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Learn Kotlin for

Android

Development

The Next Generation Language for

Modern Android Apps Programming

Learn Kotlin for

Android Development

The Next Generation Language for

Modern Android Apps Programming

Leipzig, Germany

Learn Kotlin for Android Development: e Next Generation Language for Modern

Android Apps Programming

ISBN-13 (pbk): 978-1-4842-4466-1 ISBN-13 (electronic): 978-1-4842-4467-8

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v

Table of Contents

Chapter 1: Your First Kotlin Application: Hello Kotlin ...............................................1

Setting Up an ide: Android Studio .................................................................................................1

Connecting Your Android Device ...................................................................................................3

Starting Your First Kotlin App ........................................................................................................5

Setting Up and Using Emulators ...................................................................................................6

Continuing with the HelloKotlin App ..............................................................................................8

Using the Command Line ............................................................................................................12

Chapter 2: Classes and Objects: Object Orientation Philosophy .............................15

Kotlin and Object-Oriented programming ...................................................................................18

Class Declaration ...................................................................................................................18

Exercise 1 ..............................................................................................................................19

Property Declaration ..............................................................................................................20

Exercise 2 ..............................................................................................................................21

Exercise 3 ..............................................................................................................................21

Class Initialization..................................................................................................................22

Exercise 4 ..............................................................................................................................23

Exercise 5 ..............................................................................................................................23

An Invoice in Kotlin ................................................................................................................24

More Invoice Properties .........................................................................................................25

Invoice Initialization ...............................................................................................................25

Exercise 6 ..............................................................................................................................26

Instantiation in Kotlin .............................................................................................................26

About the Author ...................................................................................................xvii

About the Technical Reviewer ................................................................................xix

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Adding Functions to Invoices .................................................................................................27

Exercise 7 ..............................................................................................................................30

The Complete Invoice Class ...................................................................................................30

A Simple Number Guessing Game ..............................................................................................31

Constructors ................................................................................................................................41

Exercise 8 ..............................................................................................................................43

Constructor Invocation ..........................................................................................................43

Exercise 9 ..............................................................................................................................43

Exercise 10 ............................................................................................................................44

Named Constructor Parameters ............................................................................................44

Exercise 11 ............................................................................................................................44

Exercise 12 ............................................................................................................................45

Constructor Default Values ....................................................................................................45

Exercise 13 ............................................................................................................................46

Exercise 14 ............................................................................................................................46

Secondary Constructors ........................................................................................................46

Exercise 15 ............................................................................................................................48

If Classes Are Not Needed: Singleton Objects .............................................................................49

Exercise 16 ............................................................................................................................51

Exercise 17 ............................................................................................................................52

If State Doesn't Matter: Companion Objects ...............................................................................52

Exercise 18 ............................................................................................................................54

Exercise 19 ............................................................................................................................55

Describing a Contract: Interfaces ................................................................................................55

Exercise 20 ............................................................................................................................61

Exercise 21 ............................................................................................................................62

Exercise 22 ............................................................................................................................62

Structuring and Packages ...........................................................................................................62

A Structured Project ..............................................................................................................63

Exercise 23 ............................................................................................................................65

Namespaces and Importing ...................................................................................................65

Exercise 24 ............................................................................................................................67

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vii Chapter 3: Classes at Work: Properties and Functions ..........................................69

Properties and Their Types ..........................................................................................................69

Simple Properties ..................................................................................................................70

Exercise 1 ..............................................................................................................................72

Property Types .......................................................................................................................72

Property Value Assignment ....................................................................................................74

Exercise 2 ..............................................................................................................................76

Literals ...................................................................................................................................76

Exercise 3 ..............................................................................................................................79

Property Visibility ...................................................................................................................79

Null Values .............................................................................................................................79

Exercise 4 ..............................................................................................................................80

Property Declaration Modiers ..............................................................................................80

Member Functions ......................................................................................................................81

Functions Not Returning Values ............................................................................................82

Exercise 5 ..............................................................................................................................84

Exercise 6 ..............................................................................................................................85

Functions Returning Values ...................................................................................................85

