Standard library

Learn extensions in the Kotlin standard library.

Standard library extensions

The Kotlin standard library ships with many extension functions. The best known are let, run, apply, also, with and the collection methods.

Example: "5".toIntOrNull()?.let { println(it * 2) }. Here toIntOrNull() is a String extension and let is a standard extension for nullable values. Calling listOf(1, 2, 3).map { it * 2 } returns [2, 4, 6]; map is also a standard extension. Standard library extensions make daily work short and readable.

map and let

Among the standard extensions, map and let are the most common. map transforms a collection; let works safely with nullable values.

Example: listOf(1, 2, 3).map { it * 2 } returns [2, 4, 6]. "5".toIntOrNull()?.let { println(it * 2) } first turns "5" into 5, then prints 10 when not null. it refers to the single lambda parameter. These extensions are standard for short, readable code.

Exercise Using map and let

In main declare val nums = listOf(1, 2, 3); print the result of nums.map { it * 2 } with println. Also add the call "5".toIntOrNull()?.let { println(it * 2) }.

Show example solution

fun main() {
    val nums = listOf(1, 2, 3)
    println(nums.map { it * 2 })
    "5".toIntOrNull()?.let { println(it * 2) }
}

Test yourself

Match the standard library extensions

"5".toIntOrNull()

String extension
?.let { ... }

Nullable extension
list.map { ... }

Collection extension
it

Lambda parameter
What does <code>listOf(1, 2, 3).map { it * 2 }</code> return?
- Throws an error
- [1, 2, 3]
- [3, 6, 9]
- [2, 4, 6]
Show answer

[2, 4, 6] — map multiplies each element by 2; the result is [2, 4, 6].

Complete the map extension

val doubled = listOf(1, 2, 3).____ { it * 2 }

Show answer

val doubled = listOf(1, 2, 3).map { it * 2 }

Next lesson: Summary - Extension functions

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