Why should a Java developer learn about functional programming (FP)? After all, hasn’t functional programming been safely hidden in academia for decades? Isn’t object-oriented programming (OOP) all we really need? This book explains why functional programming has become an important tool for the challenges of our time and how you, a Java developer, can use it to your advantage.

The recent interest in functional programming started as a response to the growing pervasiveness of concurrency as a way of scaling horizontally, through parallelism. Multithreaded programming (see, e.g., [Goetz2006]) is difficult to do well and few developers are good at it. As we’ll see, functional programming offers better strategies for writing robust, concurrent software.

An example of the greater need for horizontal scalability is the growth of massive data sets requiring management and analysis, the so-called big data trend. These are data sets that are too large for traditional database management systems. They require clusters of computers to store and process the data. Today, it’s not just Google, Yahoo!, Facebook, and Twitter who work with big data. Many organizations face this challenge.

Once you learn the benefits of functional programming, you find that it improves all the code you write. When I learned functional programming a few years ago, it re-energized my enthusiasm for programming. I saw new, exciting ways to approach old problems. The rigor of functional programming complemented the design and testing benefits of test-driven development, giving me greater confidence in my work. I learned functional programming using the Scala programming language [Scala] and co-wrote a book on Scala with Alex Payne, called Programming Scala (O’Reilly). Scala is a JVM language, a potential successor to Java, with the goal of bringing object-oriented and functional programming into one coherent whole. Clojure is the other well-known functional language on the JVM. It is a Lisp dialect that minimizes the use of OOP in favor of functional programming. Clojure embodies a powerful vision for how programming should be done.

Fortunately, you don’t have to adopt a new language to enjoy many of the benefits of functional programming. Back in early 1990s, I used an object-oriented approach in the C software I wrote, until I could use C++. Similarly, if you’re working with an object-oriented language, like Java, you can still apply many of the ideas from functional programming.

Unfortunately, much of the literature on functional programming is difficult to understand for people who are new to it. This short book offers a pragmatic, approachable introduction to functional programming. While aimed at the Java developer, the principles are general and will benefit anyone familiar with an object-oriented language.

I assume that you are well versed in object-oriented programming and you can read Java code. You’ll find some exercises at the end of each chapter to help you practice and expand on what you’ve learned.

Because this is a short introduction and because it is difficult to represent some functional concepts in Java, there will be several topics that I won’t discuss in the text, although I have added glossary entries, for completeness. These topics include currying, partial application, and comprehensions. I’ll briefly discuss several other topics, such as combinators, laziness, and monads, to give you a taste of their importance. However, fully understanding these topics isn’t necessary when you’re new to functional programming.

I hope you find functional programming as seductive as I did. Let me know how it goes!

You can learn more at the book’s catalog page (http://oreilly.com/catalog/9781449311032/).

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I want to think my editor at O’Reilly, Mike Loukides, who suggested that I write this book. Brendan McNichols and Bobby Norton provided helpful feedback on drafts of the book. Debasish Ghosh provided valuable comments on the Liskov Substitution Principle and suggested the Olin Shivers quotes on the meaning of foldLeft and foldRight [Shivers]. Daniel Spiewak provided invaluable feedback that helped clarify many of the concepts in the book, such as Monads.

I have learned a lot about functional programming from fellow developers around the world, many of whom are fellow Scala enthusiasts. Martin Odersky, Jonas Bonér, Debasish Ghosh, James Iry, Daniel Spiewak, Simon Peyton Jones, Rich Hickey, Conal Elliot, David Pollak, Paul Snively, and others have illuminated dark corners with their writing, speaking, personal conversations, and code! Finally, my fellow members of the Chicago Area Scala Enthusiasts (CASE) group have also been a source of valuable feedback and inspiration over the last several years.

Of course, any errors and omissions are mine alone.