Standards Kotlin

Kotlin Coding Standards

Core Principles

1. Explicitness: Explicit code over implicit magic
2. Readability: Readable code over clever tricks
3. Null Safety: Embrace Kotlin's null safety system
4. Immutability: Prefer val over var, immutable collections
5. Expressiveness: Use Kotlin's expressive features (data classes, sealed classes)
6. DRY: Don't Repeat Yourself - but keep it simple

General Rules

• Prefer val over var: Immutability by default
• Use data classes: For simple data holders
• Sealed classes/interfaces: For type-safe state modeling
• Early returns: Avoid deep nesting
• Descriptive names: Clear, meaningful names
• Minimal changes: Only change relevant code
• No over-engineering: Keep it simple
• Minimal comments: Self-explanatory code. Comments for "why", not "what"

Naming Conventions

Project Structure

Gradle Kotlin DSL Project (Recommended)

myproject/
├── build.gradle.kts
├── settings.gradle.kts
├── gradle.properties
├── src/
│   ├── main/
│   │   ├── kotlin/
│   │   │   └── com/example/myapp/
│   │   │       ├── Application.kt          # Main entry point
│   │   │       ├── config/
│   │   │       │   └── AppConfig.kt        # Configuration
│   │   │       ├── domain/
│   │   │       │   └── User.kt             # Domain models
│   │   │       ├── repository/
│   │   │       │   └── UserRepository.kt   # Data access
│   │   │       ├── service/
│   │   │       │   └── UserService.kt      # Business logic
│   │   │       └── api/
│   │   │           └── UserController.kt   # REST endpoints
│   │   └── resources/
│   │       ├── application.conf
│   │       └── logback.xml
│   └── test/
│       ├── kotlin/
│       │   └── com/example/myapp/
│       │       ├── service/
│       │       │   └── UserServiceTest.kt
│       │       └── repository/
│       │           └── UserRepositoryTest.kt
│       └── resources/
│           └── application-test.conf
└── README.md

Maven Project (Alternative)

myproject/
├── pom.xml
├── src/
│   ├── main/
│   │   └── kotlin/...      # Same structure as Gradle
│   └── test/
│       └── kotlin/...      # Same structure as Gradle
└── README.md

Modern Kotlin Features

Recommended: Use Kotlin 2.3.0 (latest LTS) for new projects with K2 compiler enabled by default.

K2 Compiler (Stable since 2.0)

The K2 compiler brings significant performance improvements and faster compilation times.

Features:

• Faster compilation (up to 2x)
• Better smart casts
• Improved type inference
• Unified architecture for all platforms

Enabled by default in Kotlin 2.3.0 - no configuration needed.

Data Classes

Use data classes for immutable data holders.

// Data class - automatic equals, hashCode, toString, copy, componentN
data class User(
    val id: String,
    val name: String,
    val email: String,
    val age: Int
)

// Usage
val user = User("1", "John Doe", "john@example.com", 30)

// Copy with changes
val updatedUser = user.copy(age = 31)

// Destructuring
val (id, name, email, age) = user
println("User: $name ($email)")

Sealed Classes/Interfaces (Exhaustive When)

Use sealed classes for type-safe state modeling with exhaustive when expressions.

// Sealed interface for result types
sealed interface Result<out T> {
    data class Success<T>(val data: T) : Result<T>
    data class Error(val message: String, val cause: Throwable? = null) : Result<Nothing>
    data object Loading : Result<Nothing>
}

// Exhaustive when - compiler ensures all cases are handled
fun <T> handleResult(result: Result<T>) {
    when (result) {
        is Result.Success -> println("Success: ${result.data}")
        is Result.Error -> println("Error: ${result.message}")
        Result.Loading -> println("Loading...")
        // No else needed - compiler knows all cases
    }
}

// Usage
val result: Result<User> = Result.Success(user)
handleResult(result)

Inline Value Classes (Zero-Cost Wrappers)

Use inline value classes for type-safe wrappers without runtime overhead.

