Programming Scala: Tackle Multi-Core Complexity on the Java Virtual Machine

226 Pages·2009·1.306 MB·English

by  Venkat Subramaniam

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What Readers Are Saying About Programming Scala
This books speaks directly to developers faced with the real and hard
problems of concurrency. It offers clear solutions for building actors
on the JVM.
John Heintz
Principal, Gist Labs
Venkat introduces (Java) developers to programming Scala in an easy-
to-follow, pragmatic style. This book covers Scala from basics to con-
currency, one of the most critical and most difficult topics in program-
ming today. Venkat gets to the meat without any fluff, and I highly
recommend this book to get you up to speed on Scala quickly.
Scott Leberknight
Chief architect,Near Infinity Corporation
Once again Venkat has made learning easy and fun. With his conver-
sational style, this book allows developers to quickly learn the Scala
language, its uniqueness, and how it can be best utilized in a multi-
language environment.
Ian Roughley
Consultant, Down & Around, Inc.
Multicore processors demand that developers have a solid grounding
in the functional programming concepts found at the core of Scala.
Venkat provides a great guide to get you started with this exciting new
language.
Nathaniel T. Schutta
Author, speaker, teacher
A pleasure to read! A great introduction to Scala for the experienced
Java developer! This book teaches the “Scala way” of programming
froma Java, object-oriented perspective. Very thorough yet concise.
Albert Scherer
Softwarearchitect, Follett Higher Education Group, I
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Concurrency is the next giant challenge we must face as develop-
ers, and traditional imperative languages make it too hard. Scala is a
functional language on the JVM that offers easy multithreading,con-
cise syntax, and seamless Java interop. This book guides Java devel-
opers through the important capabilities and nuances of Scala, show-
ing why so much interest is bubbling around this new language.
Neal Ford
Softwarearchitect/meme wrangler, ThoughtWorks, Inc.
Programming Scala is concise, easy to read, and thorough...one of the
best introductions to Scala currently available. It’s a must-read for the
programmer who wants to stay relevant as we enter the era of ubiqui-
tous multicore processing. This is one of the books that I will go back
to, time and again, in the coming years.
Arild Shirazi
Senior software developer, CodeSherpas, Inc.
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Programming Scala
Tackle Multicore Complexity on the JVM
Venkat Subramaniam
The Pragmatic Bookshelf
Raleigh,NorthCarolina Dallas,Texas
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Many of the designations used by manufacturers and sellers to distinguish their prod-
uctsareclaimedastrademarks.Wherethosedesignationsappearinthisbook,andThe
Pragmatic Programmers, LLC was aware of a trademark claim, the designations have
been printed in initial capital letters or in all capitals. The Pragmatic Starter Kit, The
PragmaticProgrammer,PragmaticProgramming,PragmaticBookshelfandthelinkingg
devicearetrademarksofThePragmaticProgrammers,LLC.
Every precaution was taken in the preparation of this book. However, the publisher
assumesnoresponsibility for errorsor omissions,or for damagesthatmay result from
theuseofinformation(includingprogramlistings)containedherein.
Our Pragmatic courses, workshops, and other products can help you and your team
create better software and have more fun. For more information, as well as the latest
Pragmatictitles,pleasevisitusat
http://www.pragprog.com
Copyright©2008VenkatSubramaniam.
Allrightsreserved.
Nopartofthispublicationmaybereproduced,storedinaretrievalsystem,ortransmit-
ted, in any form, or by any means, electronic,mechanical,photocopying, recording, or
otherwise,withoutthepriorconsentofthepublisher.
PrintedintheUnitedStatesofAmerica.
ISBN-10:1-934356-31-X
ISBN-13:978-1-934356-31-9
Printedonacid-freepaper.
