Found the ‘JAVA Quiz’ app on the AppStore. See http://www.appgrabber.com/app/java-quiz
As its free, thought I’d give it a go.
It says its got over 500 questions and starts a random quiz each time.
You get 30 seconds per question (can change this) and its a multiple choice Q&A.
I found it useful as its a good way of finding out which areas you know – and more likely at the moment – which areas you don’t know.