// Inline value class - no boxing overhead
@JvmInline
value class UserId(val value: String)

@JvmInline
value class Email(val value: String) {
    init {
        require(value.contains("@")) { "Invalid email" }
    }
}

// Usage - type-safe, no runtime cost
fun getUserById(id: UserId): User = TODO()
fun sendEmail(email: Email): Unit = TODO()

val userId = UserId("123")
val email = Email("user@example.com")

Context Receivers (Experimental, 2.2+)

Context receivers allow implicit parameters for cleaner DSLs.

Enable with:

// build.gradle.kts
kotlin {
    compilerOptions {
        freeCompilerArgs.add("-Xcontext-receivers")
    }
}

Usage:

interface Logger {
    fun log(message: String)
}

// Function with context receiver
context(Logger)
fun processUser(user: User) {
    log("Processing user: ${user.name}")
    // ...
}

// Call with context
val logger = object : Logger {
    override fun log(message: String) = println(message)
}

with(logger) {
    processUser(user)
}

Explicit Backing Fields (Experimental, 2.3)

Simplifies backing property pattern - define implementation type within property scope.

Enable with:

// build.gradle.kts
kotlin {
    compilerOptions {
        freeCompilerArgs.add("-Xexplicit-backing-fields")
    }
}

Before:

private val _users = MutableStateFlow<List<User>>(emptyList())
val users: StateFlow<List<User>> = _users

After:

val users: StateFlow<List<User>> field = MutableStateFlow(emptyList())

UUID API (Experimental, 2.3)

Built-in UUID support without external dependencies.

Enable with:

@OptIn(ExperimentalUuidApi::class)

Usage:

import kotlin.uuid.Uuid
import kotlin.uuid.ExperimentalUuidApi

@OptIn(ExperimentalUuidApi::class)
fun generateUserId(): Uuid {
    return Uuid.generateV4()
}

@OptIn(ExperimentalUuidApi::class)
fun parseUserId(id: String): Uuid? {
    return Uuid.parseOrNull(id)
}

// V7 UUIDs (time-based, sortable)
@OptIn(ExperimentalUuidApi::class)
fun generateTimeBasedId(): Uuid {
    return Uuid.generateV7()
}

Coroutines & Concurrency

Structured Concurrency

Always use structured concurrency - never use GlobalScope.

import kotlinx.coroutines.*

// GOOD - Structured concurrency
suspend fun fetchUserData(userId: String): UserData = coroutineScope {
    val userDeferred = async { fetchUser(userId) }
    val ordersDeferred = async { fetchOrders(userId) }

    UserData(
        user = userDeferred.await(),
        orders = ordersDeferred.await()
    )
}

// BAD - GlobalScope leaks
fun fetchUserDataBad(userId: String) {
    GlobalScope.launch {  // Don't use GlobalScope!
        // ...
    }
}

Dispatchers

Use appropriate dispatchers for different workloads.

// Dispatchers.Default - CPU-intensive work
withContext(Dispatchers.Default) {
    // Heavy computation
    processLargeDataset(data)
}

// Dispatchers.IO - I/O operations (network, disk)
withContext(Dispatchers.IO) {
    // Network call
    apiClient.fetchData()
}

// Dispatchers.Main - UI updates (Android/Desktop)
withContext(Dispatchers.Main) {
    updateUI(data)
}

// Dispatchers.Unconfined - Advanced use cases only

launch vs async

// launch - fire and forget, returns Job
fun processInBackground() = coroutineScope {
    launch {
        // No result needed
        sendNotification()
    }
}

// async - returns Deferred<T>, await for result
suspend fun fetchMultipleResources() = coroutineScope {
    val users = async { fetchUsers() }
    val orders = async { fetchOrders() }

    CombinedData(
        users = users.await(),
        orders = orders.await()
    )
}

Cancellation & Exception Handling

// Cancellation-aware code
suspend fun longRunningTask() = coroutineScope {
    repeat(100) { i ->
        ensureActive()  // Check for cancellation
        delay(100)
        println("Step $i")
    }
}

// Exception handling with supervisorScope
suspend fun fetchDataSafely(): Result<Data> = try {
    supervisorScope {
        val data = async { fetchData() }
        Result.Success(data.await())
    }
} catch (e: Exception) {
    Result.Error("Failed to fetch data", e)
}

