ConcurrentWorkExecutor.java

public class WorkExecutor {

    private final MathService mathService;

    public List<Integer> processExpensiveFunction(List<Integer> inputs, int firstK) {
        try {
            List<Integer> outputs = new ArrayList<>();
            //our basic executor
            ExecutorService executor = Executors.newCachedThreadPool();
            ExecutorCompletionService<Integer> executorService = new ExecutorCompletionService<>(executor);

            for (Integer input : inputs) {
                executorService.submit(() -> {
                    //submitting input
                    log.info("Submitting task f({}) on {}", input, Thread.currentThread().getName());
                    return mathService.process(input);
                });
            }
            executor.shutdown(); //prevent more tasks
            for (int outputCount = 1; outputCount <= firstK; outputCount++) {
                //process results as and when they are being executed.
                Integer output = executorService.take().get();
                log.info("Finished {}/{}: {} on {}", outputCount, firstK, output, Thread.currentThread().getName());
                outputs.add(output);
            }
            return outputs;

        } catch (Exception ex) {
            log.error("Error processing expensive function");
            return null;
        }
    }
}