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> ecs
           = new ExecutorCompletionService<Result>(e);
       for (Callable<Result> s : solvers)
           ecs.submit(s);
       int n = solvers.size();
       for (int i = 0; i < n; ++i) {
           Result r = ecs.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> ecs
           = new ExecutorCompletionService<Result>(e);
       int n = solvers.size();
       List<Future<Result>> futures
           = new ArrayList<Future<Result>>(n);
       Result result = null;
       try {
           for (Callable<Result> s : solvers)
               futures.add(ecs.submit(s));
           for (int i = 0; i < n; ++i) {
               try {
                   Result r = ecs.take().get();
                   if (r != null) {
                       result = r;
                       break;
                   }
               } catch (ExecutionException ignore) {}
           }
       }
       finally {
           for (Future<Result> f : futures)
               f.cancel(true);
       }

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