ExecutorCompletionService in practice

ExecutorCompletionService in practice

This is where ExecutorCompletionService steps in. It is a thin wrapper around ExecutorService that "remembers" all submitted tasks and allows you to wait for the first completed, as opposed to first submitted task. In a way ExecutorCompletionService keeps a handle to all intermediate Future objects and once any of them finishes, it's returned. Crucial API method is CompletionService.take() that blocks and waits for any underlying Future to complete. Here is the submit step with ExecutorCompletionService:

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