In order to do well on Java certifications, you need good books and a reasonable number of Practice questions before you go for exams. Practice questions and Mock Exams will help you to assess your topic-wise preparation level and help you to identify your strong and weak areas. Based on the result of the mock exams, you can concentrate on areas where you lack expertise like multi-threading is one of the tricky areas. It will also expose you to exam patterns and different types of questions you can expect in examination like multiple choice questions, rearranging code, fill in the blanks etc. Though I highly recommend commercial exam simulators like Whizlabs and Enthuware, I also suggest you take advantage of many mock questions which are freely available.