Kotlin Comes to BlueJ

Ksenia Shneyveys

Kotlin support is now available in BlueJ, one of the most established environments for teaching introductory object-oriented programming (OOP). This work is the result of a collaboration between JetBrains and the BlueJ team at King’s College London, including Professor Michael Kölling and Dr. Neil Brown, whose work has shaped programming education for decades.

For many students, programming begins in the classroom, where a teacher introduces basic concepts for the first time. Having reached more than 25 million unique learners worldwide, BlueJ is one of the most widely used environments for beginners. Its visual class diagram, object bench, and direct object interaction help students see programs as systems of objects they can create, inspect, and command.

When learning to program, students increasingly need to read, evaluate, debug, and maintain code, including code generated by AI. BlueJ helps beginners build the mental models that let them understand and trust the programs they work with by making program behavior visible and interactive – they can create objects, call methods, and observe how state changes. With Kotlin support, they can do this with less boilerplate and fewer syntactic distractions.

Why Kotlin in BlueJ

Java has played a central role in introductory OOP for many years. It is explicit, structured, and familiar to educators. At the same time, many teachers know the cost of that explicitness: students often need to write quite a lot of boilerplate before they grasp the concept.

Kotlin keeps the object-oriented model visible and surfaces a few important design choices from the beginning:

• Concise syntax reduces the amount of code students need to read and write.
• Null safety makes the possibility of missing values explicit.
• val and var help students distinguish what can change from what cannot.
• JVM interoperability keeps Kotlin close to the Java ecosystem educators already know.

What you can do with Kotlin in BlueJ

This first release focuses on the core classroom workflow. You can create, edit, compile, and run Kotlin files; define classes with properties and methods; and create objects and call their methods through the familiar BlueJ interface, including class diagrams and the object bench. 

Teaching materials

To help educators get started, we’ve prepared an onboarding guide for teaching OOP with Kotlin in BlueJ.

It’s written for teachers who already know how to teach introductory OOP and want to understand how those concepts map to Kotlin. It includes explanations, examples, and projects for classroom use. Each unit comes with example projects and practice materials that can be opened directly in BlueJ.

Thank you to the BlueJ community

Early builds of Kotlin support in BlueJ were shared with educators in the BlueJ community, and their feedback helped shape the release. Teachers tested classroom examples, reported issues, and shared how they think about Kotlin in an objects-first environment. We are especially grateful to everyone who tried the early builds and helped us understand what matters most in real teaching practice.

Try Kotlin in BlueJ

If you teach with BlueJ, we’d love for you to try Kotlin and tell us how it goes.

We are looking forward to your questions and feedback from your own classroom. Write to us at education@kotlinlang.org, and visit the Kotlin for Education page to explore more resources and join the community of Kotlin educators.

Let’s teach Kotlin – good luck, and have fun!