let, run, with, apply, also

Kotlin language Junior Kotlin 2.0+ API 24+ Reviewed 31 Oct 2026 By Best Latest Apps

Predict the output

What does this print?

fun main() {
    val a = "kotlin".let { it.length }
    val b = "kotlin".also { it.length }
    val c = StringBuilder().apply { append("hi") }.toString()
    println("$a $b $c")
}

Options6 kotlin hi6 6 hikotlin kotlin hi6 kotlin StringBuilder

Hint 1 of 1

Which of these return the receiver?

Choose an answer, then open Show answer below to check it.

Show answer

Answer: A 6 kotlin hi

Correct. let returns the lambda result, also returns the receiver, apply returns the builder.

Why not the others?

B 6 6 hi. also returns the original object, not the lambda's value.
C kotlin kotlin hi. let returns the lambda's result, the length.
D 6 kotlin StringBuilder. toString() turns the builder into "hi".

30-second answer

They differ in two ways: how the object is referenced inside (it for let/also, this for run/with/apply) and what they return (the lambda result for let/run/with, the object itself for apply/also). Use apply to configure an object, also for side effects like logging, let for null checks and transforms, and run or with to compute a result from an object.

Practise saying it · 30 s. No recording, just a timer

Explain how to pick a scope function.

Say your answer out loud, then check which of these points you covered:

it vs this
Lambda result vs receiver
apply to configure, also for side effects
let for null checks

Deep dive

x?.let { } runs only when x isn't null.

apply is ideal for builders: Intent(...).apply { putExtra(...) }.

also keeps a chain readable when you add logging or validation.

Nesting scope functions hurts readability; name things instead.

let	it	lambda result	null checks, transforms
run	this	lambda result	compute from an object
with	this	lambda result	group calls on an object
apply	this	the object	configure a builder
also	it	the object	logging, side effects

Common mistakes

Nesting let inside let inside apply.
Using apply when you need the lambda's result.
Replacing a simple if with ?.let { } ?: run { }.

Senior take & follow-ups

A two-by-two table (this/it × result/receiver) answers this faster than listing all five.

Follow-up questions

Think of your answer first, then open the hint.

Which return the receiver?
Show hint
apply and also.
Which use it?
Show hint
let and also.

Suggest a fix if something here is wrong or out of date.