Question

I’m probably missing something in the Guava API. How can I use ExecutorService.invokeAll with Futures.allAsList without casting?

My usecase is that I would like to submit a List of Callables and wait until all are executed (in parallel). I’m only interested in the list of results from the Callables.

    ListeningExecutorService executor = MoreExecutors.sameThreadExecutor(); //or any other ExecutorService
    List<Future<Object>> futures = 
        executor.invokeAll(singletonList(new Callable<Object>() {
            @Override public Object call() throws Exception {
                return 42;
            }
        }
    ));
    Iterable<ListenableFuture<Object>> listableFutures = 
        (Iterable<ListenableFuture<Object>>) (Iterable) futures;
    //would like to use "futures" here
    ListenableFuture<List<Object>> r = Futures.allAsList(listableFutures);
    System.out.println(r.get());

In a perfect world I would like to call ListeningExecutorService.allAsList(Collection<? extends Callable<T>> tasks) and get a Future<List<T>> or even List<T>.

Answer 1

Nope. You’re supposed to do the cast. It can’t be avoided, due to the way Java restricts overriding of methods.

The Javadoc for ListeningExecutorService.invokeAll states

All elements in the returned list must be ListenableFuture instances.