# CompletionService

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  
Summary: [Methods](https://developer.android.com/reference/java/util/concurrent/CompletionService#pubmethods)  

# CompletionService


`
public


interface
CompletionService
`


`


`

|---------------------------------------------|
| java.util.concurrent.CompletionService\<V\> |

|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| Known indirect subclasses [ExecutorCompletionService](https://developer.android.com/reference/java/util/concurrent/ExecutorCompletionService)\<V\> |--------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | [ExecutorCompletionService](https://developer.android.com/reference/java/util/concurrent/ExecutorCompletionService)\<V\> | A [CompletionService](https://developer.android.com/reference/java/util/concurrent/CompletionService) that uses a supplied [Executor](https://developer.android.com/reference/java/util/concurrent/Executor) to execute tasks. | |

<br />

*** ** * ** ***

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](https://developer.android.com/reference/java/util/concurrent/Executor) to actually execute the tasks, in which case the
`CompletionService` only manages an internal completion
queue. The [ExecutorCompletionService](https://developer.android.com/reference/java/util/concurrent/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*](https://developer.android.com/reference/java/util/concurrent/package-summary#MemoryVisibility)
actions taken by that task, which in turn *happen-before*
actions following a successful return from the corresponding `take()`.

## Summary

|                                                                                                                                                                                                                                         ### Public methods                                                                                                                                                                                                                                          ||
|------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| ` abstract `[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[poll](https://developer.android.com/reference/java/util/concurrent/CompletionService#poll(long,%20java.util.concurrent.TimeUnit))`(long timeout, `[TimeUnit](https://developer.android.com/reference/java/util/concurrent/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](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[poll](https://developer.android.com/reference/java/util/concurrent/CompletionService#poll())`() ` Retrieves and removes the Future representing the next completed task, or `null` if none are present.                                                                                                                                                                                         |
| ` abstract `[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[submit](https://developer.android.com/reference/java/util/concurrent/CompletionService#submit(java.lang.Runnable,%20V))`(`[Runnable](https://developer.android.com/reference/java/lang/Runnable)` task, V result) ` Submits a Runnable task for execution and returns a Future representing that task.                                                                                          |
| ` abstract `[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[submit](https://developer.android.com/reference/java/util/concurrent/CompletionService#submit(java.util.concurrent.Callable<V>))`(`[Callable](https://developer.android.com/reference/java/util/concurrent/Callable)`<V> task) ` Submits a value-returning task for execution and returns a Future representing the pending results of the task.                                                |
| ` abstract `[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[take](https://developer.android.com/reference/java/util/concurrent/CompletionService#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](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
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.

<br />

|                                           Parameters                                            ||
|-----------|--------------------------------------------------------------------------------------|
| `timeout` | `long`: how long to wait before giving up, in units of `unit` <br />                 |
| `unit`    | `TimeUnit`: a `TimeUnit` determining how to interpret the `timeout` parameter <br /> |

|                                                                                                     Returns                                                                                                      ||
|------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------|
| [Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | the Future representing the next completed task or `null` if the specified waiting time elapses before one is present <br /> |

|                                                            Throws                                                            ||
|------------------------------------------------------------------------------------------------|------------------------------|
| [InterruptedException](https://developer.android.com/reference/java/lang/InterruptedException) | if interrupted while waiting |

### poll

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public abstract Future<V> poll ()
```

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

<br />

|                                                                                  Returns                                                                                  ||
|------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------|
| [Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | the Future representing the next completed task, or `null` if none are present <br /> |

### submit

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
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.

<br />

|                              Parameters                               ||
|----------|-------------------------------------------------------------|
| `task`   | `Runnable`: the task to submit <br />                       |
| `result` | `V`: the result to return upon successful completion <br /> |

|                                                                                                           Returns                                                                                                            ||
|------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------|
| [Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | a Future representing pending completion of the task, and whose `get()` method will return the given result value upon completion <br /> |

|                                                                                Throws                                                                                ||
|-----------------------------------------------------------------------------------------------------------------------|-----------------------------------------------|
| [RejectedExecutionException](https://developer.android.com/reference/java/util/concurrent/RejectedExecutionException) | if the task cannot be scheduled for execution |
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException)                        | if the task is null                           |

### submit

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
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.

<br />

|                  Parameters                   ||
|--------|---------------------------------------|
| `task` | `Callable`: the task to submit <br /> |

|                                                                     Returns                                                                     ||
|------------------------------------------------------------------------------------|-------------------------------------------------------------|
| [Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | a Future representing pending completion of the task <br /> |

|                                                                                Throws                                                                                ||
|-----------------------------------------------------------------------------------------------------------------------|-----------------------------------------------|
| [RejectedExecutionException](https://developer.android.com/reference/java/util/concurrent/RejectedExecutionException) | if the task cannot be scheduled for execution |
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException)                        | if the task is null                           |

### take

Added in [API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels)  

```
public abstract Future<V> take ()
```

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

<br />

|                                                                  Returns                                                                   ||
|------------------------------------------------------------------------------------|--------------------------------------------------------|
| [Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | the Future representing the next completed task <br /> |

|                                                            Throws                                                            ||
|------------------------------------------------------------------------------------------------|------------------------------|
| [InterruptedException](https://developer.android.com/reference/java/lang/InterruptedException) | if interrupted while waiting |