P1.0printing,June2009
Version:2009-7-7
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Contents
1 Introduction 11
1.1 Why Scala? . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.2 What’s Scala? . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3 Functional Programming . . . . . . . . . . . . . . . . . . 19
1.4 What’s in This Book? . . . . . . . . . . . . . . . . . . . . 22
1.5 Who Is This Book For? . . . . . . . . . . . . . . . . . . . 24
1.6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 24
2 Getting Started 26
2.1 Downloading Scala . . . . . . . . . . . . . . . . . . . . . 26
2.2 Installing Scala . . . . . . . . . . . . . . . . . . . . . . . 27
2.3 Take Scala for a Ride . . . . . . . . . . . . . . . . . . . . 28
2.4 Scala on the Command Line . . . . . . . . . . . . . . . . 30
2.5 Running Scala Code as a Script . . . . . . . . . . . . . . 31
2.6 Scala from an IDE . . . . . . . . . . . . . . . . . . . . . . 32
2.7 Compiling Scala . . . . . . . . . . . . . . . . . . . . . . . 32
3 Getting Up to Speed in Scala 34
3.1 Scala as Concise Java . . . . . . . . . . . . . . . . . . . 34
3.2 Scala Classes for Java Primitives . . . . . . . . . . . . . 37
3.3 Tuples and Multiple Assignments . . . . . . . . . . . . . 38
3.4 Stringsand Multiline Raw Strings . . . . . . . . . . . . 40
3.5 Sensible Defaults . . . . . . . . . . . . . . . . . . . . . . 41
3.6 Operator Overloading . . . . . . . . . . . . . . . . . . . . 43
3.7 Scala Surprises for the Java Eyes. . . . . . . . . . . . . 45
4 Classes in Scala 53
4.1 Creating Classes. . . . . . . . . . . . . . . . . . . . . . . 53
4.2 Defining Fields, Methods, and Constructors . . . . . . 54
4.3 Extending a Class . . . . . . . . . . . . . . . . . . . . . . 57
4.4 Singleton Object . . . . . . . . . . . . . . . . . . . . . . . 58
4.5 Stand-Alone and Companion Objects . . . . . . . . . . 60
4.6 static in Scala . . . . . . . . . . . . . . . . . . . . . . . . 61
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5 Sensible Typing 63
5.1 Collections and Type Inference . . . . . . . . . . . . . . 64
5.2 The Any Type . . . . . . . . . . . . . . . . . . . . . . . . 66
5.3 More About Nothing. . . . . . . . . . . . . . . . . . . . . 67
5.4 Option Type . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.5 Method Return Type Inference . . . . . . . . . . . . . . 69
5.6 Passing Variable Arguments (Varargs) . . . . . . . . . . 70
5.7 Variance of Parameterized Type . . . . . . . . . . . . . . 71
6 Function Values and Closures 75
6.1 Moving from Normal to Higher-Order Functions . . . . 75
6.2 Function Values . . . . . . . . . . . . . . . . . . . . . . . 76
6.3 Function Values with Multiple Parameters . . . . . . . 78
6.4 Currying . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
6.5 Reusing Function Values . . . . . . . . . . . . . . . . . . 81
6.6 Positional Notation for Parameters . . . . . . . . . . . . 83
6.7 Execute Around Method Pattern . . . . . . . . . . . . . 84
6.8 Partially Applied Functions . . . . . . . . . . . . . . . . 87
6.9 Closures . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
7 Traits and Type Conversions 91
7.1 Traits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
7.2 Selective Mixins . . . . . . . . . . . . . . . . . . . . . . . 94
7.3 Decorating with Traits . . . . . . . . . . . . . . . . . . . 95
7.4 Method Late Binding in Traits . . . . . . . . . . . . . . . 97
7.5 Implicit Type Conversions . . . . . . . . . . . . . . . . . 99
8 Using Collections 103
8.1 Common Scala Collections . . . . . . . . . . . . . . . . . 103
