This is to show the difference between handling jobs with a pure ExecutorService VS a CompletionService VS a normal loop. The CompletionServiceTest class simply prints numbers at predefined intervals ( waittime = 200 that simulates a work done by the thread, for example reading or parsing a file or run calculations, etc). We do the same job in 3 different ways and we print the result together with the total execution time. import java.util.Arrays; import java.util.HashSet; import java.util.List; import java.util.Set; import java.util.concurrent.Callable; import java.util.concurrent.CompletionService; import java.util.concurrent.ExecutionException; import java.util.concurrent.ExecutorCompletionService; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; public class CompletionServiceTest { private static final int waittime = 200; private static final int numberOfThreadsInThePool = 3; private final List printRequests = Arrays.asList("1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" ); void normalLoop() { for (String image : printRequests) { try { Thread.sleep(waittime); } catch (InterruptedException e) { e.printStackTrace(); } System.out.print(image); } } void normalExecutorService() { ExecutorService executor = Executors.newFixedThreadPool(numberOfThreadsInThePool); try { Set> printTaskFutures = new HashSet>(); for (final String printRequest : printRequests) { printTaskFutures.add(executor.submit(new Printer(printRequest))); } for (Future future : printTaskFutures) { System.out.print(future.get()); } } catch (Exception e) { Thread.currentThread().interrupt(); } finally { if (executor != null) { executor.shutdownNow(); } } } void completionService() { ExecutorService executor = Executors.newFixedThreadPool(numberOfThreadsInThePool); CompletionService completionService = new ExecutorCompletionService(executor); for (final String printRequest : printRequests) { completionService.submit(new Printer(printRequest)); } try { for (int t = 0, n = printRequests.size(); t < n; t++) { Future f = completionService.take(); System.out.print(f.get()); } } catch (InterruptedException e) { Thread.currentThread().interrupt(); } catch (ExecutionException e) { Thread.currentThread().interrupt(); } finally { if (executor != null) { executor.shutdownNow(); } } } private class Printer implements Callable { private final String toPrint; public Printer(String toPrint) { this.toPrint = toPrint; } public String call() { try { Thread.sleep(waittime); } catch (InterruptedException e) { e.printStackTrace(); } return toPrint; } } public static void main(String[] args) { System.out.println("Normal Executor Service"); long start = System.currentTimeMillis(); new CompletionServiceTest().normalExecutorService(); System.out.println(); System.out.println("Execution time : " + (System.currentTimeMillis() - start)); System.out.println("Completion Service"); start = System.currentTimeMillis(); new CompletionServiceTest().completionService(); System.out.println(); System.out.println("Execution time : " + (System.currentTimeMillis() - start)); System.out.println("Normal Loop"); start = System.currentTimeMillis(); new CompletionServiceTest().normalLoop(); System.out.println(); System.out.println("Execution time : " + (System.currentTimeMillis() - start)); } } The output : Normal Executor Service 4865447106310711038976137104163378132553196955489824610105761329674254896133105110752911167871022184410147885282715959248479622229673310496981056331010958 Execution time : 9410 Completion Service 1234567891012345678910123456789101234567891012345678109123456789101234567891012345678910123456789101235647891012345678910123456789101234568791012435687109 Execution time : 9405 Normal Loop 1234567891012345678910123456789101234567891012345678910123456789101234567891012345678910123456789101234567891012345678910123456789101234567891012345678910 Execution time : 28013 As you can see the total execution time between Normal Executor Service and Completion Service is the same, while the normal loop was more than 3 times slower. This is easily explained by the fact that the normal loop was single threaded and the others 2 were run with 3 threads. The difference between Normal Executor Service and Completion Service is visible at runtime, Normal Executor Service always waits for the next thread to finish the job before printing the number, the client needs to wait until a big bunch of threads return all together. Completion Service is behaving differently, it follows a producer/consumer philosophy: as soon a thread is done, it puts the result into a non blocking queue so that the consumer can take it. I hope this simple example will help you to better clarify the completion service as it did for me.