Getting the result of the first completed parallel task in Java

In this post we saw how to delegate one or more parallel tasks to different threads and wait for all of them to complete. We pretended that 4 different computations took 1,2,3 and respectively 4 seconds to complete. If we execute each calculation one after the other on the same thread then it takes 10 seconds to complete them all. We can do a lot better by assigning each operation to a separate thread and let them run in parallel. The Future and Callable of T objects along with a thread pool make this very easy to implement.

There are situations where we only need the result from 1 parallel operation. Imagine that it’s enough to complete 1 of the four computations in the example code so that our main thread can continue. We don’t know how long each operation will take so we let them have a race. The one that is executed first returns its value and the rest are interrupted and forgotten. We’ll see how to achieve that in this post.

We’ll reuse the same CalculationService interface and its 4 implementations from the post referenced above: AdditionService, SubtractionService, MultiplicationService and DivisionService. When we waited for all of them to complete it took 4 seconds for the thread pool to return them which corresponds to the 4 seconds it took the DivisionService to complete its task. Our expectation now is that AdditionService should win since it only needs 1 second to return. The ExecutorCompletionService can help us with the implementation. ExecutorCompletionService has a generic form where we can declare the expected return type. Without specifying the type we get an Object back.

Here’s an example:

private static void waitForFirstThreadCompletion()
    CalculationService adder = new AdditionService();
    CalculationService subtractor = new SubtractionService();
    CalculationService multiplier = new MultiplicationService();
    CalculationService divider = new DivisionService();
    int firstOperand = 10;
    int secondOperand = 5;

    Instant start =;

    ExecutorService newCachedThreadPool = Executors.newCachedThreadPool();
    ExecutorCompletionService<Integer> completionService = new ExecutorCompletionService(newCachedThreadPool);
    List<Callable<Integer>> calculationTasks = new ArrayList<>();
    calculationTasks.add(() -> divider.calculate(firstOperand, secondOperand));
    calculationTasks.add(() -> subtractor.calculate(firstOperand, secondOperand));
    calculationTasks.add(() -> multiplier.calculate(firstOperand, secondOperand));    
    calculationTasks.add(() -> adder.calculate(firstOperand, secondOperand));

        Integer firstResult = completionService.take().get();
    } catch (InterruptedException | ExecutionException | NullPointerException ex)

    Instant finish =;
    Duration duration = Duration.between(start, finish);
    long seconds = duration.getSeconds();


The take() method of the completion service will wait until the first completed task has returned. Note that the order of the tasks doesn’t matter, the addition service will win the race. The above example block will complete in 1 second as expected.

The completion service has a poll() method as well. It returns null if none of the submitted tasks has returned. Thus it’s necessary to to check for nulls in a loop like in the following example:

boolean firstCompleted = false;
while (!firstCompleted)
        Future<Integer> future = completionService.poll();
        if (future != null)
            Integer firstResult = future.get();
            firstCompleted = true;
    } catch (NullPointerException | InterruptedException | ExecutionException ex)

View all posts related to Java here.


About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

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