Venkat Subramaniam - Functional Programming in Java: Harnessing the Power Of Java 8 Lambda Expressions

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To the loving memory of my grandmothers, Kuppammal and Jayalakshmi. I cherish my wonder years under your care.

Venkat Subramaniam would never be described as a waterfall sort of guy. So, when he mentioned that he was starting on a Java 8 booklong before the design of Java 8 was settledI was not at all surprised. It was clear this was going to be an agile book project.

Despite having to more than occasionally rework the text as the language and library features evolved, Venkat had a secret advantagehe knew where we were going. The Java 8 design was heavily influenced by the core principles of functional programming: functions as values, immutability, and statelessness. We didnt do this because functional programming is trendy or cool; we did it because programs that are expressed as stateless transformations on immutable data, rather than as modifications of mutable data structures, tend to be easier to read and maintain, to be less error prone, and to parallelize more gracefully.

The new features introduced in Java 8 were designed together to facilitate development of more expressive and parallel-friendly libraries. Lambda expressions reduce the syntactic overhead of encoding behavior as data; default methods allow existing libraries to evolve over time, enabling core JDK classes such as Collections to take advantage of lambda expressions; the java.util.stream package leverages these language features to offer rich aggregate operations on collections, arrays, and other data sources in a declarative, expressive, and parallel-friendly manner. All of this adds up to more powerful and performant code, as well as a more pleasant programming experience.

This book not only provides lots of examples of how to use these new features, but offers readers a peek into the underlying principles behind their design, and why they result in better code. Let Venkat be your guide to this new and improved Javayoure in for a treat.

Writing a book is like taking a road tripwe know where were heading, but some details emerge only after the start. The journey may involve detoursopportunities to explore unexpected placesand its more fun with good company. Im thankful for the great company of people on this voyage: smart reviewers, an amazing editor, a wonderful set of people at The Pragmatic Bookshelf, and a very supportive family.

I first thank the Java language team members at Oracle for their hard work to bring the functional style of programming to one of the most popular mainstream languages. Youve taken the language in the right directionnot through shortcuts and quick fixes, but by way of sound reasoning and prudent design decisions. Kudos, team.

I express my sincere gratitude to the smart developers who volunteered their personal time to review this book. Thank you, Kimberly Barnes, Fred Daoud, Raju Gandhi, Marty Hall, Praveen Kumar, Rebecca Parsons, Kirk Pepperdine, Chris Richardson, Ian Roughley, Nate Schutta, Ken Sipe, and Dan Sline. Your comments were critical yet very constructive and motivational; they helped make this book better. Im honored and humbled by Bruce Tate reviewing this book. Hes been a great mentor for me over the years. He reviewed this book multiple times, at different stages, and took the time to motivate me as to why certain changes were essential. Thank you, Bruce. Id also like to express my gratitude to Brian Goetz for reviewing the book multiple times, for the encouragement starting early on, and for kindly agreeing to write the foreword. Any errors in the book are solely mine.