping a component that displays some bitmaps in different JPanel ('Tabs').
In order to avoid loading all bitmaps i would like to be able to detect when a user is selecting a tab so that i can load the bitmap at that time (and avoid useless/inefficient load).

I have found nothing on this on any book or forums.
Any ideas ?

Thanks