Coroutine vs Thread in Kotlin: Key Differences and Usage

In Kotlin, threads are OS-managed units of execution that run tasks concurrently but are heavy and limited in number. Coroutines are lightweight, Kotlin-managed tasks that run asynchronously with minimal resource use, making them ideal for scalable concurrency.

Quick Comparison

This table summarizes the main differences between Kotlin coroutines and threads.

Factor	Thread	Coroutine
Management	Managed by OS	Managed by Kotlin runtime
Lightweight	Heavy, limited number	Very lightweight, thousands possible
Creation cost	High (memory and time)	Low (minimal overhead)
Concurrency type	True parallelism (multi-core)	Cooperative multitasking
Use case	CPU-intensive tasks	Asynchronous, non-blocking tasks
Context switching	Expensive	Cheap and fast

Key Differences

Threads are the traditional way to run code concurrently. Each thread is managed by the operating system and has its own call stack and memory. Creating and switching between threads is costly in terms of system resources, so the number of threads is usually limited.

Coroutines are Kotlin's way to write asynchronous code that looks sequential. They are managed by Kotlin's runtime and share threads, allowing thousands of coroutines to run with very low overhead. Coroutines suspend and resume cooperatively, which makes context switching much cheaper than threads.

While threads can run truly in parallel on multiple CPU cores, coroutines run on threads but switch tasks without blocking threads. This makes coroutines ideal for tasks like network calls or UI updates where waiting happens often, while threads are better for heavy CPU work.

Code Comparison

This example shows how to print numbers concurrently using a thread in Kotlin.

kotlin

import kotlin.concurrent.thread

fun main() {
    val t = thread {
        for (i in 1..5) {
            println("Thread: $i")
            Thread.sleep(100)
        }
    }
    t.join()
    println("Thread finished")
}

Output

Thread: 1
Thread: 2
Thread: 3
Thread: 4
Thread: 5
Thread finished

Coroutine Equivalent

The same task using a Kotlin coroutine with delay instead of blocking sleep.

kotlin

import kotlinx.coroutines.*

fun main() = runBlocking {
    val job = launch {
        for (i in 1..5) {
            println("Coroutine: $i")
            delay(100)
        }
    }
    job.join()
    println("Coroutine finished")
}

Output

Coroutine: 1
Coroutine: 2
Coroutine: 3
Coroutine: 4
Coroutine: 5
Coroutine finished

When to Use Which

Choose threads when you need true parallelism for CPU-heavy tasks that benefit from multiple cores, such as complex calculations or image processing.

Choose coroutines for asynchronous, non-blocking tasks like network requests, UI updates, or handling many concurrent operations efficiently without creating many threads.

Coroutines provide better scalability and lower resource use for most modern Kotlin applications, especially on Android and server-side development.

Key Takeaways

Coroutines are lightweight and managed by Kotlin, while threads are heavy and managed by the OS.

Use threads for CPU-intensive parallel tasks and coroutines for asynchronous, non-blocking operations.

Coroutines allow thousands of concurrent tasks with minimal overhead compared to threads.

Coroutines suspend without blocking threads, making them efficient for waiting tasks.

Choosing the right tool improves app performance and resource use.