# ExecutorCompletionService

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

# ExecutorCompletionService

`
public


class
ExecutorCompletionService
`  
`

extends `[Object](https://developer.android.com/reference/java/lang/Object)`


``


implements

`[CompletionService](https://developer.android.com/reference/java/util/concurrent/CompletionService)`<V>


`

|---|-----------------------------------------------------|
| [java.lang.Object](https://developer.android.com/reference/java/lang/Object) ||
| ↳ | java.util.concurrent.ExecutorCompletionService\<V\> |

<br />

*** ** * ** ***

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. This class arranges that submitted tasks are, upon completion, placed on a queue accessible using`take`. The class is lightweight enough to be suitable for transient use when processing groups of tasks.

**Usage Examples.** Suppose you have a set of solvers for a certain problem, each returning a value of some type`Result`, and would like to run them concurrently, processing the results of each of them that return a non-null value, in some method`use(Result r)`. You could write this as:  

     void solve(Executor e,
                Collection<Callable<Result>> solvers)
         throws InterruptedException, ExecutionException {
       CompletionService<Result> cs
           = new ExecutorCompletionService<>(e);
       solvers.forEach(cs::submit);
       for (int i = solvers.size(); i > 0; i--) {
         Result r = cs.take().get();
         if (r != null)
           use(r);
       }
     }

Suppose instead that you would like to use the first non-null result of the set of tasks, ignoring any that encounter exceptions, and cancelling all other tasks when the first one is ready:  

     void solve(Executor e,
                Collection<Callable<Result>> solvers)
         throws InterruptedException {
       CompletionService<Result> cs
           = new ExecutorCompletionService<>(e);
       int n = solvers.size();
       List<Future<Result>> futures = new ArrayList<>(n);
       Result result = null;
       try {
         solvers.forEach(solver -> futures.add(cs.submit(solver)));
         for (int i = n; i > 0; i--) {
           try {
             Result r = cs.take().get();
             if (r != null) {
               result = r;
               break;
             }
           } catch (ExecutionException ignore) {}
         }
       } finally {
         futures.forEach(future -> future.cancel(true));
       }

       if (result != null)
         use(result);
     }

<br />

## Summary

|                                                                                                                                                                                                                                                                                                                                        ### Public constructors                                                                                                                                                                                                                                                                                                                                        ||
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---|
| ` `[ExecutorCompletionService](https://developer.android.com/reference/java/util/concurrent/ExecutorCompletionService#ExecutorCompletionService(java.util.concurrent.Executor))`(`[Executor](https://developer.android.com/reference/java/util/concurrent/Executor)` executor) ` Creates an ExecutorCompletionService using the supplied executor for base task execution and a[LinkedBlockingQueue](https://developer.android.com/reference/java/util/concurrent/LinkedBlockingQueue)as a completion queue.                                                                                                                                                                                       |
| ` `[ExecutorCompletionService](https://developer.android.com/reference/java/util/concurrent/ExecutorCompletionService#ExecutorCompletionService(java.util.concurrent.Executor,%20java.util.concurrent.BlockingQueue<java.util.concurrent.Future<V>>))`(`[Executor](https://developer.android.com/reference/java/util/concurrent/Executor)` executor, `[BlockingQueue](https://developer.android.com/reference/java/util/concurrent/BlockingQueue)`<`[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>> completionQueue) ` Creates an ExecutorCompletionService using the supplied executor for base task execution and the supplied queue as its completion queue. |

|                                                                                                                                                                                                                                         ### Public methods                                                                                                                                                                                                                                         ||
|---------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| ` `[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[poll](https://developer.android.com/reference/java/util/concurrent/ExecutorCompletionService#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. |
| ` `[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[poll](https://developer.android.com/reference/java/util/concurrent/ExecutorCompletionService#poll())`() ` Retrieves and removes the Future representing the next completed task, or`null`if none are present.                                                                                                                                                                                           |
| ` `[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[submit](https://developer.android.com/reference/java/util/concurrent/ExecutorCompletionService#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.                                                                                          |
| ` `[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[submit](https://developer.android.com/reference/java/util/concurrent/ExecutorCompletionService#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.                                                |
| ` `[Future](https://developer.android.com/reference/java/util/concurrent/Future)`<V>` | ` `[take](https://developer.android.com/reference/java/util/concurrent/ExecutorCompletionService#take())`() ` Retrieves and removes the Future representing the next completed task, waiting if none are yet present.                                                                                                                                                                                       |

