Can I add static extension fun for third-party class in kotlin?

Question

Given a third party class ThirdClass, which doesn't have a companion object, how can I define a static extension function for it? Something like this:

fun ThirdClass.Companion.hello(){}

Answer 1

Not at the moment, but it's tracked under KT-11968: Adding statically accessible members to an existing Java class via extensions (despite the name, the discussion covers Kotlin classes/interfaces without a companion as well). See particularly the comments starting here.

Answer 2

I think what you need is to declare an object inside the class marked as companion like from this link.

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

Members of the companion object can be called by using simply the class name as the qualifier:

val instance = MyClass.create()

Answer 3

You cannot declare a static function from outside of the class definition, but you can make an extension function that will work on all instances:

fun ThirdClass.hello() = println("Hello from ${this.javaClass.name}!")

fun main() {
    ThirdClass().hello()
}

Output:

Hello from ThirdClass!