// CoroutineExceptionHandler
val handler = CoroutineExceptionHandler { _, exception ->
    println("Caught exception: $exception")
}

val scope = CoroutineScope(SupervisorJob() + handler)

Flow API

StateFlow vs SharedFlow vs MutableStateFlow

// StateFlow - single value, always has current value, conflates updates
class UserViewModel {
    private val _userName = MutableStateFlow("")
    val userName: StateFlow<String> = _userName.asStateFlow()

    fun updateUserName(name: String) {
        _userName.value = name
    }
}

// SharedFlow - event stream, can replay, doesn't conflate
class EventBus {
    private val _events = MutableSharedFlow<Event>(
        replay = 0,      // Don't replay events
        extraBufferCapacity = 64
    )
    val events: SharedFlow<Event> = _events.asSharedFlow()

    suspend fun emit(event: Event) {
        _events.emit(event)
    }
}

// Hot vs Cold flows
// StateFlow/SharedFlow = Hot (emit regardless of collectors)
// flow { } = Cold (only emit when collected)

Flow Operators

// Transform flows
val userNames: Flow<String> = users
    .map { it.name }
    .filter { it.isNotEmpty() }
    .distinctUntilChanged()

// Combine flows
val combinedData = combine(users, orders) { users, orders ->
    CombinedData(users, orders)
}

// FlatMap variants
val allOrders: Flow<Order> = users
    .flatMapConcat { user -> fetchOrders(user.id) }  // Sequential
    // .flatMapMerge { user -> fetchOrders(user.id) }  // Concurrent
    // .flatMapLatest { user -> fetchOrders(user.id) }  // Cancel previous

// Error handling
val safeData: Flow<Data> = dataFlow
    .catch { e -> emit(Data.Empty) }
    .retry(3)

Flow Collection

// Collect in coroutine
lifecycleScope.launch {
    userViewModel.userName.collect { name ->
        updateUI(name)
    }
}

// collectLatest - cancel previous collection on new emission
lifecycleScope.launch {
    searchQuery.collectLatest { query ->
        // Cancelled if new query arrives
        val results = searchRepository.search(query)
        updateResults(results)
    }
}

// Single value
val user = userFlow.first()  // First value
val user = userFlow.firstOrNull()  // Or null if empty

Flow Best Practices

1. Single source of truth: Expose StateFlow/SharedFlow, keep MutableStateFlow/MutableSharedFlow private
2. Use collectLatest for UI: Cancel previous work on new emissions
3. Use stateIn for cold-to-hot conversion:

val data: StateFlow<Data> = dataRepository.getData()
    .stateIn(
        scope = viewModelScope,
        started = SharingStarted.WhileSubscribed(5000),
        initialValue = Data.Empty
    )

Null Safety

Safe Calls, Elvis, and !! Operator

// Safe call (?.) - returns null if receiver is null
val length: Int? = name?.length

// Elvis operator (?:) - default value
val length: Int = name?.length ?: 0

// !! operator - throws NPE if null (use sparingly!)
val length: Int = name!!.length  // Only if you're 100% sure it's not null

// Safe cast (as?)
val user: User? = obj as? User

// let for null checks
name?.let { n ->
    println("Name: $n")
}

lateinit vs lazy

// lateinit - non-null var, initialized later (must be var)
class MyClass {
    lateinit var repository: Repository

    fun init(repo: Repository) {
        repository = repo
    }

    // Check if initialized
    fun isInitialized() = ::repository.isInitialized
}

// lazy - initialized on first access (must be val)
class MyClass {
    val expensiveValue: String by lazy {
        // Computed only once, on first access
        computeExpensiveValue()
    }
}

Nullable Types vs Optional

// GOOD - Use nullable types (Kotlin-native)
fun findUser(id: String): User? {
    return repository.findById(id)
}

// BAD - Don't use Java's Optional in Kotlin
fun findUserBad(id: String): Optional<User> {  // Avoid
    return Optional.ofNullable(repository.findById(id))
}

// When interoping with Java, convert at boundary
fun getUserFromJava(id: String): User? {
    return javaService.getUser(id).orElse(null)
}

Backing Properties (Standard Pattern)

Use underscore prefix for private mutable backing properties.

