How to Create Singleton in Kotlin: Simple Guide

In Kotlin, you create a singleton by using the object keyword, which defines a class with only one instance. This instance is created lazily and is thread-safe by default, so you just use object MySingleton to define it.

Syntax

The object keyword in Kotlin declares a singleton. It creates a class and its single instance at the same time. You don't need to write constructors or manage instance creation.

• object MySingleton: Defines the singleton object.
• Inside, you can add properties and functions like a normal class.
• Access members directly via MySingleton.member.

kotlin

object MySingleton {
    val name = "SingletonExample"
    fun greet() = "Hello from $name"
}

Example

This example shows a singleton that holds a counter and a greeting function. It demonstrates how to access and modify the singleton's properties.

kotlin

object CounterSingleton {
    var count = 0
    fun increment() {
        count++
    }
    fun greet() = "Count is $count"
}

fun main() {
    println(CounterSingleton.greet())
    CounterSingleton.increment()
    CounterSingleton.increment()
    println(CounterSingleton.greet())
}

Output

Count is 0
Count is 2

Common Pitfalls

Some common mistakes when creating singletons in Kotlin include:

• Trying to use a class with private constructors and manual instance management instead of object.
• Assuming object instances are created eagerly; they are lazy but thread-safe.
• Using companion objects when a full singleton object is needed.

Using object is the simplest and safest way to create a singleton in Kotlin.

kotlin

/* Wrong way: manual singleton with class */
class ManualSingleton private constructor() {
    companion object {
        private var instance: ManualSingleton? = null
        fun getInstance(): ManualSingleton {
            if (instance == null) {
                instance = ManualSingleton()
            }
            return instance!!
        }
    }
}

/* Right way: Kotlin singleton */
object KotlinSingleton {
    fun greet() = "Hello from KotlinSingleton"
}

Quick Reference

Summary tips for Kotlin singletons:

• Use object keyword to create a singleton.
• Access members directly via the object name.
• Singletons are thread-safe and lazily initialized.
• Use companion objects only for static-like members inside classes.

Key Takeaways

Use the Kotlin object keyword to create a singleton easily and safely.

Singleton objects are lazily initialized and thread-safe by default.

Access singleton members directly through the object name without creating instances.

Avoid manual singleton patterns with private constructors; Kotlin's object is simpler.

Use companion objects only for static members inside classes, not for full singletons.