Effective Java [0] is the Java book. It covers up to Java 9, and I expect the author will release another edition for Java 17 at some point.

Java 9-17 adds some major language features, but they're not so major that you'd really need a whole book. You could look up the individual JDK release notes and see what each release changed.

[0] https://www.oreilly.com/library/view/effective-java/97801346...

Effective Java [0] is the single most important book I can recommend for any Java programmer. This article [1] gives an overview of some of the primitives you can use for writing functional Java. Lombok [2] is a very common library that makes writing functional Java much more ergonomic with its `@Value` annotation, but that might not be needed anymore with Java 14's Record types [3].

The book Clean Code [4] helped me a lot to really learn how to write clean Java, and many of these ideas directly translate into writing good functional code. One of the key takeaways was just how small functions should be which incidentally is a great thing to learn when functions are your main unit of composition.

Java isn't a purely functional language, so you obviously will always have some impurity regarding state/mutation. I personally try to do the following: 1. Keep all state in some top-level class and let everything else be immutable 2. Nearly every class I write is immutable 3. Follow common OOP principles like SOLID 4. Write reactive code with heavy use of Optional and Streams

Here's [5] an example repo of board game written using those ideas.

[0] https://www.oreilly.com/library/view/effective-java/97801346... [1] https://www.baeldung.com/java-functional-programming [2] https://projectlombok.org/ [3] https://www.baeldung.com/java-record-keyword [4] https://smile.amazon.com/Clean-Code-Handbook-Software-Crafts... [5] https://github.com/harding-capstone/logic