# Class ExecutorCompletionService<V>

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

Type Parameters:
  `V` - the type of values the tasks of this service produce and consume

All Implemented Interfaces:
  `CompletionService<V>`

public class ExecutorCompletionService<V> extends Object implements CompletionService<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);
    }

Since:
  1.5

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `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.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `Future<V>`
  `poll()`
  Retrieves and removes the Future representing the next completed task, or `null` if none are present.
  `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>`
  `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.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### ExecutorCompletionService

    public ExecutorCompletionService(Executor executor)
    Creates an ExecutorCompletionService using the supplied executor for base task execution and a `LinkedBlockingQueue` as a completion queue.
    Parameters:
      `executor` - the executor to use
    Throws:
      `NullPointerException` - if executor is `null`

  - ### ExecutorCompletionService

    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` - the executor to use
      `completionQueue` - 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`

- ## Method Details

  - ### submit

    public Future<V> submit(Callable<V> task)
    Description copied from interface: `CompletionService`
    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.
    Specified by:
      `submit` in interface `CompletionService<V>`
    Parameters:
      `task` - the task to submit
    Returns:
      a Future representing pending completion of the task
    Throws:
      `RejectedExecutionException` - if the task cannot be scheduled for execution
      `NullPointerException` - if the task is null

  - ### submit

    public Future<V> submit(Runnable task, V result)
    Description copied from interface: `CompletionService`
    Submits a Runnable task for execution and returns a Future representing that task. Upon completion, this task may be taken or polled.
    Specified by:
      `submit` in interface `CompletionService<V>`
    Parameters:
      `task` - the task to submit
      `result` - the result to return upon successful completion
    Returns:
      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

  - ### take

    public Future<V> take() throws InterruptedException
    Description copied from interface: `CompletionService`
    Retrieves and removes the Future representing the next completed task, waiting if none are yet present.
    Specified by:
      `take` in interface `CompletionService<V>`
    Returns:
      the Future representing the next completed task
    Throws:
      `InterruptedException` - if interrupted while waiting

  - ### poll

    public Future<V> poll()
    Description copied from interface: `CompletionService`
    Retrieves and removes the Future representing the next completed task, or `null` if none are present.
    Specified by:
      `poll` in interface `CompletionService<V>`
    Returns:
      the Future representing the next completed task, or `null` if none are present

  - ### poll

    public Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException
    Description copied from interface: `CompletionService`
    Retrieves and removes the Future representing the next completed task, waiting if necessary up to the specified wait time if none are yet present.
    Specified by:
      `poll` in interface `CompletionService<V>`
    Parameters:
      `timeout` - how long to wait before giving up, in units of `unit`
      `unit` - a `TimeUnit` determining how to interpret the `timeout` parameter
    Returns:
      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