launch Coroutine in Kotlin

Introduction

launch Coroutine is a fundamental concept every Kotlin developer should understand. Coroutines let you write asynchronous code that reads like synchronous code, without blocking threads or nesting callbacks.

Launch starts a new coroutine without returning result. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• launch starts a new coroutine without returning result.
• It is used for fire-and-forget tasks.
• Returns Job handle for cancellation and waiting.

Syntax

launch { ... }

launch Coroutine in Kotlin Example Program in Kotlin

import kotlinx.coroutines.*

fun main(args: Array<String>) = runBlocking {
    launch {
        repeat(3) {
            println("Task $it")
            delay(50)
        }
    }
    delay(200)
}

Sample Output

Task 0
Task 1
Task 2

When to use

Use coroutines for network requests, database queries, or any work that would block the main thread if done synchronously.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. launch { shows coroutine-based concurrency — work runs without blocking the calling thread.

3. The println("Task $it") statement writes a line to the console — this produces part of the sample output below.

4. Launch starts a new coroutine without returning result.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Never block the main thread — use suspend functions and appropriate dispatchers.
• Scope coroutines with coroutineScope or supervisorScope for structured cancellation.
• Use delay instead of Thread.sleep inside coroutines.

Common Mistakes

• Launching coroutines without a scope — leaks work after the UI is destroyed.
• Using GlobalScope in application code instead of a lifecycle-aware scope.
• Calling blocking APIs directly on Dispatchers.Main.

Key Points

• launch starts a new coroutine without returning result.
• It is used for fire-and-forget tasks.
• Returns Job handle for cancellation and waiting.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Add the kotlinx-coroutines-core dependency when running coroutine examples outside Android or IntelliJ.
• Semicolons at the end of statements are optional in Kotlin.