Hi, a little background: I've been using Java for nearly 10 years in a hobbyist fashion. I started using it in College and used it as my primary language for fun coding over the years. I never really cared to study the language features in depth, but I became proficient with about 75% of the language features. Recently, I decided to get certified to help my resume, as I am not a degree holder, but IMHO, definitely possess the skills to be a professional developer. I took the OCAJP 8 exam a month ago and barely studied for it. I breezed through the official guide written by Ms. Boyarsky and Mr. Selikoff. Got a 92%. I immediately scheduled the OCPJP 8 exam and bought the official guide for that. I was very confident that I needn't study most of the exam topics that appeared prior to Java 7. I never really used NIO, the new concurrency APIs, Localization, the new Date/Time APIs, or functional programming APIs. I saw examples and tutorials over the years, but never actually used them in my projects. Mind you, I have minor experience in Scheme, Scala, and other functional languages, so I understood the concepts well, just not Java's implementation.
So, I had a month to study... and I ultimately ended up studying for roughly 4 days! In the beginning of the month, I read through the first 4 chapters of the official study guide book, then I just stopped studying. I spent most weekends working on an app for my portfolio. I had planned on spending my 3-day weekend before the exam to study the aforementioned topics in depth. I went through Enthuware's objective-wise tests and failed many of them, but I learned from their explanations. This approach was effective for me since I only needed to learn the nuances of certain topics and to get more practice with the new APIs. I studied for 10-12 hours straight both days doing Enthuware and playing with APIs in IntelliJ. I used the book as needed.
I was afraid that I didn't know the new APIs well enough that I could risk failing the test. There were so many complex Stream APIs! I knew I could deduce many correct answers if the exam didn't concentrate ONLY on testing for mundane API specifics (e.g. what exception does Files.move() throw). I completely accepted the fact that I wasn't going to be able to remember all the possible exceptions thrown by methods, or more importantly, what conditions would cause those exceptions to be thrown. Aside from that, I felt like I understood everything conceptual I needed to for the test. Or at least, so much that, I could remove the wrong answers. My impression of the first few questions invoked anxiety. They were code samples, and I was unsure of my answers. The vast majority of the test was all code samples, some small, some large. I had very few non-code questions that asked for straight-forward conceptual knowledge, and surprisingly some of them were not covered in the official OCPJP 8 text, nor Enthuware. I felt like I was guessing on more of the questions than I would have liked, which didn't sit well with me during the exam. Even worse, I'd always be able to narrow down an answer to two choices but was stuck between the two. The exam seemed to get easier with each passing question, or rather I just stopped caring since time was running out.
Ultimately, I finished with ~10 minutes left to review. Overall, the test provided good coverage of all the topics. As mentioned by others, the exam is dominated by lambdas and Streams, but I also got many Concurrency and NIO questions, so don't ignore those topics. There were some easy things I missed, such as knowing that methods could be 'final'. The questions do test multiple things at once, hence some of them were "gotchas". Ironically, I did the best on the lambdas/Streams questions.
Enthuware is a great product and should be used for preparation, but in my experience, the exam is nowhere near as hard as Enthuware. Enthuware is very tricky, and frankly, frustrating, but I highly recommend it to toughen you up. Also, shoutouts to Ms. Boyarsky and Mr. Selikoff. They wrote a fantastic study guide! Welp, hope this helps, and good luck to you all who are taking the exam!