Java ThreadPoolExexecutor using streams and Callables

Question

I have a class which implements Callable, and it has a method which overrides call and returns a Long.

I create a List of Callable<Long> as

List<Callable<Long>> callables = new ArrayList<>();
for (File fileEntry : folder.listFiles()) {
    callables.add(new DataProcessor(fileEntry));

I have

ExecutorService threadPoolExecutor = Executors.newFixedThreadPool(10);

and I call

threadPoolExecutor.invokeAll(callables)
    .stream()
    .map(future -> {
        try {
            return future.get();
        } catch (Exception e) {
            throw new IllegalStateException(e);
        }
        })
        .collect(Collectors.toLong(/* what goes here? */));

What I want to do is sum all the return values from future.get().

Also, since I am calling the invokeAll, do I still need to do a shutdown on the Executor?

Answer 1

You can use Stream.mapToLong to map the future.get as LongStream and then find the sum of the stream as:

long sum = threadPoolExecutor.invokeAll(callables)
            .stream()
            .mapToLong(future -> {
                try {
                    return future.get();
                } catch (Exception e) {
                    throw new IllegalStateException(e);
                }
            }) // LongStream
            .sum(); // sum of the stream

Note: This simplifies stream API call chains using Collectors.summingLong. It allows avoiding the creation of redundant temporary objects when traversing a collection.

Aside: You can also collect your Callables as :

List<Callable<Long>> callables = fileList.stream()
                                         .map(fileEntry -> new DataProcessor(fileEntry))
                                         .collect(Collectors.toList());

since I am calling the invokeAll, do I still need to do a shutdown on the Executor?

Yes, you would have to shut down the ExecutorService. You can also confirm the status of the same using isShutDown() API as :

System.out.println(threadPoolExecutor.isShutdown()); // would return false