--- name: java-kotlin description: Java and Kotlin programming patterns domain: programming-languages version: 1.0.0 tags: [java, kotlin, jvm, spring, android] triggers: keywords: primary: [java, kotlin, jvm, spring, springboot, gradle, maven] secondary: [android, hibernate, jpa, stream, coroutine, quarkus] context_boost: [enterprise, backend, microservice, mobile, android] context_penalty: [python, javascript, rust, go] priority: high --- # Java & Kotlin ## Overview Modern Java and Kotlin patterns for JVM development. --- ## Java Modern Features ### Records and Sealed Classes (Java 17+) ```java // Record - immutable data class public record User( String id, String email, String name, Instant createdAt ) { // Compact constructor for validation public User { Objects.requireNonNull(id); Objects.requireNonNull(email); if (!email.contains("@")) { throw new IllegalArgumentException("Invalid email"); } } // Static factory method public static User create(String email, String name) { return new User(UUID.randomUUID().toString(), email, name, Instant.now()); } } // Sealed classes - restricted hierarchy public sealed interface Shape permits Circle, Rectangle, Triangle { double area(); } public record Circle(double radius) implements Shape { @Override public double area() { return Math.PI * radius * radius; } } public record Rectangle(double width, double height) implements Shape { @Override public double area() { return width * height; } } public final class Triangle implements Shape { private final double base; private final double height; public Triangle(double base, double height) { this.base = base; this.height = height; } @Override public double area() { return 0.5 * base * height; } } ``` ### Pattern Matching ```java // Pattern matching for instanceof (Java 16+) public String describe(Object obj) { if (obj instanceof String s) { return "String of length " + s.length(); } else if (obj instanceof Integer i) { return "Integer: " + i; } else if (obj instanceof List list && !list.isEmpty()) { return "Non-empty list with " + list.size() + " elements"; } return "Unknown type"; } // Pattern matching for switch (Java 21+) public double calculateArea(Shape shape) { return switch (shape) { case Circle c -> Math.PI * c.radius() * c.radius(); case Rectangle r -> r.width() * r.height(); case Triangle t -> 0.5 * t.base() * t.height(); }; } // Guard patterns public String classify(Shape shape) { return switch (shape) { case Circle c when c.radius() > 10 -> "Large circle"; case Circle c -> "Small circle"; case Rectangle r when r.width() == r.height() -> "Square"; case Rectangle r -> "Rectangle"; default -> "Other shape"; }; } ``` ### Streams and Optionals ```java import java.util.stream.*; import java.util.Optional; public class StreamExamples { // Basic operations public List processUsers(List users) { return users.stream() .filter(u -> u.active()) .map(User::name) .sorted() .distinct() .collect(Collectors.toList()); } // Grouping public Map> groupByDomain(List users) { return users.stream() .collect(Collectors.groupingBy( u -> u.email().substring(u.email().indexOf("@") + 1) )); } // Statistics public DoubleSummaryStatistics getStats(List orders) { return orders.stream() .mapToDouble(Order::total) .summaryStatistics(); } // Parallel processing public long countLargeFiles(Path directory) throws IOException { try (Stream paths = Files.walk(directory)) { return paths .parallel() .filter(Files::isRegularFile) .filter(p -> { try { return Files.size(p) > 1_000_000; } catch (IOException e) { return false; } }) .count(); } } // Optional handling public String getUserEmail(Long userId) { return findUser(userId) .map(User::email) .filter(email -> !email.isBlank()) .orElse("unknown@example.com"); } public User getOrCreate(Long userId) { return findUser(userId) .orElseGet(() -> createUser(userId)); } } ``` --- ## Kotlin Fundamentals ### Data Classes and Null Safety ```kotlin // Data class (like Java record) data class User( val id: String = UUID.randomUUID().toString(), val email: String, val name: String, val createdAt: Instant = Instant.now() ) { init { require(email.contains("@")) { "Invalid email" } } } // Null safety fun processUser(user: User?) { // Safe call val name = user?.name // Elvis operator val displayName = user?.name ?: "Anonymous" // Smart cast after null check if (user != null) { println(user.email) // user is User, not User? } // let for null-safe operations user?.let { sendEmail(it.email) } // Not-null assertion (use sparingly) val email = user!!.email // throws if null } // Platform types from Java fun handleJavaString(javaString: String?) { // Treat Java strings as nullable val length = javaString?.length ?: 0 } ``` ### Extension Functions and Properties ```kotlin // Extension function fun String.toSlug(): String = this.lowercase() .replace(Regex("[^a-z0-9]+"), "-") .trim('-') // Extension property val String.wordCount: Int get() = this.split(Regex("\\s+")).size // Extension on nullable type fun String?.orEmpty(): String = this ?: "" // Usage val slug = "Hello World".toSlug() // "hello-world" val count = "Hello World".wordCount // 2 // Scope functions data class Config(var host: String = "", var port: Int = 0) val config = Config().apply { host = "localhost" port = 8080 } val result = user.let { it.name.uppercase() } val processedUser = user.also { log.info("Processing ${it.id}") } val transformed = with(user) { "$name <$email>" } ``` ### Coroutines ```kotlin import kotlinx.coroutines.