Kotlin

Modern Kotlin development with coroutines, null safety, and idiomatic patterns.

When to Use

• Working with .kt files
• Android app development
• JVM backend with Spring Boot/Ktor
• Multiplatform projects (KMP)

Quick Start

data class User(
    val id: String,
    val name: String,
    val email: String,
    val createdAt: Instant = Clock.System.now()
)

suspend fun fetchUser(id: String): User? {
    return apiService.getUser(id)
}

Core Concepts

Null Safety

// Nullable types
val name: String? = null

// Safe calls
val length = name?.length

// Elvis operator
val len = name?.length ?: 0

// Smart casts
if (name != null) {
    println(name.length) // Smart cast to String
}

// Not-null assertion (use sparingly)
val len = name!!.length

Data Classes & Sealed Classes

data class User(
    val id: String,
    val name: String,
    val email: String
)

sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Error(val message: String) : Result<Nothing>()
    object Loading : Result<Nothing>()
}

// Exhaustive when
fun handleResult(result: Result<User>) = when (result) {
    is Result.Success -> println(result.data)
    is Result.Error -> println(result.message)
    Result.Loading -> println("Loading...")
}

Common Patterns

Coroutines

// Suspend function
suspend fun fetchData(): List<Item> {
    return withContext(Dispatchers.IO) {
        api.fetchItems()
    }
}

// Parallel execution
suspend fun loadDashboard(): Dashboard {
    return coroutineScope {
        val user = async { fetchUser() }
        val orders = async { fetchOrders() }
        Dashboard(user.await(), orders.await())
    }
}

// Flow for streams
fun observeUsers(): Flow<List<User>> = flow {
    while (true) {
        emit(fetchUsers())
        delay(5000)
    }
}.flowOn(Dispatchers.IO)

Extension Functions

fun String.isValidEmail(): Boolean {
    return Regex("^[\\w-\\.]+@([\\w-]+\\.)+[\\w-]{2,4}\$").matches(this)
}

fun <T> List<T>.secondOrNull(): T? = getOrNull(1)

inline fun <T> Result<T>.onSuccess(action: (T) -> Unit): Result<T> {
    if (this is Result.Success) action(data)
    return this
}

Best Practices

Do:

• Use data classes for DTOs
• Prefer immutability (val over var)
• Use sealed classes for state
• Use coroutines for async work

Don't:

• Use !! without null check
• Create utility classes (use extensions)
• Block main thread with runBlocking
• Ignore cancellation in coroutines

Troubleshooting

Error	Cause	Solution
NullPointerException	Force unwrap on null	Use safe call ?.
CancellationException	Coroutine cancelled	Handle or propagate
IllegalStateException	Invalid state access	Check state before access

References

• Kotlin Official Docs
• Android Kotlin Guides