CompletionService

A service that decouples the production of new asynchronous tasks
from the consumption of the results of completed tasks. Producers
submit tasks for execution. Consumers take
completed tasks and process their results in the order they
complete. A CompletionService can for example be used to
manage asynchronous I/O, in which tasks that perform reads are
submitted in one part of a program or system, and then acted upon
in a different part of the program when the reads complete,
possibly in a different order than they were requested.

Typically, a CompletionService relies on a separate
Executor to actually execute the tasks, in which case the
CompletionService only manages an internal completion
queue. The ExecutorCompletionService class provides an
implementation of this approach.

API：

ExecutorCompletionService(Executor executor)
Creates an ExecutorCompletionService using the supplied
executor for base task execution and a
LinkedBlockingQueue as a completion queue.

ExecutorCompletionService(Executor executor,
BlockingQueue<Future> completionQueue)
Creates an ExecutorCompletionService using the supplied
executor for base task execution and the supplied queue as its
completion queue.