Exercise 7 ..............................................................................................................................86

Exercise 8 ..............................................................................................................................87

Exercise 9 ..............................................................................................................................87

Accessing Masked Properties ...............................................................................................87

Exercise 10 ............................................................................................................................88

Function Invocation ...............................................................................................................88

Exercise 11 ............................................................................................................................89

Function Named Parameters .................................................................................................89

Exercise 12 ............................................................................................................................90

Function Default Parameters .................................................................................................90

Exercise 13 ............................................................................................................................91

Function Vararg Parameters ..................................................................................................91

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Exercise 14 ............................................................................................................................93

Abstract Functions .................................................................................................................93

Polymorphism ........................................................................................................................94

Local Functions .....................................................................................................................95

Inheritance ..................................................................................................................................96

Classes Inheriting from Other Classes...................................................................................96

Constructor Inheritance .........................................................................................................97

Exercise 15 ............................................................................................................................98

Overriding Functions .............................................................................................................99

Overriding Properties ...........................................................................................................100

Exercise 16 ..........................................................................................................................100

Exercise 17 ..........................................................................................................................101

Accessing Superclass Assets ..............................................................................................101

Local Variables ..........................................................................................................................102

Exercise 18 ..........................................................................................................................103

Visibility of Classes and Class Members ...................................................................................104

Self-Reference: This ..................................................................................................................110

Converting Classes to Strings ...................................................................................................111

Exercise 19 ..........................................................................................................................112

Chapter 4: Classes and Objects: Extended Features .............................................113

Anonymous Classes ..................................................................................................................113

Inner Classes ............................................................................................................................115

Functions and Properties Outside Classes ................................................................................116

Exercise 1 ............................................................................................................................117

Importing Functions and Properties ..........................................................................................118

Exercise 2 ............................................................................................................................118

Data Classes .............................................................................................................................118

Exercise 3 ............................................................................................................................119

Exercise 4 ............................................................................................................................120

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Enumerations ............................................................................................................................120

Exercise 5 ............................................................................................................................122

Custom Property Accessors ......................................................................................................122

Exercise 6 ............................................................................................................................124

Exercise 7 ............................................................................................................................124

Exercise 8 ............................................................................................................................124

Kotlin Extensions .......................................................................................................................125

Extension Functions ............................................................................................................125

Extension Properties .................................................................................................................126

Extensions with Nullable Receivers.....................................................................................127

Encapsulating Extensions ....................................................................................................127

Functions with Tail Recursion ...................................................................................................128

Inx Operators...........................................................................................................................129

Operator Overloading ................................................................................................................130

Exercise 9 ............................................................................................................................134

Delegation .................................................................................................................................134

Chapter 5: Expressions: Operations on Data ........................................................137

Expression Examples ................................................................................................................137

Ubiquity of Expressions .............................................................................................................138

Numerical Expressions .............................................................................................................138

Exercise 1 ............................................................................................................................141

Boolean Expressions .................................................................................................................141

String and Character Expressions .............................................................................................144

Bits and Bytes ...........................................................................................................................146

Other Operators .........................................................................................................................148

Exercise 2 ............................................................................................................................150

Conversions ..............................................................................................................................150

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Chapter 6: Comments in Kotlin Files ....................................................................155

Package Comments ..................................................................................................................156

Markdown .................................................................................................................................159

Class Comments .......................................................................................................................162

Function and Property Comments .............................................................................................165

Exercise 1 ............................................................................................................................166

Generate Your Own API Documentation ....................................................................................166

Chapter 7: Structural Constructs ..........................................................................171

Ifs and Whens ...........................................................................................................................171

Ranges ......................................................................................................................................175

For and While Loops ..................................................................................................................176

Scoping Functions ....................................................................................................................177

The apply Function ....................................................................................................................178

The let Function ........................................................................................................................180

The with Function .....................................................................................................................181

The also Function ......................................................................................................................182

The run Function .......................................................................................................................182

Conditional Execution ...............................................................................................................183

Chapter 8: Exceptions: If Something Goes Wrong ................................................185

Kotlin and Exceptions ................................................................................................................185

