scala cookbook

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Alvin Alexander

Scala Cookbook

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Scala Cookbook

by Alvin Alexander

Copyright © 2013 Alvin Alexander. All rights reserved.

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August 2013: First Edition

Revision History for the First Edition:

2013-07-30: First release

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ISBN: 978-1-449-33961-6

[LSI]

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For my mom, who loves cookbooks.

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Table of Contents

Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

1. Strings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1. Testing String Equality 4

1.2. Creating Multiline Strings 6

1.3. Splitting Strings 8

1.4. Substituting Variables into Strings 9

1.5. Processing a String One Character at a Time 13

1.6. Finding Patterns in Strings 18

1.7. Replacing Patterns in Strings 21

1.8. Extracting Parts of a String That Match Patterns 22

1.9. Accessing a Character in a String 24

1.10. Add Your Own Methods to the String Class 25

2. Numbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.1. Parsing a Number from a String 32

2.2. Converting Between Numeric Types (Casting) 36

2.3. Overriding the Default Numeric Type 37

2.4. Replacements for ++ and −− 39

2.5. Comparing Floating-Point Numbers 41

2.6. Handling Very Large Numbers 43

2.7. Generating Random Numbers 45

2.8. Creating a Range, List, or Array of Numbers 47

2.9. Formatting Numbers and Currency 49

3. Control Structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.1. Looping with for and foreach 54

3.2. Using for Loops with Multiple Counters 60

3.3. Using a for Loop with Embedded if Statements (Guards) 62

3.4. Creating a for Comprehension (for/yield Combination) 63

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3.5. Implementing break and continue 65

3.6. Using the if Construct Like a Ternary Operator 71

3.7. Using a Match Expression Like a switch Statement 72

3.8. Matching Multiple Conditions with One Case Statement 76

3.9. Assigning the Result of a Match Expression to a Variable 77

3.10. Accessing the Value of the Default Case in a Match Expression 78

3.11. Using Pattern Matching in Match Expressions 79

3.12. Using Case Classes in Match Expressions 86

3.13. Adding if Expressions (Guards) to Case Statements 87

3.14. Using a Match Expression Instead of isInstanceOf 88

3.15. Working with a List in a Match Expression 89

3.16. Matching One or More Exceptions with try/catch 91

3.17. Declaring a Variable Before Using It in a try/catch/finally Block 92

3.18. Creating Your Own Control Structures 95

4. Classes and Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

4.1. Creating a Primary Constructor 100

4.2. Controlling the Visibility of Constructor Fields 104

4.3. Defining Auxiliary Constructors 108

4.4. Defining a Private Primary Constructor 112

4.5. Providing Default Values for Constructor Parameters 114

4.6. Overriding Default Accessors and Mutators 116

4.7. Preventing Getter and Setter Methods from Being Generated 119

4.8. Assigning a Field to a Block or Function 121

4.9. Setting Uninitialized var Field Types 122

4.10. Handling Constructor Parameters When Extending a Class 124

4.11. Calling a Superclass Constructor 127

4.12. When to Use an Abstract Class 129

4.13. Defining Properties in an Abstract Base Class (or Trait) 131

4.14. Generating Boilerplate Code with Case Classes 136

4.15. Defining an equals Method (Object Equality) 140

4.16. Creating Inner Classes 143

5. Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

5.1. Controlling Method Scope 148

5.2. Calling a Method on a Superclass 152

5.3. Setting Default Values for Method Parameters 154

5.4. Using Parameter Names When Calling a Method 157

5.5. Defining a Method That Returns Multiple Items (Tuples) 159

5.6. Forcing Callers to Leave Parentheses off Accessor Methods 161

5.7. Creating Methods That Take Variable-Argument Fields 163

5.8. Declaring That a Method Can Throw an Exception 165

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5.9. Supporting a Fluent Style of Programming 167

6. Objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

6.1. Object Casting 172

6.2. The Scala Equivalent of Java’s .class 174

6.3. Determining the Class of an Object 174

6.4. Launching an Application with an Object 176

6.5. Creating Singletons with object 178

6.6. Creating Static Members with Companion Objects 180

6.7. Putting Common Code in Package Objects 182

6.8. Creating Object Instances Without Using the new Keyword 185

6.9. Implement the Factory Method in Scala with apply 189

7. Packaging and Imports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

7.1. Packaging with the Curly Braces Style Notation 192

7.2. Importing One or More Members 193

7.3. Renaming Members on Import 195

7.4. Hiding a Class During the Import Process 196

7.5. Using Static Imports 197

7.6. Using Import Statements Anywhere 199

8. Traits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

8.1. Using a Trait as an Interface 203

8.2. Using Abstract and Concrete Fields in Traits 206

8.3. Using a Trait Like an Abstract Class 207

8.4. Using Traits as Simple Mixins 208

8.5. Limiting Which Classes Can Use a Trait by Inheritance 209

8.6. Marking Traits So They Can Only Be Used by Subclasses of a Certain

Type 211

8.7. Ensuring a Trait Can Only Be Added to a Type That Has a Specific

Method 213

8.8. Adding a Trait to an Object Instance 215

8.9. Extending a Java Interface Like a Trait 216

9. Functional Programming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

9.1. Using Function Literals (Anonymous Functions) 218

9.2. Using Functions as Variables 219

9.3. Defining a Method That Accepts a Simple Function Parameter 223

9.4. More Complex Functions 226

9.5. Using Closures 229

9.6. Using Partially Applied Functions 234

9.7. Creating a Function That Returns a Function 236

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9.8. Creating Partial Functions 238

9.9. A Real-World Example 242

10. Collections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

10.1. Understanding the Collections Hierarchy 246

10.2. Choosing a Collection Class 250

10.3. Choosing a Collection Method to Solve a Problem 255

10.4. Understanding the Performance of Collections 261

10.5. Declaring a Type When Creating a Collection 264

10.6. Understanding Mutable Variables with Immutable Collections 265

10.7. Make Vector Your “Go To” Immutable Sequence 266

10.8. Make ArrayBuffer Your “Go To” Mutable Sequence 268

10.9. Looping over a Collection with foreach 270

10.10. Looping over a Collection with a for Loop 272

10.11. Using zipWithIndex or zip to Create Loop Counters 276

10.12. Using Iterators 278

10.13. Transforming One Collection to Another with for/yield 279

10.14. Transforming One Collection to Another with map 282

10.15. Flattening a List of Lists with flatten 285

10.16. Combining map and flatten with flatMap 286

10.17. Using filter to Filter a Collection 289

10.18. Extracting a Sequence of Elements from a Collection 291

10.19. Splitting Sequences into Subsets (groupBy, partition, etc.) 293

10.20. Walking Through a Collection with the reduce and fold Methods 295

10.21. Extracting Unique Elements from a Sequence 300

10.22. Merging Sequential Collections 302

10.23. Merging Two Sequential Collections into Pairs with zip 304

10.24. Creating a Lazy View on a Collection 306

10.25. Populating a Collection with a Range 309

10.26. Creating and Using Enumerations 311

10.27. Tuples, for When You Just Need a Bag of Things 312

10.28. Sorting a Collection 315

10.29. Converting a Collection to a String with mkString 318

11. List, Array, Map, Set (and More). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

11.1. Different Ways to Create and Populate a List 322

11.2. Creating a Mutable List 324

11.3. Adding Elements to a List 325

11.4. Deleting Elements from a List (or ListBuffer) 328

11.5. Merging (Concatenating) Lists 330

11.6. Using Stream, a Lazy Version of a List 331

11.7. Different Ways to Create and Update an Array 333

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11.8. Creating an Array Whose Size Can Change (ArrayBuffer) 335

