I like to share my experience on preparing and taking the exam 1Z0-819 Java 11 Developer exam.
Book Selikof/Boyarsky java 8 oca and ocp books (I did these certifications in 2017 and 2018 resp.).
Oracle java trail Annotations
Oracle java trail Security
Oracle security guidelines: https://www.oracle.com/technetwork/java/seccodeguide-139067.html JPMS: Sander Mak's training on Pluralsight + his book on JPMS
Did all enthuware exams 1z0-815, 1z0-816. I always made sure that I completed the exams at an average of less then 1.8 minutes per question
1z0-815 exams first round scores: 90%+, second round: 90%+
1z0-816 exams first round scores: 80%+, second round: 90%+
Biggest challenge is the limited time you get (1.5 hours for 50 questions, i.e 1.8 minutes per question). You have to know everything very well and still several questions will be too long to analyze the code thoroughly. Selikoff's post tells which strategies to apply: https://www.selikoff.net/category/java-certification/
Total preparation time: 3 months lead time of which 4 weeks of vacation which I used for the preparation. For the rest I perpared afterwork and during the weekends.