kotlin extension method access in another kt

Question

I'm thinking about adding a global extension method to String in just one file,and wherever i use a String i can always use this extension.

But i failed to find a way to do so...i just paste the extension everywhere now.

extension here in A.kt:

class A{
    ......
    fun String.add1(): String {
        return this + "1"
    }
    ......
}

and access like this in B.kt:

class B{
    fun main(){
        ......
        var a = ""
        a.add1()
        ......
    }
}

I've tried everyting i can add like static and final but nothing worked.

Hey, this should absolutely work. Are you using IDEA? Are there no suggestions for imports? — zsmb13, Jul 13 '17 at 04:47
@zsmb13 i'm using Android Studio with kotlin plugin ver 1.1.3-2,and no import suggestions,it said Unsolved reference — Draculea, Jul 13 '17 at 04:50
Do you have an exact error message? Is it just "Unresolved reference" on the extension's name? — zsmb13, Jul 13 '17 at 04:52
Is the extension function a top-level function and all your classes either in the same package as the extension function or imported? — Naetmul, Jul 13 '17 at 04:53
@zsmb13 yes, well i mean i just put a extension fun in a kt file and try to access b.add1() in another kt file — Draculea, Jul 13 '17 at 04:55
@zsmb13 well not in same package and two kt files have no relationship — Draculea, Jul 13 '17 at 04:56
Can you tell us what the error that you're getting says exactly? — zsmb13, Jul 13 '17 at 04:57
Well, if they are in the different package, you may have to import the extension function, but I don't sure about it. — Naetmul, Jul 13 '17 at 05:00
@zsmb13 well i failed again in importing the extension [tears] — Draculea, Jul 13 '17 at 05:10
Now that you've updated the code in your post, I've figured it out, see my answer below. — zsmb13, Jul 13 '17 at 05:11

score 31 · Accepted Answer · answered Jul 13 '17 at 05:06

31

Make sure your extension function is a top level function, and isn't nested in a class - otherwise it will be a member extension, which is only accessible inside the class that it's in:

package pckg1

fun String.add1(): String {
    return this + "1"
}

Then, if your usage of it is in a different package, you have to import it like so (this should be suggested by the IDE as well):

package pckg2

import pckg1.add1

fun x() {
    var a = ""
    a.add1()
}

answered Jul 13 '17 at 05:06

zsmb13

85,752
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5

I wrote the extension inside a class! This is why i failed! – Draculea Jul 13 '17 at 05:12
2

same here, inside a class :) – noloman Nov 21 '17 at 19:33

score 13 · Answer 2 · answered Jul 13 '17 at 10:16

You can use the with-function to use a member extension outside the class where it was defined. Inside the lambda passed to with, this will refer to the instance of A you pass in. This will allow you to use extension functions defined inside A. Like this:

val a =  A()
val s = "Some string"
val result = with(a) {
    s.add1()
}
println(result) // Prints "Some string1"

kotlin extension method access in another kt

2 Answers2