JetBrains has added a Java-to-Kotlin converter extension to Visual Studio Code. It uses an LLM to propose Kotlin code from Java files, then lets you review a diff before accepting the changes. It is a conversion aid, not a guarantee of compilable, behavior-preserving output.
What the VS Code converter does
JetBrains describes the extension as sending requests to an LLM to propose Kotlin conversions. The company’s announcement calls these “idiomatic conversion suggestions”; that is a product description, not an independently verified measure of output quality. The available documentation does not establish a conversion success rate, project-wide migration completeness, or semantic equivalence.
The converter focuses on turning Java code into Kotlin. JetBrains offers a separate extension for broader Java and Kotlin language support in VS Code, discussed below.
How to convert Java to Kotlin in VS Code
- Install JetBrains’ Java to Kotlin Converter extension from the Visual Studio Marketplace.
- Open a
.javafile in VS Code, or select Java files or a folder in the Explorer. - Choose Convert to Kotlin from the editor or Explorer context menu.
- Review the side-by-side diff. Edit the proposed changes if needed, then accept or cancel them.
- Build and test the project after applying a conversion, as you would with other code changes.
The Marketplace listing describes conversion for a Java file, selected files, or a folder, with a review step before changes are accepted. It does not promise that converted code will compile or preserve behavior.
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Provider and project requirements
The Marketplace listing names these options for supplying the LLM capability: GitHub Copilot Chat, a local Ollama or LM Studio setup, or an OpenAI or OpenRouter API key. Check the current listing for provider requirements and setup details, since extension support can change.
The listing also says automatic Kotlin setup currently supports Gradle. It describes Git integration as preserving history through a rename-and-modification commit. These are capabilities stated in the listing, not independently verified results.
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How this differs from JetBrains’ broader VS Code support
Java to Kotlin Converter is for proposing conversions. The separate Java and Kotlin by IntelliJ IDEA extension provides broader language-aware editing support. It is not the converter, and installing it is not required for the conversion workflow above.
JetBrains’ documentation describes that broader extension as a free preview, with each build expiring 30 days after release. The documentation says continued use after the preview requires IntelliJ IDEA Ultimate; check its current terms before relying on that arrangement.
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JetBrains’ separate AI Assistant documentation says AI-assisted Java-to-Kotlin conversion there is experimental and does not replace a static, rule-based converter. That description concerns AI Assistant, not the internal method used by the VS Code converter.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to verify after conversion
- Inspect the diff for changes to logic, null handling, types, and API calls.
- Run the project’s build and tests, and address any compiler errors or behavioral regressions.
- Check that the project’s Kotlin and Gradle configuration fits its existing setup.
- Review the changes in version control before merging or sharing them.
These are standard review steps for generated code; JetBrains has not published a success rate or a guarantee that the extension handles every Java project correctly.
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