// Standard pattern for exposing read-only property
class UserRepository {
    private val _users = mutableListOf<User>()
    val users: List<User> get() = _users

    fun addUser(user: User) {
        _users.add(user)
    }
}

// StateFlow pattern
class UserViewModel {
    private val _state = MutableStateFlow<UiState>(UiState.Loading)
    val state: StateFlow<UiState> = _state.asStateFlow()

    fun updateState(newState: UiState) {
        _state.value = newState
    }
}

// Alternative: explicit backing fields (experimental, Kotlin 2.3+)
// Requires: -Xexplicit-backing-fields compiler flag
val users: StateFlow<List<User>> field = MutableStateFlow(emptyList())

Scope Functions

Kotlin provides five scope functions: let, run, with, apply, also. Choose based on context object reference and return value.

let - Null Checks & Transformations

// Null check with let
val length: Int? = name?.let { it.length }

// Chain with let
val result = value?.let { v ->
    processValue(v)
}?.let { processed ->
    saveResult(processed)
}

// Execute block only if non-null
user?.let { u ->
    println("User: ${u.name}")
    sendWelcomeEmail(u)
}

// Transform value
val uppercaseName = name?.let { it.uppercase() }

apply - Object Initialization

// Object initialization (returns `this`)
val user = User().apply {
    name = "John Doe"
    email = "john@example.com"
    age = 30
}

// Configure and return
val intent = Intent(context, DetailActivity::class.java).apply {
    putExtra("id", userId)
    flags = Intent.FLAG_ACTIVITY_NEW_TASK
}

// Builder pattern style
val dialog = AlertDialog.Builder(context).apply {
    setTitle("Confirm")
    setMessage("Are you sure?")
    setPositiveButton("Yes") { _, _ -> confirm() }
}.create()

also - Side Effects

// Side effects (returns `this`)
val numbers = mutableListOf(1, 2, 3).also {
    println("List created with ${it.size} elements")
}

// Debug chain
val result = processData(input)
    .also { println("Intermediate result: $it") }
    .transformData()
    .also { println("Final result: $it") }

// Multiple operations
val user = createUser().also {
    logUserCreation(it)
    sendWelcomeEmail(it)
}

run - Computations

// Compute value (returns lambda result)
val result = run {
    val x = computeX()
    val y = computeY()
    x + y
}

// Null check with run
val result = service?.run {
    fetchData()
    processData()
}

// Replace multiple statements
val hexString = run {
    val color = getColor()
    Integer.toHexString(color)
}

with - Multiple Calls on Object

// Group operations on object (returns lambda result)
val result = with(canvas) {
    drawCircle(centerX, centerY, radius)
    drawLine(startX, startY, endX, endY)
    drawText(text, x, y)
    save()
}

// Configure object
with(sharedPreferences.edit()) {
    putString("username", username)
    putInt("age", age)
    apply()
}

// Avoid repetition
val user = getUser()
with(user) {
    println("Name: $name")
    println("Email: $email")
    println("Age: $age")
}

Scope Function Selection Guide

// Use let for null checks
value?.let { println(it) }

// Use apply for object initialization (returns object)
val config = Config().apply { timeout = 30 }

// Use also for side effects (returns object)
val list = getList().also { log("Size: ${it.size}") }

// Use run for computations (returns result)
val result = run { compute() }

// Use with for grouping calls (returns result)
val formatted = with(user) { "$name ($email)" }

Collections & Sequences

List vs MutableList

// Prefer immutable collections
val users: List<User> = listOf(user1, user2, user3)

// Mutable only when needed
val mutableUsers: MutableList<User> = mutableListOf()
mutableUsers.add(user4)

// Read-only view of mutable collection
val readOnlyView: List<User> = mutableUsers

// List.of() for Java interop (immutable)
val javaList = java.util.List.of(user1, user2)

Sequence for Lazy Evaluation

Use Sequence for large collections or chained operations.