* import kotlinx.coroutines.flow.* // Suspend function suspend fun fetchUser(id: String): User { return withContext(Dispatchers.IO) { api.getUser(id) } } // Concurrent execution suspend fun fetchAllUsers(ids: List): List = coroutineScope { ids.map { id -> async { fetchUser(id) } }.awaitAll() } // Structured concurrency suspend fun processOrders(orders: List) = coroutineScope { orders.forEach { order -> launch { processOrder(order) } } } // Exception handling suspend fun safeFetch(id: String): Result = runCatching { fetchUser(id) } // Flow (cold stream) fun fetchUsers(): Flow = flow { val users = api.getAllUsers() users.forEach { user -> emit(user) delay(100) } } // Flow operators suspend fun processUserFlow() { fetchUsers() .filter { it.active } .map { it.name } .catch { e -> emit("Error: ${e.message}") } .collect { name -> println(name) } } // StateFlow for state management class UserViewModel : ViewModel() { private val _state = MutableStateFlow(UiState.Loading) val state: StateFlow = _state.asStateFlow() fun loadUser(id: String) { viewModelScope.launch { _state.value = UiState.Loading try { val user = fetchUser(id) _state.value = UiState.Success(user) } catch (e: Exception) { _state.value = UiState.Error(e.message ?: "Unknown error") } } } } ``` ### Sealed Classes and When ```kotlin // Sealed class hierarchy sealed class Result { data class Success(val data: T) : Result() data class Error(val message: String, val cause: Throwable? = null) : Result() object Loading : Result() } // Exhaustive when fun handleResult(result: Result): String = when (result) { is Result.Success -> "Success: ${result.data}" is Result.Error -> "Error: ${result.message}" Result.Loading -> "Loading..." } // Sealed interface sealed interface UiEvent { data class ShowMessage(val message: String) : UiEvent data class Navigate(val route: String) : UiEvent object GoBack : UiEvent } // When with guards fun describe(value: Any): String = when { value is String && value.isEmpty() -> "Empty string" value is String -> "String: $value" value is Int && value > 0 -> "Positive int" value is Int -> "Non-positive int" else -> "Unknown" } ``` --- ## Functional Patterns ```kotlin // Higher-order functions inline fun List.customFilter(predicate: (T) -> Boolean): List { val result = mutableListOf() for (item in this) { if (predicate(item)) { result.add(item) } } return result } // Function composition infix fun ((B) -> C).compose(other: (A) -> B): (A) -> C = { a -> this(other(a)) } val double: (Int) -> Int = { it * 2 } val addOne: (Int) -> Int = { it + 1 } val doubleThenAddOne = addOne compose double println(doubleThenAddOne(3)) // 7 // Currying fun add(a: Int): (Int) -> Int = { b -> a + b } val add5 = add(5) println(add5(3)) // 8 // Memoization fun ((T) -> R).memoize(): (T) -> R { val cache = mutableMapOf() return { key -> cache.getOrPut(key) { this(key) } } } val fibonacci: (Int) -> Long = { n: Int -> if (n <= 1) n.toLong() else fibonacci(n - 1) + fibonacci(n - 2) }.memoize() ``` --- ## DSL Building ```kotlin // Type-safe builder DSL class HtmlBuilder { private val children = mutableListOf() fun head(block: HeadBuilder.() -> Unit) { children.add(HeadBuilder().apply(block).build()) } fun body(block: BodyBuilder.() -> Unit) { children.add(BodyBuilder().apply(block).build()) } fun build() = "${children.joinToString("")}" } class BodyBuilder { private val children = mutableListOf() fun div(classes: String = "", block: BodyBuilder.() -> Unit = {}) { children.add("

${BodyBuilder().apply(block).build()}

") } fun p(text: String) { children.add("

$text

") } fun build() = children.joinToString("") } fun html(block: HtmlBuilder.() -> Unit) = HtmlBuilder().apply(block).build() // Usage val page = html { body { div("container") { p("Hello, World!") } } } // Test DSL class TestContext { fun given(description: String, block: () -> Unit) { println("Given: $description") block() } fun whenever(description: String, block: () -> Unit) { println("When: $description") block() } fun then(description: String, block: () -> Unit) { println("Then: $description") block() } } fun test(name: String, block: TestContext.() -> Unit) { println("Test: $name") TestContext().apply(block) } // Usage test("user registration") { given("a new user email") { } whenever("registering the user") { } then("user should be created") { } } ``` --- ## Related Skills - [[backend]] - Spring Boot development - [[mobile]] - Android development - [[testing]] - JUnit, Kotest

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