Type mismatch when extending CompletableFuture

Question

I am trying to extend CompletableFuture to do a thenCompose after handle , but I got a compiler error:

Type mismatch: cannot convert from CompletableFuture(Object) to CompletableFuture(U)

This is my code:

public class MyCompletableFuture<T> extends CompletableFuture<T> {

    public <U> CompletableFuture<U> handleAndCompose(BiFunction<? super T, Throwable, ? extends U> fn) {
        return super.handle(fn).thenCompose(x->x);
    }

}

For the record, I am trying to hide the thenCompose used on this reponse which is basically:

.handle((x, t) -> {
    if (t != null) {
        return askPong("Ping");
    } else {
        return x;
    }
)

Answer 1

The signature of your method is incorrect. It should be:

public <U> CompletableFuture<U> handleAndCompose(BiFunction<? super T, Throwable, ? extends CompletableFuture<U>> fn) {
    return super.handle(fn).thenCompose(x->x);
}

Note that the function given returns ? extends CompletableFuture<U> ? extends CompletableFuture<U> instead of ? extends U ? extends U . You could also accept as argument a more general CompletionStage instead of a CompletableFuture .