// List - eager evaluation (creates intermediate lists)
val result = users
    .filter { it.age > 18 }
    .map { it.name }
    .take(10)

// Sequence - lazy evaluation (no intermediate collections)
val result = users.asSequence()
    .filter { it.age > 18 }
    .map { it.name }
    .take(10)
    .toList()  // Terminal operation

// Generate infinite sequence
val fibonacci = generateSequence(1 to 1) { (a, b) -> b to a + b }
    .map { it.first }
    .take(10)
    .toList()

Collection Operations

// Transform
val names = users.map { it.name }
val adults = users.filter { it.age >= 18 }
val groups = users.groupBy { it.department }

// Reduce/Fold
val totalAge = users.sumOf { it.age }
val oldest = users.maxByOrNull { it.age }
val names = users.fold("") { acc, user -> "$acc, ${user.name}" }

// Partition
val (adults, minors) = users.partition { it.age >= 18 }

// Associate
val userById = users.associateBy { it.id }
val nameToUser = users.associateWith { it.name }

Loops on Ranges

// BAD - off-by-one prone
for (i in 0..n - 1) { }
for (i in 0 until n) { }  // Better but verbose

// GOOD - use ..< (Kotlin 1.7+)
for (i in 0..<n) { }

// Ranges
for (i in 1..10) { }        // 1 to 10 (inclusive)
for (i in 1..<10) { }       // 1 to 9 (exclusive end)
for (i in 10 downTo 1) { }  // 10 to 1 (descending)
for (i in 1..10 step 2) { } // 1, 3, 5, 7, 9

// Prefer higher-order functions over loops
// GOOD
val adults = users.filter { it.age >= 18 }

// Less idiomatic
val adults = mutableListOf<User>()
for (user in users) {
    if (user.age >= 18) {
        adults.add(user)
    }
}

String Templates

Use string templates instead of concatenation.

// Simple variable - no braces needed
val message = "Hello, $name!"

// Expression - use braces
val message = "User $name has ${children.size} children"
val message = "${user.name} (${user.age} years old)"

// Property access
val message = "Length: ${text.length}"

// Function call
val message = "Result: ${compute()}"

// BAD - concatenation
val message = "Hello, " + name + "!"

// Multiline strings with templates
val json = """
{
    "name": "$name",
    "age": $age,
    "email": "$email"
}
""".trimIndent()

// Multi-dollar strings (Kotlin 2.0+) - escape $ in raw strings
val jsonSchema = $$"""
{
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "title": "$${title ?: "unknown"}"
}
"""

Extension Functions

Extension functions add functionality to existing classes without inheritance.

When to Use Extensions

// GOOD - Add functionality to existing type
fun String.isValidEmail(): Boolean {
    return this.contains("@") && this.contains(".")
}

// Usage
val email = "user@example.com"
if (email.isValidEmail()) { }

// GOOD - Domain-specific operations
fun User.toDisplayString(): String {
    return "$name ($email)"
}

// GOOD - Collection transformations
fun List<User>.activeUsers(): List<User> {
    return filter { it.isActive }
}

Extension Best Practices

// 1. Restrict visibility (prefer private/internal)
private fun String.sanitize(): String {
    return this.trim().lowercase()
}

// 2. Place extensions in same file as class (if you own it)
// User.kt
data class User(val name: String)

fun User.greet() = "Hello, $name"

// 3. Group related extensions in separate file
// StringExtensions.kt
fun String.isValidEmail(): Boolean { }
fun String.isValidUrl(): Boolean { }

// 4. Don't overuse - prefer member functions when appropriate
class User {
    // GOOD - core behavior as member
    fun activate() { }
}

// Extension for convenience
fun User.deactivate() { }

Extension Properties

// Extension property (no backing field allowed)
val String.lastChar: Char?
    get() = this.lastOrNull()

val List<Int>.median: Double
    get() {
        val sorted = this.sorted()
        val middle = sorted.size / 2
        return if (sorted.size % 2 == 0) {
            (sorted[middle - 1] + sorted[middle]) / 2.0
        } else {
            sorted[middle].toDouble()
        }
    }

// Usage
println("Hello".lastChar)  // 'o'
println(listOf(1, 3, 2).median)  // 2.0

