Kotlin Expert
Expert guidance for Kotlin development, Android, coroutines, Kotlin Multiplatform, and modern JVM development.
Core Concepts
Kotlin Fundamentals
- Null safety
- Extension functions
- Data classes
- Sealed classes
- Coroutines and Flow
- Higher-order functions
Android Development
- Jetpack Compose
- ViewModel and LiveData
- Room database
- Retrofit networking
- Dependency injection (Hilt)
- Android lifecycle
Kotlin Multiplatform
- Shared business logic
- Platform-specific implementations
- iOS and Android targets
- Common module architecture
Modern Kotlin Syntax
// Data classes
data class User(
val id: String,
val name: String,
val email: String,
val createdAt: LocalDateTime = LocalDateTime.now()
)
// Sealed classes for type-safe states
sealed class Result<out T> {
data class Success<T>(val data: T) : Result<T>()
data class Error(val exception: Exception) : Result<Nothing>()
object Loading : Result<Nothing>()
}
// Extension functions
fun String.isValidEmail(): Boolean {
return this.contains("@") && this.contains(".")
}
// Scope functions
fun processUser(user: User) {
user.run {
println("Processing user: $name")
// 'this' refers to user
}
user.let { u ->
// 'it' or custom name refers to user
println(u.email)
}
user.apply {
// Modify properties
// Returns the object
}
}
// Null safety
fun findUser(id: String): User? {
return database.find(id)
}
val user = findUser("123")
val name = user?.name ?: "Unknown" // Elvis operator
user?.let { println(it.name) } // Safe call with let
// When expression
fun getUserStatus(user: User): String = when {
user.isActive && user.isPremium -> "Premium Active"
user.isActive -> "Active"
else -> "Inactive"
}Coroutines and Flow
import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*
class UserRepository {
private val api: UserApi
// Suspend function
suspend fun fetchUser(id: String): User {
return withContext(Dispatchers.IO) {
api.getUser(id)
}
}
// Flow for reactive streams
fun observeUsers(): Flow<List<User>> = flow {
while (true) {
val users = fetchUsers()
emit(users)
delay(5000) // Refresh every 5 seconds
}
}.flowOn(Dispatchers.IO)
// StateFlow for state management
private val _users = MutableStateFlow<List<User>>(emptyList())
val users: StateFlow<List<User>> = _users.asStateFlow()
suspend fun refreshUsers() {
_users.value = fetchUsers()
}
}
// Coroutine scopes
class UserViewModel : ViewModel() {
private val repository = UserRepository()
fun loadUsers() {
viewModelScope.launch {
try {
val users = repository.fetchUser("123")
// Update UI
} catch (e: Exception) {
// Handle error
}
}
}
// Parallel execution
suspend fun loadMultipleUsers(ids: List<String>): List<User> {
return coroutineScope {
ids.map { id ->
async { repository.fetchUser(id) }
}.awaitAll()
}
}
// Flow transformation
fun searchUsers(query: String): Flow<List<User>> {
return repository.observeUsers()
.map { users -> users.filter { it.name.contains(query, ignoreCase = true) } }
.distinctUntilChanged()
.debounce(300)
}
}Android Jetpack Compose
import androidx.compose.foundation.layout.*
import androidx.compose.material3.*
import androidx.compose.runtime.*
import androidx.compose.ui.Modifier
@Composable
fun UserListScreen(viewModel: UserViewModel = viewModel()) {
val users by viewModel.users.collectAsState()
val isLoading by viewModel.isLoading.collectAsState()
Scaffold(
topBar = { TopAppBar(title = { Text("Users") }) }
) { padding ->
if (isLoading) {
CircularProgressIndicator(
modifier = Modifier.fillMaxSize()
)
} else {
LazyColumn(
modifier = Modifier.padding(padding)
) {
items(users) { user ->
UserCard(user = user)
}
}
}
}
}
@Composable
fun UserCard(user: User) {
Card(
modifier = Modifier
.fillMaxWidth()
.padding(8.dp)
) {
Column(modifier = Modifier.padding(16.dp)) {
Text(
text = user.name,
style = MaterialTheme.typography.headlineSmall
)
Text(
text = user.email,
style = MaterialTheme.typography.bodyMedium
)
}
}
}Room Database
import androidx.room.*
import kotlinx.coroutines.flow.Flow
@Entity(tableName = "users")
data class UserEntity(
@PrimaryKey val id: String,
@ColumnInfo(name = "name") val name: String,
@ColumnInfo(name = "email") val email: String,
@ColumnInfo(name = "created_at") val createdAt: Long
)
@Dao
interface UserDao {
@Query("SELECT * FROM users")
fun getAllUsers(): Flow<List<UserEntity>>
@Query("SELECT * FROM users WHERE id = :userId")
suspend fun getUserById(userId: String): UserEntity?
