Unable to access Kotlin Extension function from Java in case it has a lambda as its parameter

Question

Kotlin Global Function:

fun Activity.showWarningDialog(title: Int, description: Int,done:()->Unit) {  
   done()
}

calling this in java:

showWarningDialog(this,R.string.alert,R.string.leave_without_saving,() -> {
                setResult(RESULT_CANCELED);
                finish();
            });

it gives the following error: Cannot resolve method 'showWarningDialog(com.us.stickermaker.backgroundRemover.CutOutActivity, int, int, <lambda expression>)'

the function is imported into the activity file, works fine if we remove the lambda parameter.

Maybe I'm missing something about how Kotlin methods are compiled, but your Kotlin method has 3 parameters, you're passing 4 parameters in your Java call — Jeroen Steenbeeke, Sep 16 '21 at 07:39
look closely, when a kotlin extension function is called in java, the "object" would be passed as a parameter in this case its an activity. — Usama Saeed US, Sep 16 '21 at 07:41
I see. I did a bit of reading on the subject, and from what I've read in [this answer](https://stackoverflow.com/a/28364983/8819761), you would need to prefix your method with the the filename it was declared in. — Jeroen Steenbeeke, Sep 16 '21 at 08:45
As i told in the question, if i remove the lambda, it works fine in java, and yes function is imported with the file name. — Usama Saeed US, Sep 16 '21 at 11:37

score 2 · Accepted Answer · answered Sep 16 '21 at 14:01

Your lambda's type in Kotlin is ()->Unit, which Java sees as kotlin.Function0<kotlin.Unit>. So your Java lambda needs to return Unit instead of void as it currently does.

showWarningDialog(this, R.string.alert, R.string.leave_without_saving, () -> {
                setResult(RESULT_CANCELED);
                finish();
                return Unit.INSTANCE;
            });

should work. The Kotlin compiler inserts returning Unit for you, but Java doesn't treat Unit specially in any way.

If you just want to call the function from Java in one or a few places, this may be good enough; otherwise the way to make it convenient to call from Java is fun interface as shown in Andrii Hridin's answer.

score -1 · Answer 2 · answered Sep 16 '21 at 07:40

-1

Instead of lambda try to use "fun interface". For example:

fun interface OnWarningDoneListener {
    fun onWarningDone()
}

fun Activity.showWarningDialog(title: Int, description: Int, onWarningDoneListener: OnWarningDoneListener) {  
   onWarningDoneListener.onWarningDone()
}

answered Sep 16 '21 at 07:40

Andrii Hridin

177
7

Usama Saeed US, did you try this solution? I checked in my project and I have the successful result – Andrii Hridin Sep 16 '21 at 07:48
in java we do thing with interfaces, but that time as passed and we want thing to be clean and easy using lambda. – Usama Saeed US Sep 16 '21 at 07:51
`fun interface` is not simple `interface`. It is special interface for lambda. Try to use – Andrii Hridin Sep 16 '21 at 07:52
Is this solution works for you? Provide feedback please – Andrii Hridin Sep 16 '21 at 08:50
I do not want to declare any interface separately, that is why lambda. – Usama Saeed US Sep 16 '21 at 11:34
What this answer doesn't explicitly say is that if you add the `fun interface` and change the parameter type, _calls_ to it use the same lambdas you have (in Java and Kotlin). Documentation: https://kotlinlang.org/docs/fun-interfaces.html – Alexey Romanov Sep 16 '21 at 16:07

Unable to access Kotlin Extension function from Java in case it has a lambda as its parameter

2 Answers2