can anyone explain the use case for companion objects? static methods/properties in java can be impl...

# getting-started

[Bernhard]

can anyone explain the use case for companion objects? static methods/properties in java can be implemented by file private variables or functions instead; I don't really see a good use case for it other than Java interop

[Sam]

Namespacing is the only thing that really springs to mind. If it's going to be accessed from outside, I'd rather write Foo.something than have a global property called somethingToDoWithFoo. Of course companion objects aren't really a great solution to that problem, hence https://youtrack.jetbrains.com/issue/KT-11968. If those proposals end up being implemented I can't see there being much use for companion objects anymore.

[Bernhard]

thank you, that sounds interesting

[Joffrey]

Another important thing is that the companion object is an object, and as such it can implement interfaces, such as factories. In that sense, the fact that the companion object has the name of the class is interesting.

[Bernhard]

You mean for use in reflection?

[Joffrey]

No just for nice API design, without actually needing to use reflection

[ephemient]

they do have some uses as singleton instances associated with their containing class

[ephemient]

kotlinx.serialization.json.Json, for example, is not meant to be a type directly instantiable by the user, for binary compatibility evolution reasons. the name refers to a builder function for an abstract class, and also that class's companion object which is a default instance

[ephemient]

it's the same as a companion object in Scala, except that it's nested in the class instead of being a separate object that happens to use the same name