Best Kotlin Open Source Repositories

Kotlin is a statically typed language that runs on the Java Virtual Machine and is fully interoperable with existing Java code, meaning Kotlin and Java classes can call each other directly within the same project. Google designated it the preferred language for Android development in 2019, and it has steadily replaced Java as the default choice for new Android codebases since then. Kotlin's syntax is more concise than Java's, with built-in null safety that distinguishes nullable and non-nullable types at compile time, reducing a common source of runtime crashes in Java and Android applications.

Beyond Android, Kotlin Multiplatform lets teams share business logic code across Android, iOS, desktop, and web targets while keeping platform-specific UI code native to each platform, an approach distinct from cross-platform frameworks like React Native or Flutter that also render the UI layer with shared code. This makes Kotlin attractive to teams that want to share logic without giving up fully native interfaces on each platform.

Kotlin's presence in open-source projects here mostly comes through mobile development. React Native and Expo include Kotlin in their Android native modules, since Android app development has largely moved to Kotlin from Java. Immich and Ente, both self-hosted photo backup applications with Android clients, use Kotlin for their native Android code, and AFFiNE and JetBrains' Compose Multiplatform framework use Kotlin as their primary implementation language for cross-platform desktop and mobile UI.