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);
 }