Object Declaration in Kotlin

Introduction

Object Declaration 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.

Object keyword creates a singleton instance. 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

• object keyword creates a singleton instance.
• Only one instance exists for the whole application.
• Object members are accessed using the object name.

Syntax

object Database { fun connect() }

Object Declaration in Kotlin Example Program in Kotlin

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

fun main(args: Array<String>) {
    Logger.log("Application started")
}

Sample Output

[LOG] Application started

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. The println("[LOG] $message") statement writes a line to the console — this produces part of the sample output below.

3. Object keyword creates a singleton instance.

4. 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

• object keyword creates a singleton instance.
• Only one instance exists for the whole application.
• Object members are accessed using the object name.
• 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.