Hi,
You could have a look at
http://www.javacertificate.com We provide you with a dynamic testcentre that concentrates on each mandatory objective, you can practice your faults until you know the objective and then move on to the next objective.
You are able to build your own mock-exam by selecting questions that are interesting to you or more difficult.
One you think you are ready for the exam have a look at the difficult mock exam, this is a compilation of the top 20 questions that our community has answered wrongly.
PS: All this is available for free. (only a small registration to enable us to keep track of your progress)
Apart from the above I can strongly recommend the javaranch and Marcus Green's Mock exams.
Hope this helps, and we keep our fingers crossed that you pass your exam.
The Javacertificate.com team.