So my understanding of extension functions is that...

# getting-started

[Jack Englund]

So my understanding of extension functions is that you can add a function to an existing library as a static function. I've made this:

fun System.currentTimeSeconds(): Long {
    return System.currentTimeMillis()/1000
 }

and yet I can't do System.currentTimeSeconds(). Am I missing a concept or can you not do extension functions on System?

[Dominaezzz]

That will only work on an instance. Like System().currentTimeSeconds()

[Dominaezzz]

What you need is this, to make that work.

fun System.Companion.currentTimeSeconds(): Long {
    return System.currentTimeMillis()/1000
}

[Dominaezzz]

But System doesn't define a companion object, since it's Java nd stuff.

[Jack Englund]

The System()... doesn't work because <init> is private.

Why does the example used here:
https://kotlinlang.org/docs/reference/extensions.html
not create an instance?

[SiebelsTim]

What do you mean? All of these examples operate on instances. There is one example that operates on the companion object.

[Luca Nicoletti]

@Jack Englund the extension functions won’t create static functions. The function created is a normal function which can be invoked on a instance of the class you’re extending

[Alowaniak]

There's an issue for it https://youtrack.jetbrains.com/issue/KT-11968

[Jack Englund]

I was primarily looking at the first example in that docs page.

[Luca Nicoletti]

val list = mutableListOf(1, 2, 3)
list.swap(0, 2) // 'this' inside 'swap()' will hold the value of 'list'
That’s exactly what we said, the swap function is called on an instance