Clojure for Java Developers

Table of Contents

Clojure for Java Developers

Credits

About the Author

About the Reviewer

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Why subscribe?

Preface

What this book covers

What you need for this book

Who this book is for

Conventions

Reader feedback

Customer support

Downloading the example code

Downloading the color images of this book

Piracy

Questions

1. Getting Started with Clojure

Getting to know Clojure

Installing Leiningen

Using a REPL

The nREPL protocol

Hello world

REPL utilities and conventions

Creating a new project

Project structure

Creating a standalone app

Using Cursive Clojure

Installing Cursive Clojure

Getting started with Clojure code and data

Lists in Clojure

Operations in Clojure

Functions in Clojure

Clojure's data types

Scalars

Collection data types

Summary

2. Namespaces, Packages, and Tests

Namespaces in Clojure

Packages in Clojure

The classpath and the classloader

Back to Clojure namespaces

Playing with namespaces

Creating a new namespace

Working with namespaces on the REPL

Testing in Clojure

Testing from the command line

Testing in IntelliJ

Summary

3. Interacting with Java

Using Maven dependencies

Clojure interop syntax

Creating an object

Calling an instance method

Calling a static method or function

Accessing inner classes

Writing a simple image namespace

Writing the tests

The let statement

Destructuring in Clojure

Sequential destructuring

Associative destructuring

Exposing your code to Java

Testing from Groovy

Proxy and reify

Summary

4. Collections and Functional Programming

Basics of functional programming

Persistent collections

Types of collections in Clojure

The sequence abstraction

Specific collection types in Clojure

Vectors

Lists

Maps

Sorted maps and hash maps

Common properties

Sets

Sorted sets and hash sets

Common properties

Union, difference, and intersection

Applying functional programming to collections

The imperative programming model

The functional paradigm

Functional programming and immutability

Laziness

Summary

5. Multimethods and Protocols

Polymorphism in Java

Multimethods in Clojure

Keyword hierarchies

isa?

parents

descendants

underive

A la carte dispatch functions

Protocols in Clojure

Records in Clojure

Summary

6. Concurrency

Using your Java knowledge

The Clojure model of state and identity

Promises

Pulsar and lightweight threads

Futures

Software transactional memory and refs

Atoms

Agents

Validators

Watchers

core.async

Why lightweight threads?

Goblocks

Channels

Transducers

Summary

7. Macros in Clojure

Lisp's foundational ideas

Macros as code modification tools

Modifying code in Java

Modifying code in Groovy

The @ToString annotation

The @TupleConstructor annotation

The @Slf4j annotation

Writing your first macro

Debugging your first macro

Quote, syntax quote, and unquoting

Unquote splicing

gensym

Macros in the real world

References

Summary

Index

Clojure for Java Developers

Clojure for Java Developers

Copyright © 2016 Packt Publishing

All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embedded in critical articles or reviews.

Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book is sold without warranty, either express or implied. Neither the author, nor Packt Publishing, and its dealers and distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book.

Packt Publishing has endeavored to provide trademark information about all of the companies and products mentioned in this book by the appropriate use of capitals. However, Packt Publishing cannot guarantee the accuracy of this information.

First published: February 2016

Production reference: 1190216

Published by Packt Publishing Ltd.

Livery Place

35 Livery Street

Birmingham B3 2PB, UK.

ISBN 978-1-78528-150-1

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Credits

Author

Eduardo Díaz

Reviewers

Ning Sun

Nate West

Commissioning Editor

Kunal Parikh

Acquisition Editor

Usha Iyer

Content Development Editors

Neeshma Ramakrishnan

Kajal Thapar

Technical Editor

Saurabh Malhotra

Copy Editor

Sneha Singh

Project Coordinator

Shweta H. Birwatkar

Proofreaders

Safis Editing

Indexer

Hemangini Bari

Production Coordinator

Shantanu N. Zagade

Cover Work

Shantanu N. Zagade

About the Author

Eduardo Díaz is a developer with a strong background in the Java language. He has a passion for functional programming and new programming paradigms. His work includes full stack development, systems design, and high volume real time data processing.

