What is the point of naming a companion object in kotlin

Question

The documentation for companion objects has the following example

class MyClass {
    companion object Factory {
        fun create(): MyClass = MyClass()
    }
}

Here Factory is the name of the companion object. It then goes on to say:

The name of the companion object can be omitted, in which case the name Companion will be used:

However there is no example that I can see that uses the name of the companion object.

Since you can only have one companion object per class (otherwise you get a Only one companion object is allowed per class error) the name feels like some pretty useless syntactic sugar to me.

What can the name of the companion object actually be used for? Why would one bother to use any name for it?

Chatoyant

check this : #38382248 – Headfirst

@Headfirst that seems to address why a companion object is called a companion object, not why you might bother to specify a name for your companion object. – Chatoyant

Answer 1

You can use the name of the companion like:

MyClass.create()           // not via companion name
MyClass.Companion.create() // via default companion name
MyClass.Factory.create()   // via companion name

The name is maybe not that important for Kotlin, because you can just access the method without knowing that there is a companion object (line one above). It is more like a personal style, if you want to make the access to such functions more explicit.

But for java interop it makes a difference, because you have to access the function via the companion name:

    MyClass.Factory.create();   // with named companion
    MyClass.Companion.create(); // with unnamed comanion

Sphagnum

So it changes the way you call the java functions slightly - but there are existing ways to change how a class gets named when exposed to the JVM in particular @JvmName - why not use those? – Chatoyant

You can use @JvmName' for functions, but not for the name of the companion-object` itself. – Sphagnum

@MichaelAnderson also, @JvmName only works on the JVM, but Kotlin also targets JavaScript and native – Inoculation

AFAIK, when importing a companion object's feature, or referring to it as a method literal (callable reference), you need to specify the companion name. – Sudbury

Answer 2

Well, companion objects in Kotlin are not just syntactic sugar. They are actually a type. They are able to do much more thing, and need not to be see as just replacement of static.

You can actually extend class or implement an interface. See an example below.

open class Super {
    open fun sayHello() {
        println("Hello")
    }
}

class Some {
    companion object Child : Super() {
        override fun sayHello() {
            super.sayHello()
            println("Hello from companion object")
        }
    }
}

fun main() {
    Some.Child.sayHello()
}

Scrubby

Answer 3

If you do not use an explicit name, the companions name is Companion, thus it can be omitted, like you already quoted.

Sometimes you may want to have an explicit name in your calls, which would be MyClass.Factory.create() in your example. For namespace reasons maybe.

I don't see a many reasons to name a companion object, either. Except if you care about Java interop with your Kotlin code. Then, you need to explicitly write the companions name.

Another reason you might care about the name is, when you define an extension function on it:

  fun MyClass.Companion.ext() = "myext"

In this case, it can be clearer when it has a name like Factory, on which specific factory methods are added via extension.

Selfabuse

I can see this as being useful if you can have more than one companion object - so that you could group the factory functions into one, the utility functions into another. Can you do that? Otherwise it feels rather pointless. – Chatoyant

In answer to my own question there - you can only have one companion object per class. You get a Only one companion object is allowed per class error otherwise. – Chatoyant

The reasons I see: Java interop and extension function definition – Selfabuse

Yes. The main purpose of having a name for companion object is to define extension functions that can be called on the class itself instead of instance of a class. – Mei

You can still use the the extension function on the companion object when using the unnamed version. The default name of Companion works fine there. So IMO its still not really a reason for the user to be able to name it. – Chatoyant

Well then there's no reason for you to name it ;-) – Selfabuse

@Michael Anderson Oh, then I misunderstand your question. Since I see you are asking "What can the name of the companion object actually be used for?" and Companion is used as a name implicitly. – Mei

Answer 4

However there is no example that I can see that uses the name of the companion object.

class Person(val name: String) { companion object Loader {
fun fromJSON(jsonText: String): Person = ... }
}
>>> person = Person.Loader.fromJSON("{name: 'Dmitry'}") >>> person.name
Dmitry
>>> person2 = Person.fromJSON("{name: 'Brent'}") >>> person2.name
Brent

Jeffryjeffy