How do I implement the singleton pattern idiomatically using Kotlin’s object declaration?

In Kotlin, the idiomatic way to implement the Singleton pattern is to use an object declaration.

object AppConfig {
    val appName = "MyApp"

    fun printConfig() {
        println("App name: $appName")
    }
}

You use it directly by its name:

fun main() {
    AppConfig.printConfig()
    println(AppConfig.appName)
}

Kotlin guarantees that an object declaration:

• has exactly one instance
• is initialized lazily, when first accessed
• is thread-safe
• can contain properties, functions, initialization blocks, and implement interfaces

Example with initialization:

object DatabaseManager {
    init {
        println("DatabaseManager initialized")
    }

    fun connect() {
        println("Connecting to database...")
    }
}

Usage:

fun main() {
    DatabaseManager.connect()
}

If you need a singleton that implements an interface:

interface Logger {
    fun log(message: String)
}

object ConsoleLogger : Logger {
    override fun log(message: String) {
        println("[LOG] $message")
    }
}

Usage:

fun main() {
    ConsoleLogger.log("Application started")
}

So instead of writing a private constructor and static getInstance() method like in Java, Kotlin’s idiomatic singleton is simply:

object MySingleton {
    fun doSomething() {
        println("Doing something")
    }
}