He has worked on every technology related problem you can imagine, as a consultant solving anything related to Java, UNIX, C, or any other strange problem you might have had.

As a developer, he has been working for around 10 years on Java, Python, Scala, Clojure, in the media, bank, and primarily communications industries.

He is currently working at Grupo Expansion, a media company, where he helps design and implement a new content delivery platform aiming to empower content editors and encourage developers to find new ways to use data.

First of all, I would like to thank Neeshma Ramakrishnan and Kajal Thapar at Packt Publishing, they are the best editors, they have an incredible amount of patience and without them, there would be nothing even close to a book.

I would also like to thank my colleagues and friends at Grupo Expansion, Lucasian Labs, and Javanes; they have helped me grow in every way imaginable. Thanks for everything!

Last but not the least, thanks to my family for always keeping up with me, cheering me up, believing in me and helping in everything I do, even when it means not seeing me for weeks or months. I truly value your support!

About the Reviewer

Ning Sun is a software engineer currently working for a China-based startup, LeanCloud, providing one-stop backend as a service for mobile apps. Being a startup engineer, he has to solve various kinds of problems and play different kinds of roles; however, he has always been an enthusiast for open source technology. He contributes to several open source projects and has also learned a lot from them.

At LeanCloud, he built a messaging system that supports tens of millions of clients per day. The system is fully powered by Clojure and its ecosystem. He has been an early member of Clojure Chinese community since 2011. He has delivered talks at several meetups and has been very active in the mailing list and open source projects. Ning created Clojure RPC library Slacker, which is widely used at LeanCloud and several other companies.

He worked on https://Delicious.com in 2013, which is known as one of the most important websites in the early Web 2.0 era.

He has been a reviewer of two books about Solr at Packt Publishing. Also, he was the reviewer of Programming Clojure, Chinese edition.

You can always find him on Github.com/sunng87 and Twitter.com/Sunng.

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Preface

In the last few years, we have seen a widespread tendency to create new languages for JVM. There are all sorts of new languages with different paradigms and different ways of working.

Clojure is one of those languages, one that we believe is worth learning.

Over the course of this book, you will learn about Clojure and how opinionated it is. You will learn why immutable objects are not only possible, but it is a good idea to use them.

You will learn about functional programming and see how it fits the concept of immutable programs.

You will understand the very powerful idea of representing your code as a data structure of the same language.

It is important to note that we will build all this knowledge on top of what you already know; this book assumes that you understand the Java language and a bit of how it works. It assumes that you understand how to create classes and objects, how to call methods, and also a bit about the JVM. We will find similarities and differences from the Java language that you already know, so you can understand how the Clojure world works.

It is said that a great programmer is not the one who knows a lot of different programming languages, it is someone who knows different programming paradigms.

Clojure brings ideas from Lisp and functional programming, which are different to what you are used to. This book will help you understand the power behind these ideas and why a language so old still exists and has a place.

Learning Lisp is one of the great pleasures of life, I hope you enjoy it!

What this book covers

Chapter 1, Getting Started with Clojure, is your first step with Clojure, from how to install an interpreter, how to use the IntelliJ plugin Cursive Clojure, and how to start writing your first lines of code in Clojure.

Chapter 2, Namespaces, Packages, and Tests, deals with how every other language needs a way to organize our code. We do it with namespaces, and we start learning by doing little experiments and by comparing with the Java packages we already know.

Chapter 3, Interacting with Java, discusses the fact that one of the most useful features of Clojure is that it can be hosted on top of other platforms. Java is probably one of the most common platforms, and here we learn how to interact with Java code and libraries and how to expose our Clojure code to Java.

Chapter 4, Collections and Functional Programming, tells us that functional programming and immutable data structures (or collections) are fundamental to programming in the Clojure language; here we understand how we can write meaningful programs using immutable data structures and how functional programming is ideal for that.

Chapter 5, Multimethods and Protocols,