How to use lambdas together with functional interfaces and directly assign lambda to var in kotlin

user_13205898 0
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I have this code:

package org.medianik.kotlindoc

fun main(){
    val creator:Creatable = toCreatable(::Created)// Ok. Compiles, and works fine
    val creator1:Creatable = ::Created // Compilation error
    val creator2:Creatable = ::Created as Creatable // ok enough to compile, but there is runtime exception
    val c = creator.create(5)
    val creator3:Creatable = toCreatable(c::doSmth1) // Ok. Compiles, works fine
    val creator4:Creatable = c::doSmth1 // Compilation error
    val creator5:Creatable = c::doSmth1 as Creatable // Again ok enough to compile, but there is runtime exception
    val creator6:Creatable = Creatable { i -> Created(i) } // Works fine, but that's not what I want
}

fun toCreatable(c:Creatable) = c

fun interface Creatable{
    fun create(i: Int) : Created
}
class Created(private var i: Int){
    fun doSmth1(add: Int): Created{
        return this.also { i+=add }
    }
}

Exception for creator2:

Exception in thread "main" java.lang.ClassCastException: class org.medianik.kotlindoc.MainKt$main$creator2$1 cannot be cast to class org.medianik.kotlindoc.Creatable (org.medianik.kotlindoc.MainKt$main$creator2$1 and org.medianik.kotlindoc.Creatable are in unnamed module of loader 'app')

Exception for creator5:

Exception in thread "main" java.lang.ClassCastException: class org.medianik.kotlindoc.MainKt$main$creator5$1 cannot be cast to class org.medianik.kotlindoc.Creatable (org.medianik.kotlindoc.MainKt$main$creator5$1 and org.medianik.kotlindoc.Creatable are in unnamed module of loader 'app')

I have no idea how to make creator1/4 work with method reference. Is there any ways to make them work? Because it seems senseless to me: if you pass method reference to a function as an argument -- it converts to interface, otherwise not. How does it work?

Are there some special casts for functions to interfaces?

functional-interface method-reference kotlin lambda
1 Answers

I copied your code into IntelliJ and also got errors in the lines

 val creator:Creatable = toCreatable(::Created)
 val creator3:Creatable = toCreatable(c::doSmth1)

The issue is, that Creatable is a named Interface that has the same Method signature like the class Created but this class does not implement the interface.

Note, a function does not implement an interface, even if they have a signature.

You could decare instead of an interface a function type. Note, this is not the same, but they can be handled almost the same.

typealias Creatable = (Int) -> Created

or you could write write a wrapper acceping such function type:

fun toCreatable(c:(Int) -> Created) = object: Creatable {
    override fun create(i: Int): Created {
        return c(i)
    }
}

this function accepts an function and returns an object that actually implements the interface.

user_27619850
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