More Exception Types ...............................................................................................................188

Throwing Exceptions Yourself ...................................................................................................189

Exercise 1 ............................................................................................................................190

Exceptions in Expressions ........................................................................................................190

Chapter 9: Data Containers ...................................................................................193

Dening and Using Arrays .........................................................................................................193

Array Instantiation .....................................................................................................................195

Exercise 1 ............................................................................................................................197

Exercise 2 ............................................................................................................................198

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Array Operations .......................................................................................................................198

Sets, Lists, and Maps ................................................................................................................200

Sets .....................................................................................................................................201

Exercise 3 ............................................................................................................................205

Lists .....................................................................................................................................205

Maps ....................................................................................................................................212

Pairs and Triples ........................................................................................................................214

Loops over Data Containers ......................................................................................................215

Sorting Arrays and Collections ..................................................................................................217

Exercise 4 ............................................................................................................................220

Exercise 5 ............................................................................................................................221

Grouping, Folding, Reducing, and Zipping .................................................................................221

Grouping ..............................................................................................................................222

Exercise 6 ............................................................................................................................223

Folding .................................................................................................................................223

Reducing .............................................................................................................................224

Exercise 7 ............................................................................................................................225

Zipping .................................................................................................................................225

Exercise 8 ............................................................................................................................227

Searching in Arrays and Collections .........................................................................................227

The Spread Operator .................................................................................................................228

Queues and Stacks: Deques .....................................................................................................229

A Statistics Class for the NumberGuess App ............................................................................231

Adding an Action Bar to The App .........................................................................................231

The Statistics Activity ..........................................................................................................237

State Housekeeping for the Statistics .................................................................................239

Communicating Between the Activities ...............................................................................241

Implementing Statistical Calculations .................................................................................242

Chapter 10: True, False, and Undecided: Nullability .............................................247

What NULL Is .............................................................................................................................247

How Nullability Gets Handled Inside Kotlin ...............................................................................249

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Chapter 11: Handling Equality ..............................................................................253

Identity in Kotlin ........................................................................................................................253

Equality in Kotlin .......................................................................................................................254

Equals and Hash Code ..............................................................................................................254

Exercise 1 ............................................................................................................................258

Exercise 2 ............................................................................................................................258

Chapter 12: Back to Math: Functional Programming ...........................................259

Kotlin and Functional Programming ..........................................................................................262

Functions Without Names: Lambda Functions ..........................................................................264

Exercise 1 ............................................................................................................................266

Exercise 2 ............................................................................................................................266

Loops Once Again .....................................................................................................................266

Functions with Receivers ..........................................................................................................268

Inline Functions .........................................................................................................................269

Filters ........................................................................................................................................271

Exercise 3 ............................................................................................................................273

Chapter 13: About Type Safety: Generics ..............................................................275

Simple Generics ........................................................................................................................277

Exercise 1 ............................................................................................................................278

Declaration-Side Variance .........................................................................................................278

Variance for Immutable Collections ..........................................................................................281

Type Projections ........................................................................................................................281

Star Projections .........................................................................................................................283

Generic Functions .....................................................................................................................283

Generic Constraints ...................................................................................................................284

Exercise 2 ............................................................................................................................287

Chapter 14: Adding Hints: Annotations.................................................................289

Annotations in Kotlin .................................................................................................................291

Annotation Characteristics ........................................................................................................292

Applying Annotations ................................................................................................................294

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Annotations with Array Parameter ............................................................................................297

Reading Annotations .................................................................................................................298

Built-in Annotations ..................................................................................................................300

Custom Annotations ..................................................................................................................302

Exercise 1 ............................................................................................................................304

Chapter 15: Using the Java and Kotlin APIs .........................................................305

Kotlin and Java Libraries ...........................................................................................................306

Using the Online Resources ......................................................................................................306

Making a Local Copy of the Documentation .............................................................................307

Chapter 16: The Collections API ...........................................................................309

Interfaces ..................................................................................................................................309

Classes ......................................................................................................................................313

Generator Functions ..................................................................................................................315

Collection and Map Setters and Removers ...............................................................................317

