How to declare parameter as function without return value?

Question

I have a function in kotlin

fun printExecutionTime(block: () -> Any) {
    run {
        val currentTimeMillis = System.currentTimeMillis()
        block()
        Logr.d("Execution time of " + block.javaClass.name + " -> " + System.currentTimeMillis().minus(currentTimeMillis))
    }
}

In java code, I want to pass void reference function as parameter, but can't bcs of return value

PerformanceKt.printExecutionTime(this::voidFunc);

One way will be to use interface

interface Action {
    fun call()
}

Is it possible to declare it in kotlin without an extra interface, so the code above will work?

Answer 1

Have you tried using Unit?

fun f(voidFunc: () -> Unit) {
    // <...>
}

---

upd: After some googling I should admit, that this solution won't work for you. There's an open issue covering the use of Java method references with Kotlin functional types.

However, there's still a workaround to use in Java (looks ugly, yes):

public Unit voidFunc() {
    // <...>
    return null;
}

Answer 2

you can using java.lang.Runnable to avoiding introduce additional interface ,for example:

fun printExecutionTime(block: Runnable) {
  run {
    val currentTimeMillis = System.currentTimeMillis()
    block.run()
    //...
  }
}

then you can using java method reference expression as below:

PerformanceKt.printExecutionTime(this::voidFunc);

void voidFunc(){...}