Kotlin Companion Objects

Before taking about companion objects, let's take an example to access members of a class.

class Person {
    fun callMe() = println("I'm called.")
}

fun main(args: Array<String>) {
    val p1 = Person()
    
    // calling callMe() method using object p1
    p1.callMe()    
}

Here, we created an object p1 of the Person class to call callMe() method. That's how things normally work.

However, in Kotlin, you can also call callMe() method by using the class name, i.e, Person in this case. For that, you need to create a companion object by marking object declaration with companion keyword.

Example: Companion objects

class Person {
    companion object Test {
        fun callMe() = println("I'm called.")
    }
}

fun main(args: Array<String>) {
    Person.callMe()
}

When you run the program, the output will be:

I'm called.

In the program, Test object declaration is marked with keyword companion to create a companion object. Hence, it is possible to call callMe() method by using the name of the class as:

Person.callMe()

The name of the companion object is optional and can be omitted.

class Person {
    
    // name of the companion object is omitted
    companion object {
        fun callMe() = println("I'm called.")
    }
}

fun main(args: Array<String>) {
    Person.callMe()
}

If you are familiar with Java, you may relate companion objects with static methods (even though how they work internally is totally different).

The companion objects can access private members of the class. Hence, they can be used to implement the factory method patterns.