| ### Inherited methods |
|-----------------------|---|
| From class` `[java.lang.Object](https://developer.android.com/reference/java/lang/Object)` ` |--------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` `[Object](https://developer.android.com/reference/java/lang/Object)          | ` `[clone](https://developer.android.com/reference/java/lang/Object#clone())`() ` Creates and returns a copy of this object.                                                                                                                                                      | | ` boolean`                                                                     | ` `[equals](https://developer.android.com/reference/java/lang/Object#equals(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` obj) ` Indicates whether some other object is "equal to" this one.                                           | | ` void`                                                                        | ` `[finalize](https://developer.android.com/reference/java/lang/Object#finalize())`() ` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.                                                          | | ` final `[Class](https://developer.android.com/reference/java/lang/Class)`<?>` | ` `[getClass](https://developer.android.com/reference/java/lang/Object#getClass())`() ` Returns the runtime class of this`Object`.                                                                                                                                                | | ` int`                                                                         | ` `[hashCode](https://developer.android.com/reference/java/lang/Object#hashCode())`() ` Returns a hash code value for the object.                                                                                                                                                 | | ` final void`                                                                  | ` `[notify](https://developer.android.com/reference/java/lang/Object#notify())`() ` Wakes up a single thread that is waiting on this object's monitor.                                                                                                                            | | ` final void`                                                                  | ` `[notifyAll](https://developer.android.com/reference/java/lang/Object#notifyAll())`() ` Wakes up all threads that are waiting on this object's monitor.                                                                                                                         | | ` `[String](https://developer.android.com/reference/java/lang/String)          | ` `[toString](https://developer.android.com/reference/java/lang/Object#toString())`() ` Returns a string representation of the object.                                                                                                                                            | | ` final void`                                                                  | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait(long,%20int))`(long timeoutMillis, int nanos) ` Causes the current thread to wait until it is awakened, typically by being*notified* or*interrupted*, or until a certain amount of real time has elapsed. | | ` final void`                                                                  | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait(long))`(long timeoutMillis) ` Causes the current thread to wait until it is awakened, typically by being*notified* or*interrupted*, or until a certain amount of real time has elapsed.                   | | ` final void`                                                                  | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait())`() ` Causes the current thread to wait until it is awakened, typically by being*notified* or*interrupted*.                                                                                             | ||
| From interface` `[java.util.concurrent.CompletionService](https://developer.android.com/reference/java/util/concurrent/CompletionService)` ` |------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` 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 constructors

### ExecutorCompletionService

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

```
public ExecutorCompletionService (Executor executor)
```

Creates an ExecutorCompletionService using the supplied executor for base task execution and a[LinkedBlockingQueue](https://developer.android.com/reference/java/util/concurrent/LinkedBlockingQueue)as a completion queue.

<br />

|                     Parameters                     ||
|------------|----------------------------------------|
| `executor` | `Executor`: the executor to use <br /> |

|                                                        Throws                                                        ||
|------------------------------------------------------------------------------------------------|----------------------|
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | if executor is`null` |

### ExecutorCompletionService

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

```
public ExecutorCompletionService (Executor executor, 
                BlockingQueue<Future<V>> completionQueue)
```

Creates an ExecutorCompletionService using the supplied executor for base task execution and the supplied queue as its completion queue.

<br />

|                                                                                                                              Parameters                                                                                                                               ||
|-------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `executor`        | `Executor`: the executor to use <br />                                                                                                                                                                                                             |
| `completionQueue` | `BlockingQueue`: the queue to use as the completion queue normally one dedicated for use by this service. This queue is treated as unbounded -- failed attempted`Queue.add`operations for completed tasks cause them not to be retrievable. <br /> |

|                                                                  Throws                                                                  ||
|------------------------------------------------------------------------------------------------|------------------------------------------|
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | if executor or completionQueue are`null` |

## Public methods

### poll

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

```
public 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) |   |

### poll

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

```
public 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 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) |   |
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException)                        |   |

### submit

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

```
public 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) |   |
| [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException)                        |   |

### take

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

```
public 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) |   |