Kotlin Academy · Lesson

expect and actual

Platform-specific implementations.

expect and actual 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.

The Need for Platform Code

Some functionality has no common implementation — getting the OS name, a UUID, or the current platform. KMM solves this with the expect/actual mechanism.

expect Declarations

In commonMain you declare an expect function or class — a contract with no body. Common code can call it.

// commonMain
expect fun platformName(): String

actual on Android

Each platform provides an actual implementation matching the signature.

// androidMain
actual fun platformName(): String = "Android " + android.os.Build.VERSION.SDK_INT

actual on iOS

The iOS source set supplies its own version, using native APIs.

// iosMain
import platform.UIKit.UIDevice

actual fun platformName(): String =
    UIDevice.currentDevice.systemName + " " + UIDevice.currentDevice.systemVersion

Using It in Common Code

Common code calls the expected function as if it existed; the compiler links the right actual per target.

// commonMain
class Platform {
    val name: String get() = platformName()
}

expect Classes

You can expect an entire class. Each platform provides an actual class with matching members.

// commonMain
expect class FileReader(path: String) {
    fun read(): String
}

Actual Class Implementations

Platforms implement the expected class using native file APIs (e.g. Java IO on Android, Foundation on iOS).

// androidMain
actual class FileReader actual constructor(private val path: String) {
    actual fun read(): String = java.io.File(path).readText()
}

expect with typealias

An actual can be a typealias to an existing platform type, avoiding a wrapper.

// commonMain
expect class Uuid

// jvm/androidMain
actual typealias Uuid = java.util.UUID

Signature Matching Rules

The actual must match the expect exactly: same name, parameters, and return type. A mismatch is a compile error, guaranteeing consistency across platforms.

When Not to Use expect/actual

Prefer simple interfaces with platform implementations injected at runtime when the abstraction is more flexible. Reserve expect/actual for small, fixed platform primitives.

A Common-Code Illustration

Outside KMM, the same idea is just calling an abstraction. Here a plain function stands in for the platform call.

fun platformName(): String = "Demo Platform"

fun main() {
    println("Running on " + platformName())
}

Quick Check

Where is an expect declaration written, and where are its actual implementations?

Recap

expect/actual bridges platform gaps:

expect declares the contract in commonMain
actual implements it per platform (or via typealias)
Signatures must match exactly
Prefer interfaces for flexible abstractions

Next: sharing business logic.

Frequently asked questions

Is the “expect and actual” lesson free?

Yes — the full text of “expect and actual” 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 “expect and actual”?

Platform-specific implementations. 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 “expect and actual” 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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