Executor Completion Service

Scenario: you want to load the tasks in parallel and then wait for the completion of all the tasks.

//Callable is different from Runnable that it returns result
private final class StringTask extends Callable<String>{
   public String call(){
      //Long operations
      return "Run";
   }
}

ExecutorService threadPool = Executors.newFixedThreadPool(4);
CompletionService<String> pool = new ExecutorCompletionService<String>(threadPool);

for(int i = 0; i < 10; i++){
   pool.submit(new StringTask());
}

//you have the result in the order they are completed and you don’t have to keep a 
//collection of Future
for(int i = 0; i < 10; i++){
   String result = pool.take().get();

   //Compute the result
}
threadPool.shutdown();

See: Java Concurrency: Executors and Thread Pools