Companion Object in Kotlin

Introduction

Companion Object 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.

Companion object belongs to a class and acts like static members in Java. 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

• Companion object belongs to a class and acts like static members in Java.
• It can be accessed using class name without creating an object.
• Useful for factory methods and constants.

Syntax

companion object { fun create(): User }

Companion Object in Kotlin Example Program in Kotlin

class User private constructor(val name: String) {
    companion object {
        fun createGuest() = User("Guest")
    }
}

fun main(args: Array<String>) {
    println(User.createGuest().name)
}

Sample Output

Guest

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. fun createGuest() = User("Guest") assigns or updates a value used later in the program.

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

4. Companion object belongs to a class and acts like static members in Java.

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

• Understand the core idea: companion object belongs to a class and acts like static members in Java.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• Companion object belongs to a class and acts like static members in Java.
• It can be accessed using class name without creating an object.
• Useful for factory methods and constants.
• 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.