CompletionService

public interface CompletionService

java.util.concurrent.CompletionService<V>

Known indirect subclasses

ExecutorCompletionService<V> A CompletionService that uses a supplied Executor to execute tasks.

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.

Memory consistency effects: Actions in a thread prior to submitting a task to a CompletionService happen-before actions taken by that task, which in turn happen-before actions following a successful return from the corresponding take().

Summary

Public methods
`abstract Future<V>`	`poll(long timeout, TimeUnit unit)` Retrieves and removes the Future representing the next completed task, waiting if necessary up to the specified wait time if none are yet present.
`abstract Future<V>`	`poll()` Retrieves and removes the Future representing the next completed task, or `null` if none are present.
`abstract Future<V>`	`submit(Runnable task, V result)` Submits a Runnable task for execution and returns a Future representing that task.
`abstract Future<V>`	`submit(Callable<V> task)` Submits a value-returning task for execution and returns a Future representing the pending results of the task.
`abstract Future<V>`	`take()` Retrieves and removes the Future representing the next completed task, waiting if none are yet present.

Public methods

poll

Added in API level 1

public abstract Future<V> poll (long timeout, 
                TimeUnit unit)

Retrieves and removes the Future representing the next completed task, waiting if necessary up to the specified wait time if none are yet present.

Parameters
`timeout`	`long`: how long to wait before giving up, in units of `unit`
`unit`	`TimeUnit`: a `TimeUnit` determining how to interpret the `timeout` parameter

Returns
`Future<V>`	the Future representing the next completed task or `null` if the specified waiting time elapses before one is present

Throws
`InterruptedException`	if interrupted while waiting

poll

Added in API level 1

public abstract Future<V> poll ()

Retrieves and removes the Future representing the next completed task, or null if none are present.

Returns
`Future<V>`	the Future representing the next completed task, or `null` if none are present

submit

Added in API level 1

public abstract Future<V> submit (Runnable task, 
                V result)

Submits a Runnable task for execution and returns a Future representing that task. Upon completion, this task may be taken or polled.

Parameters
`task`	`Runnable`: the task to submit
`result`	`V`: the result to return upon successful completion

Returns
`Future<V>`	a Future representing pending completion of the task, and whose `get()` method will return the given result value upon completion

Throws
`RejectedExecutionException`	if the task cannot be scheduled for execution
`NullPointerException`	if the task is null

submit

Added in API level 1

public abstract Future<V> submit (Callable<V> task)

Submits a value-returning task for execution and returns a Future representing the pending results of the task. Upon completion, this task may be taken or polled.

Parameters
`task`	`Callable`: the task to submit

Returns
`Future<V>`	a Future representing pending completion of the task

Throws
`RejectedExecutionException`	if the task cannot be scheduled for execution
`NullPointerException`	if the task is null

take

Added in API level 1

public abstract Future<V> take ()

Retrieves and removes the Future representing the next completed task, waiting if none are yet present.

Returns
`Future<V>`	the Future representing the next completed task

Throws
`InterruptedException`	if interrupted while waiting