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

Thanks for your answer, I think I’m stuck with Java for the time being but I least I now cater to a modern codebase. I think it’s … alright. But I miss the function being truly a first class citizen.

Those advice ring familiar or interesting. I need to give effective Java a second look. It’s been year and I’m a different dev now.

God. Those example how to archive currying or composition are painful to read. Not wonder why I never write code this way in Java. Still. I want to be more diligent in that domain because I know it pays off down the line.