11.9. Deleting Array and ArrayBuffer Elements 335

11.10. Sorting Arrays 337

11.11. Creating Multidimensional Arrays 338

11.12. Creating Maps 341

11.13. Choosing a Map Implementation 343

11.14. Adding, Updating, and Removing Elements with a Mutable Map 345

11.15. Adding, Updating, and Removing Elements with Immutable Maps 347

11.16. Accessing Map Values 349

11.17. Traversing a Map 350

11.18. Getting the Keys or Values from a Map 352

11.19. Reversing Keys and Values 352

11.20. Testing for the Existence of a Key or Value in a Map 353

11.21. Filtering a Map 354

11.22. Sorting an Existing Map by Key or Value 357

11.23. Finding the Largest Key or Value in a Map 360

11.24. Adding Elements to a Set 361

11.25. Deleting Elements from Sets 363

11.26. Using Sortable Sets 365

11.27. Using a Queue 367

11.28. Using a Stack 369

11.29. Using a Range 371

12. Files and Processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

12.1. How to Open and Read a Text File 375

12.2. Writing Text Files 381

12.3. Reading and Writing Binary Files 382

12.4. How to Process Every Character in a Text File 383

12.5. How to Process a CSV File 384

12.6. Pretending that a String Is a File 387

12.7. Using Serialization 389

12.8. Listing Files in a Directory 391

12.9. Listing Subdirectories Beneath a Directory 392

12.10. Executing External Commands 394

12.11. Executing External Commands and Using STDOUT 397

12.12. Handling STDOUT and STDERR for External Commands 399

12.13. Building a Pipeline of Commands 401

12.14. Redirecting the STDOUT and STDIN of External Commands 402

12.15. Using AND (&&) and OR (||) with Processes 404

12.16. Handling Wildcard Characters in External Commands 405

12.17. How to Run a Process in a Different Directory 406

12.18. Setting Environment Variables When Running Commands 407

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12.19. An Index of Methods to Execute External Commands 408

13. Actors and Concurrency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

13.1. Getting Started with a Simple Actor 414

13.2. Creating an Actor Whose Class Constructor Requires Arguments 418

13.3. How to Communicate Between Actors 419

13.4. Understanding the Methods in the Akka Actor Lifecycle 422

13.5. Starting an Actor 425

13.6. Stopping Actors 427

13.7. Shutting Down the Akka Actor System 432

13.8. Monitoring the Death of an Actor with watch 433

13.9. Simple Concurrency with Futures 436

13.10. Sending a Message to an Actor and Waiting for a Reply 445

13.11. Switching Between Different States with become 446

13.12. Using Parallel Collections 448

14. Command-Line Tasks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453

14.1. Getting Started with the Scala REPL 454

14.2. Pasting and Loading Blocks of Code into the REPL 459

14.3. Adding JAR Files and Classes to the REPL Classpath 461

14.4. Running a Shell Command from the REPL 462

14.5. Compiling with scalac and Running with scala 465

14.6. Disassembling and Decompiling Scala Code 466

14.7. Finding Scala Libraries 471

14.8. Generating Documentation with scaladoc 472

14.9. Faster Command-Line Compiling with fsc 479

14.10. Using Scala as a Scripting Language 480

14.11. Accessing Command-Line Arguments from a Script 483

14.12. Prompting for Input from a Scala Shell Script 485

14.13. Make Your Scala Scripts Run Faster 489

15. Web Services. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491

15.1. Creating a JSON String from a Scala Object 491

15.2. Creating a JSON String from Classes That Have Collections 495

15.3. Creating a Simple Scala Object from a JSON String 500

15.4. Parsing JSON Data into an Array of Objects 501

15.5. Creating Web Services with Scalatra 503

15.6. Replacing XML Servlet Mappings with Scalatra Mounts 507

15.7. Accessing Scalatra Web Service GET Parameters 509

15.8. Accessing POST Request Data with Scalatra 510

15.9. Creating a Simple GET Request Client 514

15.10. Sending JSON Data to a POST URL 518

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15.11. Getting URL Headers 519

