ExecutorService InvokeAll() vs ExecutorCompletionService in Java

Suppose we have list of four tasks: Task A, Task B, Task C and Task D which perform some complex computation and result into an integer value. These tasks may take random time depending upon various parameters. We can submit these tasks to executor

Suppose task B finished first followed by task C. But task A was still going on. In that case when using ExecutorService the for loop would be waiting for the result of task A to be available. So in case of ExecutorService tasks will be processed in the same order in which they were submitted

In case of CompletionService tasks will be processed in order the result becomes available, the order tasks are completed. One interesting example is where we want to download some file which can be downloaded from various mirrors. In that case we can quickly get the response from the server which is located closest to us. In that case we can get the first available result and discard the others