Table Of ContentHands-On Design Patterns
with Kotlin
Build scalable applications using traditional, reactive,
and concurrent design patterns in Kotlin
Alexey Soshin
BIRMINGHAM - MUMBAI
Hands-On Design Patterns with Kotlin
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To Lula Leus, my constant source of inspiration.
To my mentor, Lior Bar On. Without you, I would have never started writing.
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Contributors
About the author
Alexey Soshin is a software architect with 13 years of experience of making software,
mostly for JVM. He started exploring Kotlin even before Kotlin 1.0 was released, and since
then, he has been a big enthusiast of the language. He speaks about Kotlin and reactive
frameworks at various conferences and meetups and maintains a technical blog. He's also a
contributor to Vert.x, a toolkit for building reactive applications on the Java Virtual
Machine.
About the reviewers
Ranga Rao Karanam is a programmer, trainer, and architect. He is the founder of
in28Minutes—helping 200,000 learners reskill on Cloud native applications, microservices,
evolutionary design, high-quality code, DevOps, BDD, TDD, and refactoring. He loves
consulting for startups on the development of scalable component-based cloud-native
applications and following modern development practices, such as BDD,
continuous delivery, and DevOps.
Ranga likes to play cricket and tennis, and he is a regular hiker. His dream is to spend a
year hiking in the Himalayas.
Ganesh Samarthyam is a co-founder of CodeOps Technologies, a software technology,
consultancy, and training company based in Bangalore. He has 16 years of experience in the
IT industry, and his latest book, Refactoring for Software Design Smells by Morgan
Kaufmann/Elsevier, has been translated into Korean and Chinese. Ganesh loves exploring
anything and everything about technology in his free time.
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Table of Contents
Preface 1
Chapter 1: Getting Started with Kotlin 6
Basic language syntax and features 6
Multi-paradigm 7
Code structure 7
No semicolons 7
Naming conventions 8
Packages 8
Types 8
Type inference 8
val versus var 9
Comparison 9
Null safety 10
Declaring functions 11
Control flow 12
Using the if expression 13
Using the when expression 14
String interpolation 14
Classes and inheritance 15
Classes 15
Inheritance 16
Constructors 17
Properties 18
Data classes 19
More control flow – loops 20
The for loop 20
For-each loop 21
While loop 22
Extension functions 22
Introduction to design patterns 23
What are design patterns? 23
Design patterns in real life 24
Design process 24
Why use design patterns in Kotlin? 25
Summary 25
Chapter 2: Working with Creational Patterns 26
Singleton 26
Factory Method 28
Factory 28
Table of Contents
Static Factory Method 33
Advantages of the Static Factory Method 33
Caching 33
Subclassing 33
Static Factory Method in Kotlin 34
Companion object 34
Abstract Factory 36
Abstract Factory in action 36
Introduction to generics in Kotlin 37
Back to our bases 39
Making improvements 41
Builder 43
Composing an email 43
Collection types in Kotlin 44
Creating an email – first attempt 44
Creating an email – second attempt 45
Creating an email – the Kotlin way 46
Creating an email – the Kotlin way – second attempt 47
Prototype 48
Building your own PC 48
Starting from a prototype 49
Summary 50
Chapter 3: Understanding Structural Patterns 51
Decorator 51
Enhancing a class 52
Operator overloading 53
Dude, where's my map? 55
The great combinator 55
Caveats 57
Adapter 58
Different adapters 59
Adapters in the real world 60
Caveats of using adapters 60
Bridge 61
Bridging changes 63
Type aliases 64
You're in the army now 64
Constants 65
A lethal weapon 65
Composite 66
Get together 66
The Squad 67
Varargs and secondary constructors 68
Counting bullets 69
Facade 71
Keep it simple 72
[ ii ]
Table of Contents
Flyweight 73
Being conservative 73
Saving memory 75
Proxy 76
A short detour into the RMI world 77
A replacement 77
Lazy delegation 78
Summary 79
Chapter 4: Getting Familiar with Behavioral Patterns 80
Strategy 81
Fruit arsenal 82
Citizen function 83
Switching sides 84
Iterator 85
One, two... many 85
Running through the values 86
State 89
Fifty shades of State 90
State of the Nation 92
Command 93
Undoing commands 97
Chain of responsibility 98
Interpreter 100
We need to go deeper 101
A language of your own 101
Taking a break 105
Call suffix 105
Mediator 106
Trouble in the Jungle 106
The middleman 109
Flavors 110
Caveats 110
Memento 111
Remembrance 111
Visitor 114
Writing a crawler 114
Template method 117
Observer 120
Animal Choir 121
Summary 125
Chapter 5: Functional Programming 126
Why functional programming? 126
Immutability 127
[ iii ]
