Kotlin in 2025: Main New Features and Language Updates

Kotlin in 2025: Evolution Continues

Kotlin, developed by JetBrains, has long ceased to be just a "better alternative to Java for Android." In 2025, it represents a mature, multiplatform language targeting server-side development, web frontend, native applications, and of course, mobile development. The year was marked by the release of the stable version of the K2 compiler, significant improvements in the ecosystem, and new features making code even safer and more expressive. In this article, we will break down the key changes in Kotlin in 2025, backing them up with practical examples.

1. K2 Compiler: A New Era of Performance

The main event of 2024-2025 is the full transition to the K2 compiler. In 2025, it became the de facto standard. K2 not only speeds up compilation (in some projects, the improvement reached up to 200-300%) but also paves the way for new language features that were previously impossible to implement.

Key advantages of K2:

• Speed: Significantly faster compilation both in IntelliJ IDEA and Gradle.
• Smart Analysis: Improved type inference and more accurate hints in the IDE.
• Fewer Errors: K2 checks code more strictly at compile time, identifying potential issues that previously might only appear at runtime.

Example of improved type inference:

Previously, creating a collection with an explicit type required writing more code. In 2025, K2 handles this more elegantly:

fun main() {    // Old way (K1)    val listOld: MutableList<Int> = mutableListOf()        // New way (K2) - type inference works even for complex chains    val listNew = buildList {        add(1)        add(2)        // K2 understands this is MutableList<Int> without explicit declaration    }        println(listNew) // Output: [1, 2]}

2. Multiplatform: Kotlin Multiplatform (KMP) Reaches a New Level

In 2025, Kotlin Multiplatform ceased to be an experimental technology. It is now the primary tool for code sharing between platforms. Key improvements have touched stability and integration with platform APIs.

What's new in KMP in 2025:

• Stable expect/actual: The mechanism for declaring platform-dependent implementations has become more predictable and safe.
• Improved iOS compatibility: Direct integration with Swift/Objective-C libraries without additional "wrappers."
• Kotlin/Wasm (WebAssembly): Experimental support for Kotlin in the browser via WebAssembly is becoming increasingly mature, paving the way for high-performance web applications.

Example of a data model for KMP:

// Common code (commonMain)data class User(val id: Int, val name: String, val email: String)

// Platform-dependent function to get the platform nameexpect fun getPlatformName(): String

// Usage in common codefun createGreeting(user: User): String {    return "Hello, ${user.name}! You are using ${getPlatformName()}"}

// Implementation for Android (androidMain)actual fun getPlatformName(): String = "Android ${android.os.Build.VERSION.SDK_INT}"

// Implementation for iOS (iosMain)actual fun getPlatformName(): String = "iOS ${UIDevice.currentDevice.systemVersion}"

3. Improvements in Syntax and Standard Library

Kotlin developers do not forget about the "little things" that make code cleaner and safer. In 2025, several long-awaited improvements were added to the language.

Stable Context Receivers:

This feature, which was experimental for a long time,