When Expressions Advanced

When with Guards (Kotlin 1.7+)

// Guard conditions in when
sealed interface Status {
    data class Ok(val info: Info) : Status
    data class Error(val code: Int) : Status
}

fun handleStatus(status: Status): String {
    return when (status) {
        is Status.Ok if (status.info.isEmpty()) -> "no information"
        is Status.Ok -> "success: ${status.info}"
        is Status.Error if (status.code >= 500) -> "server error"
        is Status.Error -> "client error: ${status.code}"
    }
}

// Guard with multiple conditions
fun classify(value: Int): String {
    return when (value) {
        in 0..10 if (value % 2 == 0) -> "small even"
        in 0..10 -> "small odd"
        in 11..100 -> "medium"
        else -> "large"
    }
}

When as Expression

// Prefer when as expression (not statement)
// GOOD
val result = when (x) {
    0 -> "zero"
    in 1..10 -> "small"
    else -> "large"
}

// BAD
var result: String
when (x) {
    0 -> result = "zero"
    in 1..10 -> result = "small"
    else -> result = "large"
}

Generics Basics

Generic Functions

// Generic function
fun <T> singletonList(item: T): List<T> {
    return listOf(item)
}

val list = singletonList(42)        // List<Int>
val names = singletonList("Alice")  // List<String>

// Multiple type parameters
fun <K, V> mapOf(key: K, value: V): Map<K, V> {
    return mapOf(key to value)
}

// Type constraints
fun <T : Comparable<T>> max(a: T, b: T): T {
    return if (a > b) a else b
}

Generic Classes

// Generic class
class Box<T>(val value: T) {
    fun get(): T = value
}

val intBox = Box(42)
val stringBox = Box("hello")

// Multiple type parameters
class Pair<A, B>(val first: A, val second: B)

val pair = Pair(1, "one")

Variance (in, out)

// out - covariant (producer)
interface Producer<out T> {
    fun produce(): T
}

// in - contravariant (consumer)
interface Consumer<in T> {
    fun consume(item: T)
}

// Example
class ListWrapper<out T>(private val list: List<T>) {
    fun get(index: Int): T = list[index]  // OK - produces T
    // fun add(item: T) { }  // Error - cannot consume T
}

// Covariance allows
val strings: Producer<String> = object : Producer<String> {
    override fun produce() = "Hello"
}
val anys: Producer<Any> = strings  // OK - String is subtype of Any

Reified Type Parameters

// inline + reified for runtime type access
inline fun <reified T> isInstance(value: Any): Boolean {
    return value is T
}

val result = isInstance<String>("hello")  // true
val result2 = isInstance<Int>("hello")    // false

// Useful for type-safe casts
inline fun <reified T> Any.asOrNull(): T? {
    return this as? T
}

val str: String? = obj.asOrNull<String>()

Formatting & Code Style

Follow Kotlin official coding conventions for consistent code formatting.

Indentation & Braces

// 4 spaces (not tabs)
// Opening brace at line end (Java-style)
if (condition) {
    doSomething()
}

// Single-line if can omit braces
if (condition) return

// Multi-line conditions: indent by 4 spaces
if (!component.isSyncing &&
    !hasErrors()
) {
    proceed()
}

Whitespace Rules

// Space around binary operators
val sum = a + b
val result = x * y

// NO space for range operator
for (i in 0..10) { }

// NO space for unary operators
val x = -5
val y = a++

// Space after control flow keywords
if (condition) { }
when (x) { }
for (i in list) { }

// NO space before ( in calls/constructors
foo(1, 2)
User(name, age)

// NO spaces around . or ?.
user.name
user?.email

// Space after //
// This is a comment

Trailing Commas

Encouraged at declaration/call sites (makes diffs cleaner):

// Function parameters
fun process(
    name: String,
    age: Int,
    email: String,  // trailing comma
) { }

// Arguments
process(
    name = "John",
    age = 30,
    email = "john@example.com",  // trailing comma
)

// Collections
val list = listOf(
    "apple",
    "banana",
    "cherry",  // trailing comma
)

