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inline

Learn performance optimization with inline.

inline

The inline keyword makes a function be inlined directly at the call site. It reduces the lambda creation overhead in higher-order functions.

Example: inline fun repeatN(times: Int, action: (Int) -> Unit) { for (i in 1..times) action(i) }. Calling repeatN(3) { println("Run $it") } prints three times. Thanks to inline, no lambda object is created; the code is inlined at the call site. This helps performance. But each inline function can grow code size; it is used for small functions.

The repeatN function

An inline higher-order function inlines the lambda at the call site. The action(i) call is expanded at compile time.

Example: inline fun repeatN(times: Int, action: (Int) -> Unit) { for (i in 1..times) action(i) }. Calling repeatN(3) { println("Run $it") } prints Run 1 Run 2 Run 3. it is the lambda's single parameter. inline functions are common in collection methods; forEach, map and filter are inline in the standard library.

Practice The repeatN inline function

Define inline fun repeatN(times: Int, action: (Int) -> Unit) whose body does for (i in 1..times) action(i). In main call repeatN(3) { println("Run $it") }.

Starter code

Main.kt
// Define the repeatN inline function
// takes times and action, iterates with for
// call it three times in main

Sample solution

Main.kt
inline fun repeatN(times: Int, action: (Int) -> Unit) {
    for (i in 1..times) action(i)
}

fun main() {
    repeatN(3) { println("Run $it") }
}

Expected output

Run 1
Run 2
Run 3

Questions and answers

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

1 What does the <code>inline</code> keyword provide?

  • Hiding the function
  • Pinning the function
  • Recursion of the function
  • Inlining the function body at the call site Correct answer

Explanation: inline inlines the function body at the call site.

2 Match the inline parts

Term Meaning
inline fun repeatN inline signature
action: (Int) -> Unit Function parameter
for (i in 1..times) Loop
action(i) Lambda call

3 Complete the inline function definition

Question 
inline ____ repeatN(times: Int, action: (Int) -> Unit) {
    for (i in 1..times) action(i)
}
Answer 
inline fun repeatN(times: Int, action: (Int) -> Unit) {
    for (i in 1..times) action(i)
}

Explanation: It is written as inline fun repeatN.

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