Kotlin with Gradle, Android and Multiplatform

Gradle

A typical JVM project:

// build.gradle.kts
plugins {
    kotlin("jvm") version "2.4.21"
    application
}

repositories { mavenCentral() }

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.11.0")
    testImplementation(kotlin("test"))
}

kotlin { jvmToolchain(21) }
application { mainClass = "AppKt" }

Keep versions in a version catalog (gradle/libs.versions.toml) once a project has several modules.

Where Kotlin runs

• Android: Kotlin with Jetpack Compose for UI is the default for new apps (see the Android course).
• Servers: Spring Boot has first-class Kotlin support; Ktor is JetBrains' own coroutine-based framework.
• Kotlin Multiplatform (KMP): share business logic between Android, iOS, desktop and web in a commonMain source set, with platform code in androidMain, iosMain and so on. Compose Multiplatform shares UI too.

// commonMain: shared by every platform
expect fun platformName(): String

fun greeting() = "Hello from ${platformName()}"

// androidMain
actual fun platformName() = "Android"

// iosMain
actual fun platformName() = "iOS"

The shared code is ordinary Kotlin:

// Shared business logic: no platform APIs, so it can live in commonMain
data class CartItem(val name: String, val price: Double, val quantity: Int)

fun cartTotal(items: List<CartItem>, couponPercent: Int = 0): Double {
    val subtotal = items.sumOf { it.price * it.quantity }
    return subtotal * (100 - couponPercent) / 100
}

fun main() {
    val items = listOf(CartItem("mug", 12.0, 2), CartItem("tea", 6.5, 1))
    println("%.2f".format(cartTotal(items)))
    println("%.2f".format(cartTotal(items, couponPercent = 10)))
}

30.50
27.45