@Insert(onConflict = OnConflictStrategy.REPLACE)
suspend fun insertUser(user: UserEntity)
@Update
suspend fun updateUser(user: UserEntity)
@Delete
suspend fun deleteUser(user: UserEntity)
@Query("DELETE FROM users WHERE id = :userId")
suspend fun deleteUserById(userId: String)
}
@Database(entities = [UserEntity::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
abstract fun userDao(): UserDao
companion object {
@Volatile
private var INSTANCE: AppDatabase? = null
fun getDatabase(context: Context): AppDatabase {
return INSTANCE ?: synchronized(this) {
val instance = Room.databaseBuilder(
context.applicationContext,
AppDatabase::class.java,
"app_database"
).build()
INSTANCE = instance
instance
}
}
}
}Dependency Injection with Hilt
import dagger.Module
import dagger.Provides
import dagger.hilt.InstallIn
import dagger.hilt.components.SingletonComponent
import javax.inject.Singleton
@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {
@Provides
@Singleton
fun provideRetrofit(): Retrofit {
return Retrofit.Builder()
.baseUrl("https://api.example.com")
.addConverterFactory(GsonConverterFactory.create())
.build()
}
@Provides
@Singleton
fun provideUserApi(retrofit: Retrofit): UserApi {
return retrofit.create(UserApi::class.java)
}
}
@HiltAndroidApp
class MyApplication : Application()
@AndroidEntryPoint
class MainActivity : ComponentActivity() {
@Inject lateinit var repository: UserRepository
}Kotlin Multiplatform
// commonMain
expect class Platform() {
val name: String
}
expect fun platformSpecificFunction(): String
// androidMain
actual class Platform actual constructor() {
actual val name: String = "Android ${android.os.Build.VERSION.SDK_INT}"
}
actual fun platformSpecificFunction(): String = "Android implementation"
// iosMain
actual class Platform actual constructor() {
actual val name: String = UIDevice.currentDevice.systemName()
}
actual fun platformSpecificFunction(): String = "iOS implementation"
// Shared business logic
class UserService {
suspend fun fetchUser(id: String): User {
// Shared logic works on all platforms
return api.getUser(id)
}
}Best Practices
Kotlin Style
- Use val over var when possible
- Leverage null safety features
- Use data classes for DTOs
- Prefer extension functions
- Use sealed classes for type-safe states
- Follow naming conventions
Coroutines
- Use appropriate dispatchers (IO, Main, Default)
- Handle cancellation properly
- Avoid GlobalScope
- Use structured concurrency
- Prefer Flow over LiveData
- Use StateFlow for state
Android
- Follow MVVM architecture
- Use Jetpack Compose for UI
- Implement proper lifecycle handling
- Use dependency injection
- Handle configuration changes
- Implement proper error handling
Anti-Patterns
❌ Using!! (non-null assertion) ❌ GlobalScope.launch ❌ Blocking main thread ❌ Not handling coroutine cancellation ❌ Tight coupling ❌ God classes ❌ Ignoring memory leaks
Resources
- Kotlin Docs: https://kotlinlang.org/docs/
- Android Developers: https://developer.android.com/kotlin
- Kotlin Coroutines: https://kotlinlang.org/docs/coroutines-overview.html
- Jetpack Compose: https://developer.android.com/jetpack/compose