Implementing Function Interface using Lambda Expression in Kotlin

Question

@FunctionalInterface
interface Interf {
    fun m1(num: Int)
}

fun main() {
    val a: Interf = { 34 -> println("Hello world !!") }
}

Upon compilation getting this error

Unexpected tokens (use ';' to separate expressions on the same line)

Is Kotlin lambda function syntax is bit different from Java Lambda Expression?

Answer 1

First of all, this will not compile in java as well:

@FunctionalInterface
interface Interf {
    void m1(int num);
}

class Main {
    public static void main(String[] args) {
        Interf f =  34 -> System.out.println("Hello world !!"); //compilation error here
    }
}

With pretty the same error:

error: ';' expected
        Interf f =  34 -> System.out.println("Hello world !!");
                      ^

To make it correct, name of the lambda parameter should be changed, so that it become a valid java identifier.

For instance, this will compile:

Interf f =  x -> System.out.println("Hello world !!");

Now, returning back to Kotlin. Version 1.4 introduces syntax for SAM conversions:

fun interface Interf {
    fun m1(num: Int)
}

It could be instantiated with:

val a = Interf { println("Hello world !!") }

Convention of implicitly declared it parameter referring to lambda only parameter is preserved here.