Kotlin for Java Developers: Confidently transition from Java to Kotlin through hands-on examples and idiomatic Kotlin practices

Written by José Dimas Luján Castillo and Ron Veen - seasoned developers with deep expertise in Java and Kotlin - this practical guide teaches Kotlin through side-by-side Java comparisons, real-world examples, and clear explanations of modern language features.

Key Features

• Learn Kotlin by drawing direct comparisons to core Java concepts
• Understand null safety, coroutines, extension functions, and DSLs through practical examples
• Use Kotlin confidently in backend, Android, and cross-platform development
• Purchase of the print or Kindle book includes a free PDF eBook

Book Description

This book is a hands-on programming guide for Java developers who want to gain practical, production-ready knowledge of Kotlin. Whether you’re maintaining Java systems or adopting Kotlin for Android, backend, or cross-platform projects, it will help you bridge the gap by building on your existing understanding of Java’s syntax and design principles.

You’ll begin by learning why Kotlin was created and how it improves on Java – exploring types, expressions, control flow, and null handling while mapping each concept to familiar Java constructs. Each topic is introduced with clear purpose, showing how Kotlin’s concise syntax, safety features, and expressive APIs lead to cleaner, more maintainable code.

As you progress, you’ll master object-oriented and functional programming, extension functions, smart casting, and interoperability with Java. The second half of the book dives into generics, data and sealed classes, coroutines for concurrency, and DSL design – demonstrating when and why to use these features to build scalable, elegant solutions.

By the end, you’ll not only write idiomatic Kotlin but also understand the reasoning behind its design, enabling you to craft modern, reliable applications with confidence.

What you will learn

• Write Kotlin programs using variables, functions, and control flow
• Model real-world data with classes, properties, and constructors
• Handle missing data safely with nullable types and safe calls
• Simplify logic using lambdas and extension functions
• Work with Kotlin collections and sequences to process data efficiently
• Use Kotlin with Java, Gradle, and Maven for seamless project builds
• Write non-blocking code using coroutines and Async flows
• Create custom DSL for your projects

Who this book is for

This book is for software developers who are proficient in Java and want to learn Kotlin for professional application development. It’s especially relevant for Android engineers, JVM backend developers, and full-stack programmers who maintain or extend Java systems. Familiarity with core Java syntax and object-oriented programming is expected.

Table of Contents

1. An Introduction to Kotlin and Its Basic Types
2. Interoperability, Gradle, and Maven
3. Null and Non-Nullable Types
4. Extension Functions and the Apply Function
5. Object-Oriented Programming Basics
6. Generics and Variance
7. Data and Sealed Classes
8. Basics of Functional Programming
9. Lambda
10. Collections and Sequences
11. Coroutines
12. Synchronous and Asynchronous Programming
13. Kotlin Testing
14. Domain-Specific Languages in Kotlin

José Dimas Luján has 15 years of experience as a software developer, mobile developer, Android, iOS, Flutter, as well as Tech Lead, for different companies around the world developing more than 500 applications for mobile devices. In addition, he has more than 7 development books and at the same time taught classes at more than 25 universities throughout Latin America on programming, mobile development, electronic business, and development of technological projects. He has been working for the last 10 years with technologies such as Java, Kotlin, Python, Rust, Dart, mostly oriented to mobile development. With the clear idea that sharing what he knows is learning twice.

Ron Veen is a seasoned software engineer with over 20 years of experience in the JVM and Java ecosystem. An Oracle Certified Java Programmer and a certified Sun Business Component Developer, he has worked across domains from mainframes to microservices. A Java enthusiast with a keen interest in Kotlin, Ron serves as a Special Agent and Lead Developer at Team Rockstars IT and speaks regularly at international conferences. He is the author of books on Java Cloud-native migrations with Jakarta EE and on advanced concurrency topics such as Virtual Threads, Structured Concurrency, and Scoped Values.