Does Kotlin have something like static variable inside a function?

Question

I'm wondering if it has something that works like static variable inside a function in C.

In C language we have this:

void next_x()
{
    static int x = 0;
    x++;
}

Variable x is declared and initialized inside a function. As far I know C - it can be used only in the scope of this function and it is initialized only in first call of this function.

I need something like this in Kotlin. I have code similar to this:

private val x: Int = 0

fun getNextX() : Int {
    x++;
    return x;
}

and I would like to have something like this:

fun getNextX() : Int {
    static val x: Int = 0 // this is not Kotlin code
    x++;
    return x;
}

I want to:

Limit x variable scope to emphasize that this object is only used by this function and protect it from changes from outside
Initialize it only once
Keep value/state between function calls

Example above was simplified. In fact I need something like this for ArrayList with limited scope, but retaining state.

I realize that we have singleton pattern which is almost perfect for such needs (except limited scope), but maybe Kotlin offers something else?

Does this answer your question? how do you declare static property in kotlin? — Marcin Orlowski
– Marcin Orlowski, Commented Nov 21, 2022 at 21:08
No, Kotlin doesn't feature static local variables. In any case, their usefulness would be somewhat limited because such a function would not be thread-safe. — Klitos Kyriacou
– Klitos Kyriacou, Commented Nov 21, 2022 at 21:11
@MarcinOrlowski No, for two reasons. Reason 1: static variable in C is something diffrent from Java static variable. Reason 2: You cannot declare static variable inside a method in Java. — Kamil
– Kamil, Commented Nov 21, 2022 at 21:13

Sweeper · Accepted Answer · 2022-11-21 21:17:09Z

5

There isn't anything similar to that in Kotlin, but to simulate a similar effect, you can declare an object with all your static variables as properties, and put the function body in an invoke operator:

object NextX {
    private var x = 0
    operator fun invoke() = x++
}

You basically create an object, but syntactically, it works like a function that you can call:

fun main() {
    println(NextX()) // 0
    println(NextX()) // 1
    println(NextX()) // 2
}

If you think of the entire object as your function, then x is indeed only accessible within the "function".

edited Nov 21, 2022 at 21:17

answered Nov 21, 2022 at 21:14

Sweeper

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1 Comment

Kamil Over a year ago

That's pretty close to my idea. I will experiment with that and see if such thing would be useful.

Willi Mentzel · Accepted Answer · 2022-11-21 21:59:29Z

3

Functions in Kotlin are stateless, so this is not possible out of the box.

You can get creative though, by having an object with a function which returns the function you desire.

val foo: () -> Int = object {
  var x = 0
  fun createFunc() = fun() = x++
}.createFunc()

foo() // 0
foo() // 1

Note how x doesn't even have to be private, because the object itself cannot be referenced.

This would meet your criteria.

edited Nov 21, 2022 at 21:59

answered Nov 21, 2022 at 21:28

Willi Mentzel

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