A small piece of advice is to make sure you're motivated before diving in. Debugging a race condition, just as an example, can be quite involved and consume a lot of your time and energy to even reproduce.

https://thecomputersciencebook.com/book/concurrent-programmi...

[1]: ISBN 978-0321349606

There was a Java-specific edition: https://www.os-book.com/OS8/os8j/index.html

Maybe that is a good side project.

2) The Art of Multiprocessor Programming by Herlihy, Shavit et al. - https://shop.elsevier.com/books/the-art-of-multiprocessor-pr... The classic must-study book.

Both the above are not language specific but do have examples in Java.

(I actually authored a few of those problems - they cover the classic scenarios like producer-consumer, dining philosophers, etc.)

The cool stuff in concurrency is not having to deal with it imoand recognizing when its not essential. Also I hope you mean concurrency not parallelism. The second one is a bit more manageable.

If not, I suggest reading this blog post: https://prettygoodblog.com/p/what-threads-are-part-2

(disclaimer: I wrote this)