Kotlin

Kotlin addresses Java's limitations with a more concise, expressive syntax while maintaining full Java interoperability.

Advantages:
• Concise syntax: less boilerplate than Java
• Null safety: eliminate NullPointerException
• Extension functions: add methods to existing classes
• Smart casts: automatic type checking
• Coroutines: simplified async programming
• Data classes: automatic equals(), toString(), hashCode()

Kotlin introduces modern language features common in functional and expressive languages.

Features:
• Higher-order functions and lambdas
• Destructuring: unpack data easily
• When expressions: powerful pattern matching
• Sealed classes: restricted inheritance
• Scope functions: with, apply, let, also
• Operator overloading
• Infix functions for DSLs

Kotlin is the official language for Android development, recommended by Google.

Android Features:
• Kotlin coroutines: replace callbacks with structured concurrency
• Compose: declarative UI framework for Android
• Jetpack integration: seamless with Android libraries
• Official support: first-class IDE support in Android Studio
• Performance: minimal runtime overhead
• Interoperability: use existing Java libraries

Kotlin runs on the JVM, JavaScript, and native platforms, enabling diverse applications.

Platforms:
• JVM: backend services, microservices
• Android: mobile applications
• JavaScript: browser and Node.js
• Native: iOS and Linux compilation
• Multiplatform: code sharing across targets

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

// ── Structured concurrency ──────────────────────────── suspend fun loadDashboard(): Dashboard = coroutineScope { val users = async { fetchUsers() } val stats = async { fetchStats() } val alerts = async { fetchAlerts() } // All run in parallel, wait for all to complete Dashboard(users.await(), stats.await(), alerts.await()) }

// ── Flow cold, lazy asynchronous streams ──────────── fun userStream(query: String): Flow> = flow { while (true) { emit(api.searchUsers(query)) delay(5_000) // poll every 5 seconds } }

// Collect with operators viewModelScope.launch { searchQuery .debounce(300) .distinctUntilChanged() .flatMapLatest { q -> userStream(q) } .catch { e -> emit(emptyList()) } .collectLatest { users -> _uiState.update { it.copy(users = users) } } }

// ── StateFlow in ViewModel ──────────────────────────── class UserViewModel : ViewModel() { private val _state = MutableStateFlow(UserUiState()) val state: StateFlow = _state.asStateFlow()

fun loadUser(id: String) { viewModelScope.launch { _state.update { it.copy(isLoading = true) } runCatching { repository.getUser(id) } .onSuccess { user -> _state.update { it.copy(user = user, isLoading = false) } } .onFailure { e -> _state.update { it.copy(error = e.message, isLoading = false) } } } } } ```

@Composable
fun ProductScreen(vm: ProductViewModel = hiltViewModel()) {
    val state by vm.state.collectAsStateWithLifecycle()

LazyColumn( contentPadding = PaddingValues(16.dp), verticalArrangement = Arrangement.spacedBy(12.dp) ) { items(state.products, key = { it.id }) { product -> ProductCard( product = product, onAddToCart = { vm.addToCart(product) }, modifier = Modifier.animateItemPlacement() ) } } }

@Composable fun ProductCard( product: Product, onAddToCart: () -> Unit, modifier: Modifier = Modifier ) { Card( modifier = modifier.fillMaxWidth(), elevation = CardDefaults.cardElevation(2.dp) ) { Row( modifier = Modifier.padding(16.dp), verticalAlignment = Alignment.CenterVertically ) { AsyncImage( model = product.imageUrl, contentDescription = product.name, modifier = Modifier.size(80.dp).clip(RoundedCornerShape(8.dp)) ) Spacer(Modifier.width(12.dp)) Column(Modifier.weight(1f)) { Text(product.name, style = MaterialTheme.typography.titleMedium) Text("$" + product.price, style = MaterialTheme.typography.bodySmall, color = MaterialTheme.colorScheme.primary) } IconButton(onClick = onAddToCart) { Icon(Icons.Default.AddShoppingCart, contentDescription = "Add to cart") } } } } ```