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I learned that using "empty" does not work here since the List is wrapped in a ListDataModel, and I need to use rowCount on that instead to test for empty.
But something weird was still going on. In the debugger window I could see my variable m_availableCars and listed as being an Array - i.e. showing two empty brackets.
Changing the name (i.e. removing m_) got my code working.
But so, I thought. After restarting JBoss I could not reproduce the problem, and switching back to the "m_" notation worked.
So - problem is solved.