Beginner 7 min readKotlin 2.0
Kotlin Companion Object — Factory Methods and Constants
A companion object is a singleton tied to a class. Access its members with ClassName.member — similar to Java static members.
What You Will Learn
- Declare companion objects
- Store constants and factory methods
- Implement interfaces on companions
- Compare with Java static
- Use @JvmStatic for Java interop
Declaring a Companion
Place companion object inside a class body. There can be only one companion per class.
Constants and Factory
kotlin
class User private constructor(val name: String) {
companion object {
const val MAX_NAME = 50
fun create(name: String): User {
require(name.length <= MAX_NAME)
return User(name)
}
}
}
fun main() {
val u = User.create("Juned")
println("${u.name} (max ${User.MAX_NAME})")
}Output
Juned (max 50)
Private constructor forces creation through the companion factory. const val is compile-time constant.
Best Practice: Use companion factories when construction logic is non-trivial.
Companion Implements Interface
Companions can implement interfaces — useful for plugin-style APIs and testing.
Companion as Factory Interface
kotlin
interface Parser {
fun parse(input: String): Int
}
class NumberParser private constructor() {
companion object : Parser {
override fun parse(input: String) = input.toInt()
}
}
fun main() {
println(NumberParser.parse("42"))
}Output
42
Call parse via NumberParser.parse because the companion implements Parser.
Practice Exercise
Exercisemultiple choice
How do you access a companion object member?
Quick Quiz
Quick Quiz
How many companion objects can a class have?
Frequently Asked Questions
Related Tutorials
Last updated: 2026-05-19Kotlin 2.0
Written by KotlinGuide Editorial Team · Reviewed by KotlinGuide Technical Review