Java 11 certification tips and thoughts
I have just become Oracle Certified Professional: Java SE 11 Developer.
Since I was already Java 8 certified, it was enough for me to pass only an upgrade exam 1Z0-817. Normally, if you do not have the previous certification, you would have to take 2 exams:
The exam itself is not that difficult, but make sure you study and practice before it. Complete list of topics can be found here.
I can recommend paying more attention to Java modules. I am not sure about you, but I don't have much experience with modules. I did some research when this feature was released. But I have never worked closely with it. However, at the exam you are expected to deal with "real life" situations. Don't just read the study material - practice. By practicing I mean creating a simple module app, migrating old jars to module system and familiarising yourself with jdeps tool.
Another topic to look closer at is Java File I/O (NIO.2). Again, as web developer dealing mostly with REST API and microservices I don't have enough expertise in this domain, so if this is the case for you too, make sure to refresh your knowledge in this area.
One more important skill is to be able to read and understand code during the exam. Sounds trivial, doesn't it? But please remember that normally we use an IDE with all the amazing features like autocompletion and syntax highlighting. On the contrary, during the exam you see a piece of code on a white screen and have to identify the correct option. It is quite a bizarre feeling at first. Normally Intellij and/or compiler does it for you. You have to practice this skill and be extra careful when choosing your answer.
I can recommend an extremely useful guide by Mikalai Zaikin, which helped me a lot.
And finally, is it worth it? Well personally for me - yes, it is. Not the certificate itself, without knowledge it is just a useless piece of paper. The most valuable part of certification is the preparation. This is the part where you really acquire more knowledge and new skills that you will be able to use in your work.