commonMain
- Kotlin course
- Expert
- Lesson 26/47
- 5 min
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.
Practice Write commonGreet
Define fun commonGreet(): String. It should return "Hello from commonMain". In main, write println(commonGreet()).
Starter code
// add the commonGreet function
fun main() {
// print the commonMain greeting
}Sample solution
fun commonGreet(): String = "Hello from commonMain"
fun main() {
println(commonGreet())
}Expected output
Hello from commonMain
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the expect declaration
____ fun platformName(): Stringexpect fun platformName(): StringExplanation: expect declares a contract in commonMain.
2 Match the commonMain parts
| Term | Meaning |
|---|---|
commonMain |
Shared source set |
expect |
Contract |
actual |
Implementation |
kotlinx.coroutines |
Shared library |
3 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 Correct answer
Explanation: commonMain holds shared code; platform code goes into target source sets.