How to concurrently process elements in a Collection in Java

Question

I need to process elements in some Collection instance concurrently. In other words instead of iterating a Collection instance

for (Someclass elem : coll){
     process(elem);
}

I’d like to process those elements concurrently. Say, something like ConcurrentCollectionExecutor(coll, new Callable{…}, numberOfThreads). Also, a number of simultaneous threads should be fixed.

Any flexible pattern already exists?

Answer 1

Make the process method a run() method in a class called MyRunnable that implements Runnable and whose constructor takes elem as input and stores it as an instance variable. Then use:

ExecutorService executor = Executors.newFixedThreadPool(numberOfThreads);
for (Someclass elem : coll){
   Runnable worker = new MyRunnable(elem);
   executor.execute(worker);
}

Answer 2

A good solution would be:

1. instantiate an ArrayBlockingQueue containing the elements to process
2. instantiate an ExecutorService to execute your processing concurrently
3. instantiate your Runnables giving them the ArrayBlockingQueue as parameter
4. Implement the run method: while there elements in the queue, poll them and process them
5. Submit your Runnables to the ExecutorService

The code:

BlockingQueue<Someclass> toProcess = 
    new ArrayBlockingQueue<Someclass>(coll.size(), false, coll);
ExecutorService es = Executors.newFixedThreadPool(numberOfThreads);
for(int count = 0 ; count < numberOfThreads ; ++c) {
    es.submit(new MyRunnable(toProcess));
}


private static class MyRunnable() implements Runnable {
    private final BlockingQueue<Someclass> toProcess;

    public MyRunnable(BlockingQueue<Someclass> toProcess) {
        this.toProcess = toProcess;
    }

    @Override
    public void run() {
        Someclass element = null;
        while((element = toProcess.poll()) != null) {
            process(element);
        }
    }
}