Scala Cookbook

722 Pages·2013·16.2 MB·English

by  Alvin Alexander

Scala Cookbook
Alvin Alexander
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]
For my mom, who loves cookbooks.

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
v
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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