Kotlin

Kotlin Core Knowledge

Reference: coroutines.md for structured concurrency, Flow, channels. advanced.md for sealed hierarchies, generics variance, context parameters, KSP.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: kotlin.

Null Safety

var name: String = "Alice"        // Non-nullable
var maybe: String? = null         // Nullable

val len = maybe?.length ?: 0      // Safe call + Elvis
val forced = maybe!!.length       // Throws NPE if null — avoid

// Smart casts
fun describe(x: Any?): String = when (x) {
    is String -> "String of length ${x.length}"  // smart-cast to String
    null -> "null"
    else -> x.toString()
}

Data Classes

data class User(
    val id: Long,
    val name: String,
    val email: String,
    val active: Boolean = true,
)

val u = User(1, "Alice", "alice@example.com")
val updated = u.copy(active = false)
val (id, name) = u   // destructuring

Auto-generates equals, hashCode, toString, copy, componentN. Avoid for entities with identity (use regular class).

Sealed Classes & When

sealed interface Result<out T> {
    data class Success<T>(val value: T) : Result<T>
    data class Error(val cause: Throwable) : Result<Nothing>
    data object Loading : Result<Nothing>
}

fun <T> handle(r: Result<T>): String = when (r) {
    is Result.Success -> "ok: ${r.value}"
    is Result.Error -> "error: ${r.cause.message}"
    Result.Loading -> "loading"
    // exhaustive — no else needed
}

sealed confines hierarchy to same module/package. Prefer sealed interface over sealed class when no shared state.

Scope Functions — When to Use Which

val view = TextView(context).apply {
    text = "Hello"
    textSize = 16f
}.also { log("created: $it") }

Coroutines — Essentials

import kotlinx.coroutines.*

suspend fun fetchUser(id: Long): User {
    delay(100)            // suspend, doesn't block thread
    return User(id, "...", "...")
}

fun main() = runBlocking {
    val user = async { fetchUser(1) }
    val posts = async { fetchPosts(1) }
    println("${user.await()} -> ${posts.await()}")
}

Rules:

• suspend functions can only be called from coroutine or another suspend
• Use withContext(Dispatchers.IO) for blocking I/O
• Use coroutineScope { } for parent-child cancellation propagation
• Never use GlobalScope in production

See coroutines.md for structured concurrency, Flow, channels, exception handling.

Flow (Cold Streams)

fun ticker(): Flow<Int> = flow {
    var i = 0
    while (true) {
        emit(i++)
        delay(1000)
    }
}

// Collect
viewModelScope.launch {
    ticker()
        .map { it * 2 }
        .filter { it % 4 == 0 }
        .collect { println(it) }
}

StateFlow (hot, single-value) for UI state. SharedFlow for one-shot events.

Extension Functions

fun String.toSlug(): String =
    this.lowercase().replace(Regex("[^a-z0-9]+"), "-").trim('-')

"Hello World!".toSlug()   // "hello-world"

Resolved statically. Don't override; can't access private members of receiver.

Generics & Variance

class Box<out T>(val value: T)         // covariant: Box<Cat> is Box<Animal>
class Sink<in T> { fun put(v: T) {} }  // contravariant: Sink<Animal> is Sink<Cat>

inline fun <reified T> isType(x: Any) = x is T  // reified: T available at runtime

Idiomatic Patterns

// Single-expression functions
fun double(x: Int) = x * 2

// Default + named args (replace builders)
fun connect(host: String, port: Int = 80, secure: Boolean = false) {}
connect(host = "localhost", secure = true)

// Destructuring maps
for ((key, value) in map) { /* ... */ }

// require / check / error for preconditions
fun divide(a: Int, b: Int): Int {
    require(b != 0) { "b must be non-zero" }
    return a / b
}

Anti-Patterns

Build & Tooling

// build.gradle.kts
plugins {
    kotlin("jvm") version "2.2.0"
    kotlin("plugin.serialization") version "2.2.0"
    id("com.google.devtools.ksp") version "2.2.0-1.0.x"
}

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.10.0")
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.7.0")
}

kotlin {
    compilerOptions {
        jvmTarget.set(JvmTarget.JVM_21)
        freeCompilerArgs.add("-Xcontext-parameters")  // Kotlin 2.2+
    }
}

Kotlin 2.x Highlights

• K2 compiler stable (default in 2.x): faster, better diagnostics
• Context parameters (replaces deprecated context receivers)
• Stable data object for singletons in sealed hierarchies
• expect/actual classes stable (multiplatform)
• @OptIn annotations for preview APIs

Testing

// Kotest (preferred for KMP)
class UserTest : StringSpec({
    "valid email" {
        User(1, "x", "x@y.z").email shouldContain "@"
    }
})

// JUnit 5 + kotest matchers
@Test
fun `coroutine result`() = runTest {
    val result = fetchUser(1)
    result.id shouldBe 1L
}

For coroutine testing use kotlinx-coroutines-test (runTest, TestDispatcher, virtual time).

When NOT to Use This Skill