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

kotlin-expert

Kotlin Expert

Core Concepts

Kotlin Fundamentals

Android Development

Kotlin Multiplatform

Modern Kotlin Syntax

Coroutines and Flow

Android Jetpack Compose

Room Database

Dependency Injection with Hilt

Kotlin Multiplatform

Best Practices

Kotlin Style

Coroutines

Android

Anti-Patterns

Resources