Deep Kotlin tutorial covering commonMain, platform source sets, expect, actual with examples, diagram, mistakes, and interview answers.

Kotlin Multiplatform: commonMain, expect/actual, and Platform Code

What is Kotlin Multiplatform: commonMain, expect/actual, and Platform Code?

Kotlin Multiplatform: commonMain, expect/actual, and Platform Code is an important Kotlin topic from 1.9 Kotlin Multiplatform Overview. In this lesson we focus only on the concepts that belong together: commonMain, platform source sets, expect, actual, shared domain logic. This keeps the lesson deep enough for interviews but not mixed with unrelated ideas.

Why students should learn it

• Understand Kotlin Multiplatform: commonMain, expect/actual, and Platform Code in simple language
• Use commonMain in real Kotlin or Android code
• Explain Kotlin Multiplatform: commonMain, expect/actual, and Platform Code in interviews with examples and trade-offs

How it works

Kotlin Multiplatform: commonMain, expect/actual, and Platform Code: commonMain, platform source sets, expect, actual, shared domain logic works through Kotlin's type system. The compiler uses type information to warn you before runtime. When the type says a value may be missing, changing, or unknown because it came from Java, your code must handle that explicitly.

1. Identify what problem Kotlin Multiplatform: commonMain, expect/actual, and Platform Code: commonMain, platform source sets, expect, actual, shared domain logic solves.
2. Write the smallest Kotlin snippet that demonstrates it.
3. Add one Android or JVM use case where it matters.
4. Compare it with a similar concept so the difference is clear.
5. Finish by naming one common mistake and the safer alternative.

Interview tip: keep the answer simple first, then add example, trade-off, and mistake.

Types / Classification

• commonMain: core concept in this lesson
• platform source sets: core concept in this lesson
• expect: core concept in this lesson
• actual: core concept in this lesson
• shared domain logic: core concept in this lesson
• Non-null type: value should exist
• Nullable type: value can be missing
• Safe access: handles absence

Important differences

Diagram

flowchart TD
A["1.9 Kotlin Multiplatform Overview"] --> B["Kotlin Multiplatform: commonMain, expect/actual, and Platform Code"]
B --> C["Why it exists"]
C --> D["How it works in Kotlin"]
D --> E["Android or JVM example"]
E --> F["Mistakes and interview answer"]

Examples

Safe nullable handling

fun displayName(name: String?): String {
return name?.trim()?.takeIf { it.isNotEmpty() } ?: "Guest"
}

val is not deep immutability

val names = mutableListOf("Asha")
names.add("Riya") // object still changes

When to use it

Use it when: Use this when Kotlin code has to call into an existing Java library, generate code at build time, or share business logic across Android, iOS, and backend targets.

Avoid it when: Avoid leaking platform-specific APIs, such as Android's Context, into shared/common code - keep the boundary explicit.

Common mistakes

• Using !! to silence compiler warnings
• Thinking val makes a mutable object immutable
• Trusting Java platform types without checks
• Returning null when a sealed result would explain failure
• Smart-casting mutable properties that can change

Interview questions

Read each answer in four parts: simple answer, code example, trade-off, and common mistake.

fun displayName(name: String?): String {
return name?.trim()?.takeIf { it.isNotEmpty() } ?: "Guest"
}

val names = mutableListOf("Asha")
names.add("Riya") // object still changes

fun displayName(name: String?): String {
return name?.trim()?.takeIf { it.isNotEmpty() } ?: "Guest"
}

fun displayName(name: String?): String {
return name?.trim()?.takeIf { it.isNotEmpty() } ?: "Guest"
}

val names = mutableListOf("Asha")
names.add("Riya") // object still changes

Practice task

Write one Kotlin example for Kotlin Multiplatform: commonMain, expect/actual, and Platform Code. Then explain it using: what is it, how it works, where it is used, mistake to avoid, and one interview answer.