In this article, you’re going to learn what companion object exactly is, how it behaves and what you can use it for in Kotlin!

Why you should use it?

The key to using companion object is that you treat it like a ‘companion’ of a given class/interface . From a technical perspective, it’s a special object associated directly with a class instead of its instances, meaning no matter how many instances you create, there’ll always be only 1 companion object. It allows you to write more readable, performant, and maintainable code. You can use it for:

• Holding constants associated with a class
• Writing utility functions
• Sharing resources
• Creating factory methods

How does it work?

Example

Let’s work on an example so things are more straightforward. Here’s how you can define companion object :

// It could be an interface as well
class Example {
    companion object {
        const val TIMEOUT_MILLIS = 50
    } 
}

In the case above, we have a nameless companion object that’s associated with Example class . Inside it, you can write functions, define values and variables, etc., but remember that they’ll work like a Singleton.

Usage

class Example {
    fun timeout() {
        Thread.sleep(TIMEOUT_MILLIS)
    }
    companion object {
        const val TIMEOUT_MILLIS = 50
    } 
}

fun main() {
    Thread.sleep(Example.TIMEOUT_MILLIS) // Used outside
}

So there’s an additional benefit: sometimes you’ll need a method that is associated with a given class but used outside of it as well and with companion object you can do it.

What are the key characteristics?

1. It’s public by default, but you can scope it to private
2. Assigned only to a class or interface , we cannot assign it to an object as it does not apply to standalone objects (it’s not a class).
3. There can only be one companion object per class/interface
4. Under the hood companion object needs a name that’s Companion by default but can be easily changed, f.e companion object YourName {...}

Additionally, it can access private members of a class — it sounds mysterious, but an example should clear things up:

class Example {
    private var age: Int = 0

    companion object {
        fun updateAgeRandomly(example: Example) {
            example.age = Random.nextInt()
            println(example.age)
        }
    } 
}

This article is previously published on proandroiddev.com.