Added in API level 1

Summary: Ctors | Methods | Inherited Methods

ExecutorCompletionService

public class ExecutorCompletionService
extends Object implements CompletionService<V>

java.lang.Object
↳	java.util.concurrent.ExecutorCompletionService<V>

A CompletionService that uses a supplied 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);
 }

Summary

Public constructors
`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<V>> completionQueue)` Creates an ExecutorCompletionService using the supplied executor for base task execution and the supplied queue as its completion queue.

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

Inherited methods

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(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(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()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

From interface


        
          java.util.concurrent.CompletionService

`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 constructors

ExecutorCompletionService

Added in API level 1

public ExecutorCompletionService (Executor executor)

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

Parameters
`executor`	`Executor`: the executor to use

Throws
`NullPointerException`	if executor is `null`

ExecutorCompletionService

Added in API level 1

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.

Parameters
`executor`	`Executor`: the executor to use
`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.

Throws
`NullPointerException`	if executor or completionQueue are `null`

Public methods

poll

Added in API level 1

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.

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`

poll

Added in API level 1

public 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 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`
`NullPointerException`

submit

Added in API level 1

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.

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

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

Throws
`RejectedExecutionException`
`NullPointerException`

take

Added in API level 1

public 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`