Here's a list of loads of mock exams, mostly free:
http://www.certpal.com/
https://coderanch.com/t/546876/sr/certification/links-OCJP-mock-test-preparation
Don't worry about it too much, do more mock exams and for everything you learn put some code down in as many stupid and seemingly pointless ways as you can to
test your hypotheses (e.g. can you pass an int variable to function taking a short, is a null
string an instance of a string) and it'll stick better.
Just remember that you can be a brilliant
Java programmer for 15 years and fail the exam, there's no shame. The exam isn't about showing your ability as a programmer, all certification exams are mainly about finding ways to make you fail by using obscure and irrelevent coding practices, misdirection and obfuscation. I imagine by reading the book you'll have already noticed this. In my opinion there should be absolutely no questions on the exam resulting in a compiler error.