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What are coroutines?

Learn the coroutine and async programming concept.

What are coroutines?

A coroutine is a lightweight thread that can suspend its work and resume later. A single thread can host thousands of coroutines with few resources; each coroutine takes a few hundred bytes.

Coroutines work with suspend functions: while waiting, they do not block the thread and let other work run. This replaces callback hell with clean code. The kotlinx.coroutines library provides tools like runBlocking, launch and async. In this lesson you will see the basic flow.

suspend and structured concurrency

A suspend function can pause and resume its work. While paused the thread is not blocked; other coroutines can run. delay is the simplest suspend function: it waits for a given time.

Structured concurrency ensures every coroutine has a scope. Child coroutines complete before the scope ends. This prevents resource leaks. runBlocking bridges the blocking world to the suspending world and waits for all coroutines inside.

Practice Run your first coroutine

Add import kotlinx.coroutines.*. Write fun main() = runBlocking { println("Hello coroutine") }. runBlocking opens a coroutine scope and blocks the main thread.

Starter code

Main.kt
import kotlinx.coroutines.*

// Define main with runBlocking and print

Sample solution

Main.kt
import kotlinx.coroutines.*

fun main() = runBlocking {
    println("Hello coroutine")
}

Expected output

Hello coroutine

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What is a coroutine?

  • A lightweight thread Correct answer
  • A heavy thread
  • A class type
  • A data structure

Explanation: A coroutine is a lightweight thread that can be suspended.

2 Complete the delay import

Question 
import kotlinx.coroutines.____

suspend fun greet() { delay(100) }
Answer 
import kotlinx.coroutines.delay

suspend fun greet() { delay(100) }

Explanation: delay is a suspend function.

3 Match the coroutine concepts

Term Meaning
coroutine Lightweight task
suspend Can pause
delay Wait
lightweight Few resources

All lessons in Intermediate

1 Classes in depth

  1. Primary constructor
  2. Secondary constructor
  3. Properties
  4. lateinit and lazy
  5. companion object
  6. Summary - Classes

2 Inheritance

  1. open
  2. override
  3. super
  4. abstract
  5. interface
  6. Summary - Inheritance

3 Data class

  1. data class
  2. copy
  3. equals and hashCode
  4. toString
  5. Destructuring
  6. Summary - Data class

4 Project: Helper library

  1. Module design
  2. String helpers
  3. Number helpers
  4. Date helpers
  5. Putting it together

5 Sealed class

  1. sealed class
  2. when with sealed
  3. Sealed and data
  4. Sealed and state
  5. Sealed and result
  6. Summary - Sealed class

6 Object declaration

  1. object
  2. companion object
  3. Object expression
  4. companion and interface
  5. object vs class
  6. Summary - Object declaration

7 Extension functions

  1. Extension functions
  2. Extension properties
  3. Nullable extensions
  4. companion extensions
  5. Standard library
  6. Summary - Extension functions

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Generics

  1. Generic class
  2. Generic functions
  3. Variance
  4. Type constraints
  5. Reified
  6. Summary - Generics

10 Higher-order functions

  1. Function types
  2. Lambda
  3. it
  4. inline
  5. Scope functions
  6. Summary - Higher-order functions

11 Project: Library app

  1. Model design
  2. Library class
  3. Checking out
  4. Search and filter
  5. Putting it together

12 Advanced collections

  1. map and filter
  2. fold and reduce
  3. groupBy and partition
  4. sortedBy and sortedWith
  5. associate and flatMap
  6. Summary - Advanced collections

13 Delegation

  1. by
  2. Delegated properties
  3. by map
  4. vetoable
  5. Custom delegates
  6. Summary - Delegation

14 Value class

  1. value class
  2. Type safety
  3. Methods
  4. Generic value class
  5. value class vs data class
  6. Summary - Value class

15 Coroutine basics

  1. What are coroutines?
  2. launch
  3. suspend
  4. runBlocking
  5. async/await
  6. Summary - Coroutine basics

16 Flow basics

  1. Flow
  2. flow builder
  3. collect
  4. map and filter
  5. flowOn
  6. Summary - Flow basics

17 File I/O

  1. File
  2. readText
  3. writeText
  4. readLines
  5. appendText
  6. Summary - File I/O

18 Packages and modules

  1. package
  2. import
  3. internal
  4. Gradle
  5. kotlinx
  6. Summary - Packages

19 Testing

  1. JUnit
  2. @Test
  3. Assertions
  4. @BeforeEach
  5. MockK
  6. Summary - Testing

20 Intermediate final

  1. Project design
  2. OOP model
  3. Collections
  4. File I/O
  5. Putting it together