Return interface implementation with Kotlin and lambda

Question

I have this simple interface :

interface ValidationBehavior {
    fun onValidated()
}

This interface is used in one function of a class :

private enum class BehaviorEnum {
    IDLE,
    NAVIGATEBACK
}

private fun getBehavior(payloadBehavior: String) : ValidationBehavior {
    when(BehaviorEnum.valueOf(payloadBehavior)) {
        BehaviorEnum.IDLE -> return object: ValidationBehavior {
            override fun onValidated() {
                // do some stuff
            }
        }
    }
}

My question is : is there a way to simplify the return statement with a lambda ? I try some stuff like this but it doesn't work :

return ValidationBehavior{ () -> //do some stuff }

EDIT :

Kotlin supports SAM interfaces now so declaring it like so :

fun interface ValidationBehavior {
    fun operator onValidated()
}

allows us to write :

return ValidationBehavior { //do some stuff }

Kirill Rakhman · Accepted Answer · 2015-11-20 12:23:07Z

7

No, interfaces written in Kotlin cannot be instantiated with a lambda, that only works for interfaces written in Java. If you want to use lambdas in Kotlin, use the functional type, like in your case () -> Unit instead of ValidationBehavior.

Alternatively, write a method that takes a functional type and wraps it in a ValidationBehavior:

interface ValidationBehavior {

    companion object {
        inline operator fun invoke(fn: () -> Unit) = object: ValidationBehavior {
            override fun onValidated() = fn()
        }
    }

    fun onValidated()
}

private fun getBehavior(payloadBehavior: String) : ValidationBehavior {
    when(BehaviorEnum.valueOf(payloadBehavior)) {
        BehaviorEnum.IDLE -> return ValidationBehavior { /* do stuff */ }
    }
}

answered Nov 20, 2015 at 12:23

Kirill Rakhman

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3 Comments

tufekoi Over a year ago

would it be acceptable to create the interface in Java and use it in Kotlin ? In order to avoid to create the inline function and being able to use the interface directly with lambdas

Kirill Rakhman Over a year ago

If you or your team find it okay, then I guess it would be ok. However consider just dropping the interface and using the functional type.

voddan Over a year ago

any use of java-interop for hacking kotlin is not-idiomatic, obviously. A factory function is the best solution

Marine Droit · Accepted Answer · 2022-05-17 10:30:10Z

0

I know this question is quite old, but there is a way to do it in Kotlin. Look at fun interface kotlin there is a simple way to do what you want.

answered May 17, 2022 at 10:30

Marine Droit

1718 bronze badges

2 Comments

tufekoi Over a year ago

Yes, I edited my post a few months ago saying exactly that.

Marine Droit Over a year ago

Oh right, didn't see sorry

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