8.2 Using a Set . . . . . . . . . . . . . . . . . . . . . . . . . . 104
8.3 Using a Map . . . . . . . . . . . . . . . . . . . . . . . . . 106
8.4 Using a List . . . . . . . . . . . . . . . . . . . . . . . . . 108
8.5 The for Expression . . . . . . . . . . . . . . . . . . . . . 113
9 Pattern Matching and Regular Expressions 116
9.1 Matching Literals and Constants . . . . . . . . . . . . . 116
9.2 Matching a Wildcard . . . . . . . . . . . . . . . . . . . . 117
9.3 Matching Tuples and Lists . . . . . . . . . . . . . . . . . 118
9.4 Matching with Types and Guards . . . . . . . . . . . . . 119
9.5 Pattern Variables and Constants in case Expressions . 120
9.6 Pattern Matching XML Fragments . . . . . . . . . . . . 121
9.7 Matching Using case Classes . . . . . . . . . . . . . . . 121
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9.8 Matching Using Extractors. . . . . . . . . . . . . . . . . 124
9.9 Regular Expressions . . . . . . . . . . . . . . . . . . . . 128
9.10 Regular Expressions as Extractors . . . . . . . . . . . . 129
10 Concurrent Programming 131
10.1 Promote Immutability. . . . . . . . . . . . . . . . . . . . 131
10.2 Concurrency Using Actor. . . . . . . . . . . . . . . . . . 133
10.3 Message Passing . . . . . . . . . . . . . . . . . . . . . . . 137
10.4 The Actor Class . . . . . . . . . . . . . . . . . . . . . . . 139
10.5 The actor Method . . . . . . . . . . . . . . . . . . . . . . 141
10.6 receive and receiveWithin Methods . . . . . . . . . . . . 144
10.7 react and reactWithin Methods . . . . . . . . . . . . . . 146
10.8 loop and loopWhile . . . . . . . . . . . . . . . . . . . . . 151
10.9 Controlling Thread of Execution . . . . . . . . . . . . . 153
10.10 Choosing Among the Receive Methods . . . . . . . . . . 154
11 Intermixing with Java 156
11.1 Using Scala Classes in Scala . . . . . . . . . . . . . . . 156
11.2 Using Java Classes in Scala . . . . . . . . . . . . . . . . 159
11.3 Using Scala Classes in Java . . . . . . . . . . . . . . . . 161
11.4 Extending Classes . . . . . . . . . . . . . . . . . . . . . . 165
12 Unit Testing with Scala 167
12.1 Using JUnit . . . . . . . . . . . . . . . . . . . . . . . . . 167
12.2 Using ScalaTest . . . . . . . . . . . . . . . . . . . . . . . 169
12.3 Start with a Canary Test . . . . . . . . . . . . . . . . . . 169
12.4 Using Runner . . . . . . . . . . . . . . . . . . . . . . . . 170
12.5 Asserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
12.6 Exception Tests . . . . . . . . . . . . . . . . . . . . . . . 174
12.7 Sharing Code Between Tests. . . . . . . . . . . . . . . . 176
12.8 Functional Style with FunSuite . . . . . . . . . . . . . . 178
12.9 Running ScalaTests Using JUnit . . . . . . . . . . . . . 179
13 Exception Handling 183
13.1 Exception Handling . . . . . . . . . . . . . . . . . . . . . 183
13.2 Mind the Catch Order . . . . . . . . . . . . . . . . . . . 186
14 Using Scala 187
14.1 The Net Asset Application . . . . . . . . . . . . . . . . . 187
14.2 Getting Users’ Input . . . . . . . . . . . . . . . . . . . . 187
14.3 Reading and WritingFiles . . . . . . . . . . . . . . . . . 188
14.4 XML as a First-Class Citizen. . . . . . . . . . . . . . . . 190
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14.5 Reading and WritingXML . . . . . . . . . . . . . . . . . 193
14.6 Getting Stock Prices from the Web . . . . . . . . . . . . 196
14.7 Making the Net Asset Application Concurrent . . . . . 199
14.8 Putting a GUI on the Net Asset Application . . . . . . . 201
A Web Resources 211
B Bibliography 213
Index 215
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