Refactoring Kotlin Code: Improving Architecture and Performance

The book "Refactoring Kotlin Code" is a practical guide for developers who want to turn working but "dirty" code into an elegant, maintainable, and high-performance system. Unlike abstract textbooks, this book focuses on specific problems every Kotlin developer faces: bloated classes, logic duplication, tangled call chains, and inefficient use of the language's features.

Who this book is for:

• Android developers using Kotlin in production projects (1+ year of experience);
• Backend engineers writing in Ktor or Spring Boot with Kotlin;
• Team leads and technical leaders responsible for codebase quality;
• Juniors who want to learn to write "correct" code from the start without relearning.

What you will learn from the book:

• Identifying "code smells": learn to find typical antipatterns in Kotlin — chains of let/apply, overuse of null-safety, redundant data classes, and incorrect use of coroutines.
• Refactoring catalog: over 30 specific techniques — from simple variable renaming to decomposing monolithic functions using sealed classes and extension functions.
• Working with inheritance: how to replace class hierarchies with composition using delegation (by) and marker interfaces.
• Performance optimization: refactoring inline functions, eliminating unnecessary allocations with value classes, and properly using Sequence instead of List.
• Testing refactoring: strategies for writing safe tests before and after changes, using mutation testing to verify correctness.

Book structure: Each chapter follows the principle "problem → analysis → solution → refactoring → code review." At the end of chapters are practical tasks with real code snippets from open-source projects (Anko, Ktor, standard libraries).

Why this matters: Kotlin is an expressive language, but its flexibility often leads to chaos. Without a systematic approach to refactoring, the codebase turns into "spaghetti" of scope functions and overloaded operators. This book gives you the toolkit to keep code clean, reduce technical debt, and accelerate the introduction of new features.

Bonus: A chapter on refactoring automation — how to configure IntelliJ IDEA and Detekt to detect issues at build time, plus a 20-point code review checklist.