Singleton Pattern in Kotlin

Introduction

Singleton Pattern is a fundamental concept every Kotlin developer should understand. Object declarations and companion objects provide singletons, factory methods, and static-like members without the ceremony of Java static blocks.

Singleton ensures only one instance of a class exists. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Singleton ensures only one instance of a class exists.
• Kotlin object declaration provides singleton by default.
• It is thread-safe and concise.

Syntax

object Config { val appName = "Demo" }

Singleton Pattern in Kotlin Example Program in Kotlin

object AppConfig {
    const val APP_NAME = "Kotlin Learning"
}

fun main(args: Array<String>) {
    println(AppConfig.APP_NAME)
}

Sample Output

Kotlin Learning

When to use

Use object for true singletons; use companion object for factory methods and constants tied to a class.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. const val APP_NAME = "Kotlin Learning" assigns or updates a value used later in the program.

3. The println(AppConfig.APP_NAME) statement writes a line to the console — this produces part of the sample output below.

4. Singleton ensures only one instance of a class exists.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• Singleton ensures only one instance of a class exists.
• Kotlin object declaration provides singleton by default.
• It is thread-safe and concise.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.