Kotlin for Java Developers

Introduction

The book «Kotlin for Java Developers» (author — Svetlana Isakova, 2017, 300 pages) is not just another textbook on the Kotlin language. It is a bridge thrown from the familiar world of Java to the modern, concise, and safe world of Kotlin. Written by one of the leading language developers at JetBrains, the book offers a structured and deep look at Kotlin, without requiring the reader to completely abandon previous experience, but rather using it as a foundation.

Svetlana Isakova is a well-known speaker and course author, so her writing style is characterized by clarity, logic, and an abundance of practical examples. The book was published in 2017, when Kotlin was just beginning to gain popularity (official Android support appeared in the same year), but its content remains relevant today, as it covers the fundamental concepts of the language that do not become obsolete.

What this book is about

The book is structured as a sequential immersion into Kotlin from the perspective of a Java developer. The author does not explain the basics of programming — it is assumed that the reader is proficient in Java. Instead, she focuses on the differences and advantages of Kotlin.

The main topics covered in the book:

• Basic syntax and data types — how to declare variables, functions, classes, how val differs from var, how string templates and ranges work.
• Null Safety — one of the key features of Kotlin: safe work with nullable types, operators ?., !!, ?: (Elvis).
• Higher-order functions and lambdas — how Kotlin turns functions into first-class objects, using map, filter, fold and other standard functions.
• Classes and objects — data classes, sealed classes, the object keyword, companion objects, delegation.
• Extensions — the mechanism for adding new functions to existing classes without inheritance.
• Collections and sequences — differences between List, MutableList, Sequence, lazy evaluation.
• Coroutines — although the book was published before the official release of coroutines, it lays the foundation for understanding asynchrony.
• Interoperability with Java — how to call Java code from Kotlin and vice versa, annotations @JvmStatic, @JvmOverloads, features of platform types.

Each chapter contains code examples that can be immediately tried out in IntelliJ IDEA or Android Studio. At the end of the chapters, there are exercises for self-assessment.

Who this book is for

The target audience is Java developers of any level who want to quickly and effectively master Kotlin. The book will be useful for:

• Backend developers using Spring Boot (Kotlin is fully compatible with Java frameworks).
• Android developers transitioning to Kotlin (the official language for Android since 2019).
• Students and teachers studying modern JVM languages.
• Those who want to understand the philosophy of Kotlin and its differences from Java.

The book is not suitable for absolute beginners in programming — it assumes knowledge of OOP, exceptions, generics, and basic patterns.

What the reader will learn

After reading the book, the reader will be able to:

• Write concise and expressive code in Kotlin, reducing volume by 2–3 times compared to Java.
• Use null safety to prevent NullPointerException at compile time.
• Apply functional approaches: lambdas, chains of operations on collections, higher-order functions.
• Create data classes and sealed classes for domain modeling.
• Extend the functionality of third-party libraries using extension functions.
• Organize code using ob