Kotlin Syntax for whenCompleteAsync(BiConsumer,Executor)

Question

How do I implement a function in Kotlin for below method signature. I am facing issues getting the syntax correct.

public CompletableFuture<T> whenCompleteAsync(
        BiConsumer<? super T, ? super Throwable> action, Executor executor)

Answer 1

The analogue to Java's BiConsumer<? super T, ? super Throwable> generic type would be written in Kotlin as BiConsumer<in T, in Throwable>. The in keyword indicates that the generic type T is contravariant.

This is how whenCompleteAsync function could be implemented in Kotlin using such method signature:

fun <T> whenCompleteAsync(
    action: BiConsumer<in T, in Throwable>, 
    executor: Executor
): CompletableFuture<T> {
    val future = CompletableFuture<T>()

    executor.execute {
        try {
            val result = future.get()
            action.accept(result, null)
        } catch (e: Throwable) {
            action.accept(null, e)
        }
    }

    return future
}

Usage:

val future = whenCompleteAsync(
    BiConsumer { result, ex ->
        if (ex != null) {
            // Handle exception
        } else {
            // Handle result
        }
    },
    Executors.newSingleThreadExecutor()
)

---

In Kotlin no need to use BiConsumer interface unless it is required exactly for some reason. Instead it is possible to use Kotlin function with two arguments and simplify the example:

fun <T> whenCompleteAsync(
    action: (T?, Throwable?) -> Unit,
    executor: Executor
): CompletableFuture<T> {
    val future = CompletableFuture<T>()

    executor.execute {
        try {
            val result = future.get()
            action(result, null)
        } catch (e: Throwable) {
            action(null, e)
        }
    }

    return future
}

val future = whenCompleteAsync(
    { result, ex ->
        if (ex != null) {
            // Handle exception
        } else {
            // Handle result
        }
    },
    Executors.newSingleThreadExecutor()
)