kotlin-patterns

# Kotlin Development Patterns Idiomatic Kotlin patterns and best practices for building robust, efficient, and maintainable applications. ## When to Use - Writing new Kotlin code - Reviewing Kotlin code - Refactoring existing Kotlin code - Designing Kotlin modules or libraries - Configuring Gradle Kotlin DSL builds ## How It Works This skill enforces idiomatic Kotlin conventions across seven key areas: null safety using the type system and safe-call operators, immutability via `val` and `copy()` on data classes, sealed classes and interfaces for exhaustive type hierarchies, structured concurrency with coroutines and `Flow`, extension functions for adding behaviour without inheritance, type-safe DSL builders using `@DslMarker` and lambda receivers, and Gradle Kotlin DSL for build configuration. ## Examples **Null safety with Elvis operator:** ```kotlin fun getUserEmail(userId: String): String { val user = userRepository.findById(userId) return user?.email ?: "unknown@example.com" } ``` **Sealed class for exhaustive results:** ```kotlin sealed class Result<out T> { data class Success<T>(val data: T) : Result<T>() data class Failure(val error: AppError) : Result<Nothing>() data object Loading : Result<Nothing>() } ``` **Structured concurrency with async/await:** ```kotlin suspend fun fetchUserWithPosts(userId: String): UserProfile = coroutineScope { val user = async { userService.getUser(userId) } val posts = async { postService.getUserPosts(userId) } UserProfile(user = user.await(), posts = posts.await()) } ``` ## Core Principles ### 1. Null Safety Kotlin's type system distinguishes nullable and non-nullable types. Leverage it fully. ```kotlin // Good: Use non-nullable types by default fun getUser(id: String): User { return userRepository.findById(id) ?: throw UserNotFoundException("User $id not found") } // Good: Safe calls and Elvis operator fun getUserEmail(userId: String): String { val user = userRepository.findById(userId) return user?.email ?: "unknown@example.com" } // Bad: Force-unwrapping nullable types fun getUserEmail(userId: String): String { val user = userRepository.findById(userId) return user!!.email // Throws NPE if null } ``` ### 2. Immutability by Default Prefer `val` over `var`, immutable collections over mutable ones. ```kotlin // Good: Immutable data data class User( val id: String, val name: String, val email: String, ) // Good: Transform with copy() fun updateEmail(user: User, newEmail: String): User = user.copy(email = newEmail) // Good: Immutable collections val users: List<User> = listOf(user1, user2) val filtered = users.filter { it.email.isNotBlank() } // Bad: Mutable state var currentUser: User? = null // Avoid mutable global state val mutableUsers = mutableListOf<User>() // Avoid unless truly needed ``` ### 3. Expression Bodies and Single-Expression Functions Use expression bodies for concise, readable functions. ```kotlin // Good: Expression body fun isAdult(age: Int): Boolean = age >= 18 fun formatFullName(first: String, last: String): String = "$first $last".trim() fun User.displayName(): String = name.ifBlank { email.substringBefore('@') } // Good: When as expression fun statusMessage(code: Int): String = when (code) { 200 -> "OK" 404 -> "Not Found" 500 -> "Internal Server Error" else -> "Unknown status: $code" } // Bad: Unnecessary block body fun isAdult(age: Int): Boolean { return age >= 18 } ``` ### 4. Data Classes for Value Objects Use data classes for types that primarily hold data. ```kotlin // Good: Data class with copy, equals, hashCode, toString data class CreateUserRequest( val name: String, val email: String, val role: Role = Role.USER, ) // Good: Value class for type safety (zero overhead at runtime) @JvmInline value class UserId(val value: String) { init { require(value.isNotBlank()) { "UserId cannot be blank" } } } @JvmInline value class Email(val value: String) { init { require('@' in value) { "Invalid email: $value" } } } fun getUser(id: UserId): User = userRepository.findById(id) ``` ## Sealed Classes and Interfaces ### Modeling Restricted Hierarchies ```kotlin // Good: Sealed class for exhaustive when sealed class Result<out T> { data class Success<T>(val data: T) : Result<T>() data class Failure(val error: AppError) : Result<Nothing>() data object Loading : Result<Nothing>() } fun <T> Result<T>.getOrNull(): T? = when (this) { is Result.Success -> data is Result.Failure -> null is Result.Loading -> null } fun <T> Result<T>.getOrThrow(): T = when (this) { is Result.Success -> data is Result.Failure -> throw error.toException() is Result.Loading -> throw IllegalStateException("Still loading") } ``` ### Sealed Interfaces for API Responses ```kotlin sealed interface ApiError { val message: String data class NotFound(override val message: String) : ApiError data class Unauthorized(override val message: String) : ApiError data class Validation( override val message: String, val field: String, ) : ApiError data class Internal( override val message: String, val cause: Throwable? = null, ) : ApiError } fun ApiError.toStatusCode(): Int = when (this) { is ApiError.NotFound -> 404 is ApiError.Unauthorized -> 401 is ApiError.Validation -> 422 is ApiError.Internal -> 500 } ``` ## Scope Functions ### When to Use Each ```kotlin // let: Transform nullable or scoped result val length: Int? = name?.let { it.trim().length } // apply: Configure an object (returns the object) val user = User().apply { name = "Alice" email = "alice@example.com" } // also: Side effects (returns the object) val