Expression Bodies

Prefer expression bodies for simple functions:

// GOOD - expression body
fun double(x: Int) = x * 2

// BAD - block body for simple function
fun double(x: Int): Int {
    return x * 2
}

// Expression body with line break
fun longFunctionName(
    arg1: String,
    arg2: String
) = processArguments(arg1, arg2)

Named Arguments

Use named arguments for clarity:

// Multiple parameters of same type
drawSquare(x = 10, y = 10, width = 100, height = 100)

// Boolean parameters
setVisibility(visible = true, animated = false)

// Skip default parameters
createUser(name = "John")  // age has default

Testing Fundamentals

JUnit 5 + kotlin.test

import kotlin.test.*
import org.junit.jupiter.api.*

class UserServiceTest {
    private lateinit var userService: UserService
    private lateinit var repository: UserRepository

    @BeforeEach
    fun setup() {
        repository = InMemoryUserRepository()
        userService = UserService(repository)
    }

    @Test
    fun `should return user when id exists`() {
        // Given
        val user = User("1", "John", "john@example.com", 30)
        repository.save(user)

        // When
        val result = userService.findById("1")

        // Then
        assertNotNull(result)
        assertEquals("John", result.name)
    }

    @Test
    fun `should return null when user not found`() {
        // When
        val result = userService.findById("999")

        // Then
        assertNull(result)
    }

    @ParameterizedTest
    @ValueSource(strings = ["", "  ", "\t"])
    fun `should throw when id is blank`(id: String) {
        assertFailsWith<IllegalArgumentException> {
            userService.findById(id)
        }
    }
}

Mockk for Mocking

import io.mockk.*
import kotlin.test.*

class UserServiceTest {
    private val repository = mockk<UserRepository>()
    private val userService = UserService(repository)

    @Test
    fun `should call repository when finding user`() {
        // Given
        val user = User("1", "John", "john@example.com", 30)
        every { repository.findById("1") } returns user

        // When
        val result = userService.findById("1")

        // Then
        verify { repository.findById("1") }
        assertEquals("John", result?.name)
    }

    @Test
    fun `should handle repository exception`() {
        // Given
        every { repository.findById(any()) } throws RuntimeException("DB error")

        // When/Then
        assertFailsWith<RuntimeException> {
            userService.findById("1")
        }
    }
}

Coroutines Testing

import kotlinx.coroutines.test.*
import kotlin.test.*

class UserViewModelTest {
    @Test
    fun `should fetch user on init`() = runTest {
        // Given
        val repository = FakeUserRepository()
        val viewModel = UserViewModel(repository)

        // When
        advanceUntilIdle()

        // Then
        assertEquals(UserState.Success(user), viewModel.state.value)
    }

    @Test
    fun `should emit loading state while fetching`() = runTest {
        // Given
        val repository = SlowUserRepository()
        val viewModel = UserViewModel(repository)
        val states = mutableListOf<UserState>()

        // Collect states
        backgroundScope.launch {
            viewModel.state.toList(states)
        }

        // When
        advanceUntilIdle()

        // Then
        assertEquals(
            listOf(UserState.Loading, UserState.Success(user)),
            states
        )
    }
}

Build Tool Awareness

Gradle Kotlin DSL (build.gradle.kts) - Recommended

// build.gradle.kts
plugins {
    kotlin("jvm") version "2.3.10"
    application
}

group = "com.example"
version = "1.0.0"

repositories {
    mavenCentral()
}

dependencies {
    // Kotlin standard library (automatically added in 2.3+)
    // implementation(kotlin("stdlib"))

    // Coroutines
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.8.0")

    // Testing
    testImplementation(kotlin("test"))
    testImplementation("org.junit.jupiter:junit-jupiter:5.10.0")
    testImplementation("io.mockk:mockk:1.13.8")
    testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.8.0")
}

kotlin {
    jvmToolchain(21)  // Use Java 21

    compilerOptions {
        freeCompilerArgs.add("-Xcontext-receivers")  // Enable experimental features
        freeCompilerArgs.add("-Xexplicit-backing-fields")
    }
}

tasks.test {
    useJUnitPlatform()
}

application {
    mainClass.set("com.example.ApplicationKt")
}

Gradle Dependencies: implementation vs api

dependencies {
    // implementation - internal dependency, not exposed to consumers
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.8.0")

