After Java Collections, the next, most crucial API in Java is about multithreading and concurrency, and I firmly believe that if you want to be a competent Java developer, you must have a solid understanding and command of Java Concurrency API.

You should not only have an in-depth understanding of fundamental concepts like Thread, Runnable, Object locking, and Synchronization, but you should also be familiar with concepts like deadlock, livelock, race conditions, and how to deal with them.

You should also learn about advanced Java concepts like synchronizers added on Java 5 and subsequent versions, I mean CyclicBarrier, CountDownLatch, Phaser, and CompleteableFuture, etc., along with Futures and how to perform the async operation in Java.

I know, it’s a lot of stuff, and that’s why I suggest you join an in-depth course like Java Concurrency in Practice bundle from Heinz Kabutz, a Java Champion, and authority when it comes to concurrency and design patterns. This course is a bit expensive, but you will learn a lot more concepts in-depth, which makes it completely worth your time and money.