user = createUser(request).also { logger.info("Created user: ${it.id}") } // run: Execute a block with receiver (returns result) val result = connection.run { prepareStatement(sql) executeQuery() } // with: Non-extension form of run val csv = with(StringBuilder()) { appendLine("name,email") users.forEach { appendLine("${it.name},${it.email}") } toString() } ``` ### Anti-Patterns ```kotlin // Bad: Nesting scope functions user?.let { u -> u.address?.let { addr -> addr.city?.let { city -> println(city) // Hard to read } } } // Good: Chain safe calls instead val city = user?.address?.city city?.let { println(it) } ``` ## Extension Functions ### Adding Functionality Without Inheritance ```kotlin // Good: Domain-specific extensions fun String.toSlug(): String = lowercase() .replace(Regex("[^a-z0-9\\s-]"), "") .replace(Regex("\\s+"), "-") .trim('-') fun Instant.toLocalDate(zone: ZoneId = ZoneId.systemDefault()): LocalDate = atZone(zone).toLocalDate() // Good: Collection extensions fun <T> List<T>.second(): T = this[1] fun <T> List<T>.secondOrNull(): T? = getOrNull(1) // Good: Scoped extensions (not polluting global namespace) class UserService { private fun User.isActive(): Boolean = status == Status.ACTIVE && lastLogin.isAfter(Instant.now().minus(30, ChronoUnit.DAYS)) fun getActiveUsers(): List<User> = userRepository.findAll().filter { it.isActive() } } ``` ## Coroutines ### Structured Concurrency ```kotlin // Good: Structured concurrency with coroutineScope suspend fun fetchUserWithPosts(userId: String): UserProfile = coroutineScope { val userDeferred = async { userService.getUser(userId) } val postsDeferred = async { postService.getUserPosts(userId) } UserProfile( user = userDeferred.await(), posts = postsDeferred.await(), ) } // Good: supervisorScope when children can fail independently suspend fun fetchDashboard(userId: String): Dashboard = supervisorScope { val user = async { userService.getUser(userId) } val notifications = async { notificationService.getRecent(userId) } val recommendations = async { recommendationService.getFor(userId) } Dashboard( user = user.await(), notifications = try { notifications.await() } catch (e: CancellationException) { throw e } catch (e: Exception) { emptyList() }, recommendations = try { recommendations.await() } catch (e: CancellationException) { throw e } catch (e: Exception) { emptyList() }, ) } ``` ### Flow for Reactive Streams ```kotlin // Good: Cold flow with proper error handling fun observeUsers(): Flow<List<User>> = flow { while (currentCoroutineContext().isActive) { val users = userRepository.findAll() emit(users) delay(5.seconds) } }.catch { e -> logger.error("Error observing users", e) emit(emptyList()) } // Good: Flow operators fun searchUsers(query: Flow<String>): Flow<List<User>> = query .debounce(300.milliseconds) .distinctUntilChanged() .filter { it.length >= 2 } .mapLatest { q -> userRepository.search(q) } .catch { emit(emptyList()) } ``` ### Cancellation and Cleanup ```kotlin // Good: Respect cancellation suspend fun processItems(items: List<Item>) { items.forEach { item -> ensureActive() // Check cancellation before expensive work processItem(item) } } // Good: Cleanup with try/finally suspend fun acquireAndProcess() { val resource = acquireResource() try { resource.process() } finally { withContext(NonCancellable) { resource.release() // Always release, even on cancellation } } } ``` ## Delegation ### Property Delegation ```kotlin // Lazy initialization val expensiveData: List<User> by lazy { userRepository.findAll() } // Observable property var name: String by Delegates.observable("initial") { _, old, new -> logger.info("Name changed from '$old' to '$new'") } // Map-backed properties class Config(private val map: Map<String, Any?>) { val host: String by map val port: Int by map val debug: Boolean by map } val config = Config(mapOf("host" to "localhost", "port" to 8080, "debug" to true)) ``` ### Interface Delegation ```kotlin // Good: Delegate interface implementation class LoggingUserRepository( private val delegate: UserRepository, private val logger: Logger, ) : UserRepository by delegate { // Only override what you need to add logging to override suspend fun findById(id: String): User? { logger.info("Finding user by id: $id") return delegate.findById(id).also { logger.info("Found user: ${it?.name ?: "null"}") } } } ``` ## DSL Builders ### Type-Safe Builders ```kotlin // Good: DSL with @DslMarker @DslMarker annotation class HtmlDsl @HtmlDsl class HTML { private val children = mutableListOf<Element>() fun head(init: Head.() -> Unit) { children += Head().apply(init) } fun body(init: Body.() -> Unit) { children += Body().apply(init) } override fun toString(): String = children.joinToString("\n") } fun html(init: HTML.() -> Unit): HTML = HTML().apply(init) // Usage val page = html { head { title("My Page") } body { h1("Welcome") p("Hello, World!") } } ``` ### Configuration DSL ```kotlin data class ServerConfig( val host: String = "0.0.0.0", val port: Int = 8080, val ssl: SslConfig? = null, val database: DatabaseConfig? = null, ) data class SslConfig(val certPath: String, val keyPath: String) data class DatabaseConfig(val url: String, val maxPoolSize: Int = 10) class ServerConfigBuilder { var host: String = "0.0.0.0" var port: Int = 8080 private var ssl: SslConfig? = null private var database: DatabaseConfig? = null fun ssl(certPath: String, keyPath: String) { ssl = SslConfig(certPath, keyPath) } fun database(url: String, maxPoolSize: Int = 10) { database = DatabaseConfig(url, maxPoolSize) }