I just found CompletionService in [this blog post][1]. However, this does&#39;t really showcases the advantages of CompletionService over a standard ExecutorService. The same code can be written with either. So, when is a CompletionService useful?

Can you give a short code sample to make it crystal clear? For example, this code sample just shows where a CompletionService is not needed (=equivalent to ExecutorService)


        ExecutorService taskExecutor = Executors.newCachedThreadPool();
        //        CompletionService&lt;Long&gt; taskCompletionService =
        //                new ExecutorCompletionService&lt;Long&gt;(taskExecutor);
        Callable&lt;Long&gt; callable = new Callable&lt;Long&gt;() {
            @Override
            public Long call() throws Exception {
                return 1L;
            }
        };

        Future&lt;Long&gt; future = // taskCompletionService.submit(callable);
            taskExecutor.submit(callable);

        while (!future.isDone()) {
            // Do some work...
            System.out.println(&quot;Working on something...&quot;);
        }
        try {
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }


  [1]: http://www.chrisellsworth.com/blogs/devblog/archive/2006/10/31/Java-Concurrency-API-Example.aspx

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When should I use a CompletionService over an ExecutorService?