Disclaimer: I got this book as a review-copy from O’Reilly as part of their reviewer programme.
In general, I think the author’s primary goal was to help object oriented developers working with the JVM to take a step outside their comfort zone and explore functional programming without leaving the JVM completely. Introducing Java developers to mapping, filtering, and reducing now that Java 8 finally has some less verbose ways to handle that is definitely a good idea, but the book kind of just does some cherry picking here.
For example, you have to wait until page 137 for this sentence:
Functional architectures embrace immutability at the core level, leveraging it as much as possible. Embracing immutability is high on the list of ways to think like a functional programmer.
… which should perhaps have been in the first chapter. The whole idea of a functional actually being side-effect-free is also only hinted upon throughout some chapters but that’s it. Same goes for the term “higher-order function” which is used throughout the book without but only actually described on page 122.
The second big problem I have with this book, though, are the examples. The author tries to come up with simple examples but then always repeats them for Scala and Groovy and if possible Java. At first this seemed like a good approach but already after the second chapter it felt rather tedious and distracting. Scala and Closure are just such different beasts from Java that (despite the author trying very hard to help here) you definitely benefit from knowing at least the basics of each language for some of the examples. Sticking with the JVM throughout the book might have not been the best choice after all.
Chapter 2 which introduces map, reduce, and filter also suffers from quite a lot of repetition. Basically these basic functions are described multiple times throughout the chapter which really harms the reading flow.
Book’s website on oreilly.com
While I think the book had good intentions for me it is too focused on the JVM environment. The last chapter indicated that the author also had experience with other things outside the JVM and .NET world so it would have been nice to see some of this here. That, the unfortunate order of introducing functional characteristics and the length of the mostly repeating examples that distract from the actual functional concepts while focusing too much on the implementation make this really hard to recommend.
Over the years I've written quite a few reviews 🙂 You can find them at /reviews/.