I used a mock tests from https://enthuware.com/java-certification-mock-exams/oracle-certified-associate/ocajp-1z0-808 for preparation. It costs 10$ but it worth it. More on reddit.com

I have a different opinion from the other folks here. I would say yes, but maybe not for the reasons you may have in mind. I did the Sun Certified Java Programmer certification several years ago because I was a Rails dev and believed it would help me break into getting a Java job (no CS or stem degree either). In the end, I don’t know if it made much difference getting an interview, but what I learned during the several months I spent diligently, humbly studying for that test has — in addition to getting me through some crappy-interview Java trivia questions — made me a much better Java dev. Things I didn’t know as a Java noob but learned for the test were e.g. the difference between checked and unchecked exceptions, threading, I/O, stringbuilder vs stringbuffer, generics, when to override hashcode, vagaries around enums, the difference between boxed types and primitives, how references work, and a ton of other stuff that I use every day. And I still work with developers who make mistakes that they would never have made if they had embarked on a formal, comprehensive study of the language. I used a book by Kathy Sierra (this was for Java 6 — don’t know if it’s been updated), and I took detailed notes, without which I wouldn’t have retained much. The book’s contents were great, but the practice tests weren’t so good. If you end up using her book, don’t beat yourself up if the tests are confusing — it’s the books fault (otherwise, an excellent resource). The actual SCJP exam, OTOH, was very clear and fair. Good luck in any case. More on reddit.com

I have ocp certification in Java and honestly, it felt like they are trying to make a human compiler out of you. You have to answer questions like "what is the result of this program?". However they wrote Class with a capital C so the code doesn't compile.... My opinion is, if you want to learn Java, just practice and make some personal projects. However I also know that quite some companies will like to see a piece of paper signed by Oracle... More on reddit.com

Question on reason for the exam and what's the benefit of having the cert? More on reddit.com