apply and with: Configure and Return
Use apply and with to configure objects and understand their return values.
apply and with: Configure and Return is a free Kotlin Academy lesson on CoddyKit. This is lesson 1 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, and your progress syncs across the web and the CoddyKit app. The Kotlin Academy course includes 4 lessons in total.
Scope Function Recap
Kotlin's scope functions (apply, also, let, run, with) let you execute a block in the context of an object. Each differs by receiver (this vs it) and return value.
apply Basics
apply { ... } runs the block with this bound to the receiver and returns the receiver itself. Perfect for object configuration.
class Person {
var name: String = ""
var age: Int = 0
}
fun main() {
val p = Person().apply {
name = "Ada"
age = 35
}
println("${p.name}, ${p.age}")
}Why apply Is Useful
Without apply, you'd need a temporary variable to configure and then return it. apply compresses this to a single expression.
class Config {
var host: String = ""
var port: Int = 0
}
fun makeConfig(): Config {
// Without apply:
// val c = Config(); c.host = "x"; c.port = 8080; return c
return Config().apply {
host = "localhost"
port = 8080
}
}
fun main() {
val c = makeConfig()
println("${c.host}:${c.port}")
}apply in a Chain
apply returns the receiver, so it chains naturally into further method calls.
class Builder {
val parts = mutableListOf<String>()
}
fun main() {
val result = Builder().apply {
parts.add("hello")
parts.add("world")
}.parts.joinToString(" ")
println(result) // hello world
}with Basics
with(receiver) { ... } runs the block with this = receiver and returns the lambda result (NOT the receiver). Useful when you want a derived value.
class Rect(val w: Int, val h: Int)
fun main() {
val area = with(Rect(3, 4)) {
w * h
}
println(area) // 12
}apply vs with
apply returns the receiver, perfect for builders. with returns the lambda result, perfect for computing from a receiver.
class Box(var width: Int = 0, var height: Int = 0)
fun main() {
val box = Box().apply { width = 10; height = 20 } // returns Box
val area = with(box) { width * height } // returns Int
println("$box area=$area")
}apply with Lists
Configure a mutable collection inline.
fun main() {
val list = mutableListOf<String>().apply {
add("a")
add("b")
add("c")
}
println(list)
}apply Returns the Same Type
The return type of apply is always the receiver type. No need for an explicit return.
class Sb {
private val buf = StringBuilder()
fun append(s: String): Sb { buf.append(s); return this }
fun build(): String = buf.toString()
}
fun main() {
val s = Sb().apply {
append("Hello, ")
append("Kotlin!")
}.build()
println(s)
}with Reusing Multiple Members
Inside with, all members of the receiver are in scope — no obj. prefix needed.
class Person(val name: String, val age: Int)
fun main() {
val description = with(Person("Ada", 35)) {
"Name: $name, Age: $age, AgeNext: ${age + 1}"
}
println(description)
}Nested apply
You can nest apply inside another apply for hierarchical configuration. Use sparingly.
class Address {
var city: String = ""
var zip: String = ""
}
class User {
var name: String = ""
var address: Address = Address()
}
fun main() {
val u = User().apply {
name = "Ada"
address.apply {
city = "Istanbul"
zip = "34000"
}
}
println("${u.name} in ${u.address.city}")
}When NOT to Use apply
If you don't need to return the receiver, prefer plain construction with named arguments or let/run. Don't use apply just for variety.
class Config(val host: String, val port: Int)
fun main() {
// Named arguments often beat builder + apply:
val cfg = Config(host = "localhost", port = 8080)
println("${cfg.host}:${cfg.port}")
}Quick Check
What does apply return after running its block?
Recap
apply runs a block with this = receiver and returns the receiver — perfect for object configuration. with runs a block with this = receiver and returns the lambda result — perfect for derived values. Use named arguments when configuration is simple.
Learn Kotlin with an AI tutor — free
Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.
- Courses
- 51
- Lessons
- 203
Frequently Asked Questions
Is the “apply and with: Configure and Return” lesson free?
Yes — any 3 lessons of the Kotlin Academy learning path, including “apply and with: Configure and Return”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Kotlin Academy course includes 4 lessons in total.
What will I learn in “apply and with: Configure and Return”?
Use apply and with to configure objects and understand their return values. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Kotlin Academy?
No prior experience is required. Kotlin Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.
How long does the “apply and with: Configure and Return” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Kotlin Academy lesson?
Yes. Every Kotlin Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- apply and with: Configure and Return
- let and run: Transform and Scope
- also: Side Effects Without Changing the Receiver
- Choosing the Right Scope Function: Decision Guide