The Ultimate Guide to Successfully Adopting Kotlin in a Java-Dominated Environment

Adopting Kotlin in a Java-centric company is not about flipping a switch or rewriting everything “the right way”. It’s about people, timing, risk, and trust.

Over the last four weeks, we’ve published a series of blog posts by Urs Peter, covering all of these aspects of migrating to Kotlin. In this post, we’ll tie the series together and give you one file you can hand to any engineer, tech lead, or manager to help them make the change.

Use this post as a map, and when you spot a topic that resonates, check out the PDF for detailed instructions, side‑by‑side code examples, and concrete migration patterns.

The journey in five parts

Kotlin adoption starts with a spark: one or two engineers who feel that Kotlin could make their code a bit clearer, safer, or easier to work with in the long run.

The guide walks you through a five-stage process to make this happen.

Part 1: Getting Started With Kotlin for Java Developers

We’re starting small. And for good reason: You want to introduce Kotlin in a way that can’t possibly hurt production – by using your test suite.

This part shows you how to:

• Wire Kotlin into an existing Java project.
• Use tools like Kotest and MockK.
• Use Kotlin’s null safety and collections in code you run every day.

The goal here is not to become an expert, but to answer a simple question: “Does working with this language feel better?”

Part 2: Evaluating Kotlin in Real Projects

Once tests feel comfortable, you’ll inevitably start asking yourself, “Can we trust this in production?”

Part 2 explores two paths:

• Starting a new service in Kotlin (often with Spring Boot).
• Adding Kotlin modules to an existing Java system.

You’ll see how to avoid the “Java in Kotlin syntax” trap, teaching you how to use extension functions instead of static helpers, nullable types instead of Optional, and data classes and immutability instead of boilerplate models and builders.

Part 3: Growing Kotlin Adoption in Your Company

Part 3 looks at the human side: how to get your colleagues on board with Kotlin by piquing their interest rather than pushing it on them.

This part will give you practical ways to:

• Provide small, focused examples of Java and Kotlin that respect people’s existing code.
• Share a starter repository with Kotlin, linting, tests, and CI already set up.
• Hold short clinics, pairing sessions, and chats to give your colleagues a chance to ask you questions.
• Build an in‑house Kotlin community that doesn’t depend on a single proponent.

The aim is simple: make it easy for others to say, “Let me try this on my next task.”

Part 4: Helping Decision‑Makers Say Yes to Kotlin

Persuading your tech-savvy employees is one thing – but what about your colleagues who ask, “How does Kotlin benefit us as a business?”

Part 4 helps you explain the benefits of Kotlin in terms of business value:

• Less code to read and maintain.
• Fewer defects thanks to null safety and safer defaults.
• No re-writing required, as Kotlin has full Java interop and existing frameworks still work.
• Developer happiness and hiring – people want to work with thoughtful, cutting-edge tools.
• Predictable costs for training, gradual migration, and knowledge‑sharing.

This part is written to help you have honest conversations with managers and architects who are accountable for risk, not just syntax.

Part 5: Scaling Kotlin Adoption Across Your Organization

Okay – let’s say you’ve managed to bring managers on board and teams are enjoying coding in Kotlin. Now to address the final question: “We have a lot of Java. How do we migrate without breaking things or burning people out?”

Part 5 focuses on strategy at scale. It suggests:

• Treating systems differently based on lifecycle:

• Leave end‑of‑life apps as they are.

• Default new builds to Kotlin.

• Migrate active systems step by step, not as big‑bang rewrites.

• Using the right tools for the job:

• IDE conversion.

• Null‑safety annotations like JSpecify’s.

• AI‑assisted refactoring where tests are strong.

• Rule‑based automation for large codebases.

• Capturing everything in a Kotlin “house playbook” so teams don’t have to re-learn it all from scratch.

Who this guide is for

The guide is written for people who currently work in Java codebases and care about moving them forward responsibly:

• Java developers who want safer, more expressive tools without throwing away their experience.
• Backend and platform engineers running Spring Boot or similar stacks who need a clear, low‑risk path.
• Tech leads and architects who are responsible for consistency across many services and teams.
• Engineering managers and decision‑makers who need a transparent view of costs, benefits, and risks.

Download the complete series as a PDF

The PDF version brings everything together:

• All five parts of the journey.
• Side‑by‑side Java and Kotlin examples.
• Migration patterns, pitfalls to avoid, and success factors at scale.

Take a look and let us know what you think in the comments. We can’t wait to hear about how you use it.

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