Kotlin函数类型在运行时的Java类型是什么？

Question

If I have the Kotlin method bellow and call it from Java, what is the return type that I get in Java?

fun ff(): (Int, Int) -> Int {
    return { x, y -> x + y }
}

Answer 1

The Kotlin function types (T1, ..., TN) -> R are represented by the FunctionN<T1, ..., TN, R> types of the Kotlin runtime.

There is a specification document describing this aspect of the language design at its current state.

It says, in particular, that there are 23 function types from Function0 to Function22 in the Kotlin runtime for JVM (shipped as the kotlin-runtime library), looking like this:

package kotlin.jvm.functions

interface Function1<in P1, out R> : kotlin.Function<R> {
    fun invoke(p1: P1): R
}

The extension function types TR.(T1, ..., TN) -> R have the same runtime representation as normal functional types with the receiver prepended to the parameters list, namely (TR, T1, ..., TN) -> R .

Functions with more than 22 parameters are represented at runtime by a single type:

package kotlin.jvm.functions

interface FunctionN<out R> : kotlin.Function<R> {
    val arity: Int
    fun invokeVararg(vararg p: Any?): R
}

In Kotlin/JS, functional values are represented in the way that is natural to JS, ie they are compiled to a function definition and a reference to it:

function main$lambda(s1, s2) {
    println(s1 + s2);
    return Unit;
}

function main(args) {
    var f = main$lambda;
    /* ... */
}