Kotlin: Interface ... does not have constructors

Question

I am converting some of my Java code to Kotlin and I do not quite understand how to instantiate interfaces that are defined in Kotlin code. As an example, I have an interface (defined in Java code):

public interface MyInterface {
    void onLocationMeasured(Location location);
}

And then further in my Kotlin code I instantiate this interface:

val myObj = new MyInterface { Log.d("...", "...") }

and it works fine. However, when I convert MyInterface to Kotlin:

interface MyInterface {
    fun onLocationMeasured(location: Location)
}

I get an error message: Interface MyListener does not have constructors when I try to instantiate it - though it seems to me that nothing has changed except syntax. Do I misunderstand how interfaces work in Kotlin?

Answer 1

Your Java code relies on SAM conversion - an automatic conversion of a lambda into an interface with a single abstract method. SAM conversion is currently not supported for interfaces defined in Kotlin. Instead, you need to define an anonymous object implementing the interface:

val obj = object : MyInterface {
    override fun onLocationMeasured(location: Location) { ... }
}

Answer 2

SAM conversion is supported for interfaces defined in Kotlin since 1.4.0

What's New in Kotlin 1.4.0

Before Kotlin 1.4.0, you could apply SAM (Single Abstract Method) conversions only when working with Java methods and Java interfaces from Kotlin. From now on, you can use SAM conversions for Kotlin interfaces as well. To do so, mark a Kotlin interface explicitly as functional with the fun modifier.

SAM conversion applies if you pass a lambda as an argument when an interface with only one single abstract method is expected as a parameter. In this case, the compiler automatically converts the lambda to an instance of the class that implements the abstract member function.

So the example in Your question will look something like this:

fun interface MyInterface {
    fun onLocationMeasured(location: String)
}

fun main() {
    val myObj = MyInterface { println(it) }

    myObj.onLocationMeasured("New York")
}