Deterministic Getters ................................................................................................................318

Collection and Map Characteristics ..........................................................................................320

Traversing Collections and Maps ..............................................................................................321

Transformations ........................................................................................................................322

Exercise 1 ............................................................................................................................326

Exercise 2 ............................................................................................................................327

Exercise 3 ............................................................................................................................327

Exercise 4 ............................................................................................................................327

Filtering .....................................................................................................................................327

Exercise 5 ............................................................................................................................328

Changing the Mutability ............................................................................................................328

Element Checks ........................................................................................................................329

Exercise 6 ............................................................................................................................329

Finding Elements ......................................................................................................................329

Exercise 7 ............................................................................................................................331

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Aggregating, Folding, and Reducing .........................................................................................331

Exercise 8 ............................................................................................................................335

Joining ......................................................................................................................................335

Zipping ......................................................................................................................................337

Windowing ................................................................................................................................339

Sequences ................................................................................................................................340

Operators ..................................................................................................................................343

Chapter 17: More APIs ..........................................................................................347

The Math API .............................................................................................................................347

The Date and Time API, API Level 25 or Less ............................................................................349

The Date and Time API, API Level 26 or Greater ........................................................................352

Local Dates and Times.........................................................................................................353

Instants ................................................................................................................................357

Offset Dates and Times .......................................................................................................358

Zoned Dates and Times .......................................................................................................360

Duration and Periods ...........................................................................................................362

Clock ....................................................................................................................................366

Exercise 1 ............................................................................................................................367

Input and Output .......................................................................................................................367

Creating Some Test Files .....................................................................................................368

File Names ...........................................................................................................................368

Listing Directories ...............................................................................................................369

Writing to Files ....................................................................................................................371

Reading from Files ..............................................................................................................372

Deleting Files .......................................................................................................................373

Working with Temporary Files .............................................................................................374

More File Operations ...........................................................................................................375

Reading URLs ......................................................................................................................377

Using Reection ........................................................................................................................378

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Regular Expressions .................................................................................................................382

Patterns ...............................................................................................................................383

Determining Matches ..........................................................................................................386

Exercise 2 ............................................................................................................................386

Splitting Strings ...................................................................................................................387

Extracting Substrings ..........................................................................................................388

Replacing .............................................................................................................................390

Chapter 18: Working in Parallel: Multithreading ..................................................393

Basic Multithreading the Java Way ...........................................................................................393

Advanced Multithreading the Java Way ....................................................................................402

Special Concurrency Collections .........................................................................................402

Locks ...................................................................................................................................403

Atomic Variable Types ..........................................................................................................406

Executors, Futures, and Callables .......................................................................................407

Exercise 1 ............................................................................................................................414

Kotlin Coroutines .......................................................................................................................414

Basic Coroutines ..................................................................................................................419

Coroutine Context ................................................................................................................425

What a delay( ) Does ............................................................................................................426

What Is a Suspending Function? .........................................................................................426

Waiting for Jobs...................................................................................................................427

Canceling Coroutines ...........................................................................................................428

Timeouts ..............................................................................................................................429

Dispatchers .........................................................................................................................430

Exception Handling ..............................................................................................................432

Exercise 2 ............................................................................................................................435

Chapter 19: Using External Libraries ....................................................................437

Adding External Libraries ..........................................................................................................437

Dependency Management ........................................................................................................441

Unresolved Local Dependencies ...............................................................................................442

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External Libraries and Nullability ..............................................................................................442

Creating Your Own Library ........................................................................................................443

Chapter 20: XML and JSON ...................................................................................447

XML Processing ........................................................................................................................447

Reading XML Data ...............................................................................................................449

Altering XML Data ................................................................................................................453

Creating New DOMs ............................................................................................................458

JSON Processing .......................................................................................................................458

JSON Helper Functions ........................................................................................................459

Reading and Writing JSON Data ..........................................................................................464

Creating New JSON Trees ....................................................................................................466

Appendix ...............................................................................................................469

Solutions to the Exercises .........................................................................................................469

Chapter 2 ..................................................................................................................................469

Chapter 3 ..................................................................................................................................482

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