    // api - exposed to consumers (only in library modules)
    api("com.example:shared-models:1.0.0")

    // compileOnly - needed only at compile time
    compileOnly("org.jetbrains:annotations:24.0.0")

    // runtimeOnly - needed only at runtime
    runtimeOnly("com.h2database:h2:2.2.220")
}

Maven (pom.xml) - Alternative

<project>
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.example</groupId>
    <artifactId>myapp</artifactId>
    <version>1.0.0</version>

    <properties>
        <kotlin.version>2.3.10</kotlin.version>
        <kotlin.compiler.jvmTarget>21</kotlin.compiler.jvmTarget>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib</artifactId>
            <version>${kotlin.version}</version>
        </dependency>

        <dependency>
            <groupId>org.jetbrains.kotlinx</groupId>
            <artifactId>kotlinx-coroutines-core</artifactId>
            <version>1.8.0</version>
        </dependency>

        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-test-junit5</artifactId>
            <version>${kotlin.version}</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <sourceDirectory>src/main/kotlin</sourceDirectory>
        <testSourceDirectory>src/test/kotlin</testSourceDirectory>

        <plugins>
            <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <version>${kotlin.version}</version>
            </plugin>
        </plugins>
    </build>
</project>

Recommended Tooling

ktlint Usage

# Install ktlint
brew install ktlint  # macOS
# Or download from: https://github.com/pinterest/ktlint

# Format code
ktlint -F "src/**/*.kt"

# Check code
ktlint "src/**/*.kt"

# Gradle plugin
# build.gradle.kts
plugins {
    id("org.jlleitschuh.gradle.ktlint") version "12.0.3"
}

detekt Usage

// build.gradle.kts
plugins {
    id("io.gitlab.arturbosch.detekt") version "1.23.0"
}

detekt {
    buildUponDefaultConfig = true
    allRules = false
    config.setFrom(files("$projectDir/config/detekt.yml"))
}

dependencies {
    detektPlugins("io.gitlab.arturbosch.detekt:detekt-formatting:1.23.0")
}

Production Best Practices

1. Immutability by default - Use val over var, immutable collections
2. Null safety - Embrace nullable types, avoid !! operator
3. Sealed classes for state - Type-safe state modeling with exhaustive when
4. Structured concurrency - Always use coroutineScope, never GlobalScope
5. Flow for streams - Use StateFlow/SharedFlow for reactive state
6. Data classes for DTOs - Automatic equals, hashCode, toString, copy
7. Inline value classes - Type-safe wrappers without runtime cost
8. Explicit over implicit - Clear, readable code over clever tricks
9. Early returns - Reduce nesting, improve readability
10. Descriptive naming - Functions and properties should explain intent
11. Prefer expressions - Use when, if as expressions when possible
12. Use extension functions - Add functionality without inheritance
13. Coroutine cancellation - Always handle cancellation properly
14. Test coroutines properly - Use runTest and TestDispatcher
15. ktlint + detekt - Automate code quality checks in CI

Comments - Less is More

// BAD - redundant comment
// Get user from repository
val user = repository.findById(id)

// GOOD - self-explanatory code, no comment needed
val user = repository.findById(id)

// GOOD - comment explains WHY (not obvious)
// Rate limit: API allows max 100 requests per minute per client
rateLimiter.acquire()

// GOOD - KDoc for public API
/**
 * Fetches user by ID. Returns null if user not found.
 *
 * @param id User ID (non-blank)
 * @throws IllegalArgumentException if id is blank
 */
fun findUserById(id: String): User?

References

• Kotlin Language Spec: https://kotlinlang.org/spec/
• Kotlin Coding Conventions: https://kotlinlang.org/docs/coding-conventions.html
• Kotlin Coroutines Guide: https://kotlinlang.org/docs/coroutines-guide.html
• Effective Kotlin by Marcin Moskała
• Kotlin in Action by Dmitry Jemerov & Svetlana Isakova