Design Patterns in Kotlin: A Practical Guide

This book is a practical guide to applying classic and modern design patterns in the Kotlin language. Unlike abstract textbooks, each pattern is examined through the lens of real-world tasks: from organizing code in Android applications to building server-side solutions with Ktor.

The book begins with the fundamental SOLID and GRASP principles, and then systematically breaks down 23 GoF patterns, adapted to Kotlin idioms. You will see how language features such as sealed classes, data classes, extension functions, and coroutines allow you to implement patterns more elegantly and safely than in Java.

What you will learn:

• How to replace the classic factory with a companion object using the invoke operator
• Why coroutines make Observer and Mediator simpler and more efficient
• How to use sealed classes to implement State and Visitor without boilerplate
• Ways to implement dependency injection using inline reified
• Patterns for working with coroutines: Channel, Flow, SharedFlow

Who this book is for:

• Kotlin developers with basic experience looking to improve the architecture of their projects
• Java programmers transitioning to Kotlin and seeking the "right" idioms
• Android developers striving to write clean, testable, and scalable code
• Students and course graduates who want to understand how to apply theory in practice

Each chapter contains: a problem statement, a UML diagram, a step-by-step implementation in Kotlin, a comparison with Java equivalents, and an "Antipatterns" section — typical mistakes and how to avoid them. All examples are accompanied by unit tests and are available in a GitHub repository.

By the end of the book, you will not just memorize the names of patterns — you will learn to think in Kotlin, choose the right architectural solutions, and write code that is easy to maintain and evolve. This is a must-read for anyone who wants to move from "just working code" to professional development.