commonMain

Learn shared code with commonMain.

What is commonMain?

commonMain is the source set in KMP that holds shared code. Code written here can be used by every target.

Inside commonMain you may use the Kotlin stdlib, data classes, functions and expect declarations. Platform-specific APIs (iOS frameworks, JVM classes) cannot be used here. Instead you write a contract with expect, and each target implements it with actual. This keeps shared logic in one place.

commonMain dependencies

Dependencies for commonMain are added in Gradle with a commonMain { dependencies { ... } } block. Libraries added here are shared by every target.

For instance the Ktor client, kotlinx.coroutines or kotlinx.serialization can be added in commonMain. Platform libraries go into the matching target source set. This split keeps shared logic clean. Only multiplatform-compatible libraries may be used in commonMain.

Exercise Write commonGreet

Define fun commonGreet(): String. It should return "Hello from commonMain". In main, write println(commonGreet()).

Show example solution

fun commonGreet(): String = "Hello from commonMain"
fun main() {
    println(commonGreet())
}

Test yourself

Complete the expect declaration

____ fun platformName(): String

Show answer

expect fun platformName(): String

Match the commonMain parts

commonMain

Shared source set
expect

Contract
actual

Implementation
kotlinx.coroutines

Shared library
What is the role of the commonMain source set?
```
commonMain is the hub of sharing.
```
- Write platform code
- Freeze classes
- Write tests
- Hold shared code
Show answer

Hold shared code — commonMain holds shared code; platform code goes into target source sets.

Next lesson: expect/actual

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