15.12. Setting URL Headers When Sending a Request 520

15.13. Creating a GET Request Web Service with the Play Framework 521

15.14. POSTing JSON Data to a Play Framework Web Service 524

16. Databases and Persistence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527

16.1. Connecting to MySQL with JDBC 528

16.2. Connecting to a Database with the Spring Framework 530

16.3. Connecting to MongoDB and Inserting Data 533

16.4. Inserting Documents into MongoDB with insert, save, or += 537

16.5. Searching a MongoDB Collection 539

16.6. Updating Documents in a MongoDB Collection 542

16.7. Accessing the MongoDB Document ID Field 544

16.8. Deleting Documents in a MongoDB Collection 545

16.9. A Quick Look at Slick 547

17. Interacting with Java. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549

17.1. Going to and from Java Collections 549

17.2. Add Exception Annotations to Scala Methods to Work with Java 554

17.3. Using @SerialVersionUID and Other Annotations 556

17.4. Using the Spring Framework 557

17.5. Annotating varargs Methods 560

17.6. When Java Code Requires JavaBeans 562

17.7. Wrapping Traits with Implementations 565

18. The Simple Build Tool (SBT). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569

18.1. Creating a Project Directory Structure for SBT 570

18.2. Compiling, Running, and Packaging a Scala Project with SBT 574

18.3. Running Tests with SBT and ScalaTest 579

18.4. Managing Dependencies with SBT 581

18.5. Controlling Which Version of a Managed Dependency Is Used 584

18.6. Creating a Project with Subprojects 586

18.7. Using SBT with Eclipse 588

18.8. Generating Project API Documentation 590

18.9. Specifying a Main Class to Run 591

18.10. Using GitHub Projects as Project Dependencies 593

18.11. Telling SBT How to Find a Repository (Working with Resolvers) 595

18.12. Resolving Problems by Getting an SBT Stack Trace 596

18.13. Setting the SBT Log Level 597

18.14. Deploying a Single, Executable JAR File 597

18.15. Publishing Your Library 601

18.16. Using Build.scala Instead of build.sbt 602

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18.17. Using a Maven Repository Library with SBT 604

18.18. Building a Scala Project with Ant 606

19. Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 611

19.1. Creating Classes That Use Generic Types 614

19.2. Creating a Method That Takes a Simple Generic Type 617

19.3. Using Duck Typing (Structural Types) 618

19.4. Make Mutable Collections Invariant 620

19.5. Make Immutable Collections Covariant 622

19.6. Create a Collection Whose Elements Are All of Some Base Type 624

19.7. Selectively Adding New Behavior to a Closed Model 627

19.8. Building Functionality with Types 630

20. Idioms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 635

20.1. Create Methods with No Side Effects (Pure Functions) 636

20.2. Prefer Immutable Objects 644

20.3. Think “Expression-Oriented Programming” 647

20.4. Use Match Expressions and Pattern Matching 650

20.5. Eliminate null Values from Your Code 654

20.6. Using the Option/Some/None Pattern 658

Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 667

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Preface

This is a cookbook of problem-solving recipes about Scala, the most interesting pro‐

gramming language I’ve ever used. The book contains solutions to more than 250 com‐

mon problems, shown with possibly more than 700 examples. (I haven’t counted, but I

suspect that’s true.)

There are a few unique things about this book:

• As a cookbook, it’s intended to save you time by providing solutions to the most

common problems you’ll encounter.

• Almost all of the examples are shown in the Scala interpreter. As a result, whether

you’re sitting by a computer, on a plane, or reading in your favorite recliner, you

get the benefit of seeing their exact output. (Which often leads to, “Ah, so that’s how

that works.”)

• The book covers not only the Scala language, but also has large chapters on Scala

tools and libraries, including SBT, actors, the collections library (more than 100

pages), and JSON processing.

Just prior to its release, the book was updated to cover Scala 2.10.x and SBT 0.12.3.

The Scala Language

My (oversimplified) Scala elevator pitch is that it’s a child of Ruby and Java: it’s light,

concise, and readable like Ruby, but it compiles to class files that you package as JAR

files that run on the JVM; it uses traits and mixins, and feels dynamic, but it’s statically

typed. It uses the Actor model to simplify concurrent programming so you can keep

those multicore processors humming. The name Scala comes from the word scalable,

and true to that name, it’s used to power the busiest websites in the world, including

Twitter, Netflix, Tumblr, LinkedIn, Foursquare, and many more.

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