kotlin-patterns

Kotlin Patterns Skill

Project Structure

Gradle KTS Multi-Module Layout

project-root/
├── build.gradle.kts
├── settings.gradle.kts
├── gradle/libs.versions.toml
├── app/
│   ├── build.gradle.kts
│   └── src/{main,test}/kotlin/com/example/app/
├── domain/
│   └── src/main/kotlin/com/example/domain/
│       ├── model/
│       ├── repository/
│       └── usecase/
└── infrastructure/
    └── src/main/kotlin/com/example/infra/

settings.gradle.kts

rootProject.name = "my-project"
dependencyResolutionManagement {
    versionCatalogs { create("libs") { from(files("gradle/libs.versions.toml")) } }
}
include(":app", ":domain", ":infrastructure")

Module build.gradle.kts

plugins {
    alias(libs.plugins.kotlin.jvm)
    alias(libs.plugins.kotlin.serialization)
}
dependencies {
    implementation(project(":domain"))
    implementation(libs.kotlinx.coroutines.core)
    testImplementation(libs.bundles.testing)
}
kotlin { jvmToolchain(21) }

---

Idioms / Code Style

Data Classes + Value Classes

data class User(
    val id: UserId,
    val name: String,
    val email: String,
    val role: Role = Role.USER,
) {
    init {
        require(name.isNotBlank()) { "Name must not be blank" }
        require(email.contains("@")) { "Invalid email format" }
    }
}

@JvmInline
value class UserId(val value: String)  // Zero-overhead type-safe ID
enum class Role { ADMIN, USER, GUEST }

Sealed Interfaces for Domain Modeling

sealed interface PaymentResult {
    data class Success(val transactionId: String, val amount: Money) : PaymentResult
    data class Declined(val reason: String) : PaymentResult
    data class Error(val exception: Throwable) : PaymentResult
}

// Exhaustive when -- compiler enforces all branches
fun handlePayment(result: PaymentResult): String = when (result) {
    is PaymentResult.Success -> "Paid: ${result.amount}"
    is PaymentResult.Declined -> "Declined: ${result.reason}"
    is PaymentResult.Error -> "Error: ${result.exception.message}"
}