kotlinx

kotlinx is the namespace of Kotlin's official extension libraries. They provide capabilities beyond the standard library that are often needed. The best known are kotlinx-coroutines-core (async programming), kotlinx-serialization-json (JSON processing) and kotlinx-datetime (date and time).

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

To use these libraries you first add them as a dependency in build.gradle.kts. Then import brings the types into the file. The kotlinx packages are updated in parallel with Kotlin's release rhythm; keeping version numbers compatible matters.

Some common kotlinx libraries: kotlinx-coroutines-core for coroutines and Flow, kotlinx-serialization-json for JSON conversion, kotlinx-datetime for date and time. Each is a separate dependency; adding only what you need keeps compile time short.

import kotlinx.coroutines.delay
import kotlinx.coroutines.runBlocking

fun main() = runBlocking {
    delay(100)
    println("ready")
}

Here delay and runBlocking are imported from kotlinx.coroutines. Each dependency version must be compatible with the Kotlin version; mismatched versions cause compile errors. Version catalogs are a common tool to manage this compatibility.