Kotlin Academy · Lesson

apply and also

Configure and side-effect.

apply and also is a free Kotlin Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Configuring Objects

apply and also are scope functions that return the object itself rather than the lambda result. This makes them perfect for configuration and chaining.

The apply Function

apply runs a block with the object as this and returns the object. It is the classic tool for initializing an object after creation.

fun main() {
    val builder = StringBuilder().apply {
        append("Hello")
        append(", ")
        append("World")
    }
    println(builder.toString())
}

Why apply Returns the Object

Because apply returns the receiver, you can assign the configured object directly to a variable.

data class Person(var name: String = "", var age: Int = 0)

fun main() {
    val p = Person().apply {
        name = "Alice"
        age = 30
    }
    println(p)
}

apply Uses this

Inside apply, the object is the receiver, so you access its properties directly without any prefix.

data class Config(var host: String = "", var port: Int = 0)

fun main() {
    val config = Config().apply {
        host = "localhost"
        port = 8080
    }
    println("${config.host}:${config.port}")
}

The also Function

also runs a block with the object as it and returns the object. It is used for side effects such as logging or validation that do not change the object.

fun main() {
    val numbers = mutableListOf(1, 2, 3)
    numbers
        .also { println("Before: $it") }
        .add(4)
    println("After: $numbers")
}

also Uses it

Because also exposes the object as it, it is convenient when you still want to refer to the outer this or just keep the reference explicit.

fun main() {
    val value = 42
        .also { println("Logging value: $it") }
    println(value)
}

apply vs also

The difference mirrors run vs let:

apply uses this and is best for configuring properties.
also uses it and is best for side effects on the object.

Both return the original object.

data class Box(var size: Int = 0)

fun main() {
    val box = Box()
        .apply { size = 5 }
        .also { println("Created box with size ${it.size}") }
    println(box)
}

Chaining apply and also

Since both return the object, you can chain them to first configure and then perform side effects in a fluent pipeline.

data class Account(var owner: String = "", var balance: Int = 0)

fun main() {
    val account = Account()
        .apply { owner = "Sam" }
        .apply { balance = 100 }
        .also { println("New account: $it") }
    println(account.balance)
}

also for Validation

Use also to insert a check in the middle of a chain without breaking the flow.

fun main() {
    val result = listOf(1, 2, 3, 4)
        .filter { it % 2 == 0 }
        .also { check(it.isNotEmpty()) { "No even numbers" } }
    println(result)
}

Returning the Object Enables Pipelines

The shared trait of apply and also is that they never interrupt a chain, because they always hand back the same object you started with.

fun main() {
    val text = StringBuilder()
        .apply { append("a") }
        .also { it.append("b") }
        .apply { append("c") }
    println(text)
}

Choosing apply or also

Reach for apply when initializing or mutating an object using its own members. Reach for also when performing an action that uses the object but does not configure it, like logging, debugging, or validation.

Quick Check

Which value do apply and also return?

Recap

You learned the two object-returning scope functions:

apply uses this for configuring an object.
also uses it for side effects on an object.

Both return the object, enabling clean chains. Next you will group operations with with.

Frequently asked questions

Is the “apply and also” lesson free?

Yes — the full text of “apply and also” is free to read here on the web, and the Kotlin Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Kotlin Academy course, upgrade to CoddyKit PRO.

What will I learn in “apply and also”?

Configure and side-effect. 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; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “apply and also” 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.

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