Wait for multiple AsyncTask to complete

Question

I am parallelizing my operation by splitting it in the exact number of cores available and then, by start the same number of AsyncTask, performing the same operation but on

Answer 1

You could also simply decrement a counter in a shared object as part of onPostExecute. As onPostExecute runs on the same thread (the main thread), you won't have to worry about synchronization.

UPDATE 1

The shared object could look something like this:

public class WorkCounter {
    private int runningTasks;
    private final Context ctx;

    public WorkCounter(int numberOfTasks, Context ctx) {
        this.runningTasks = numberOfTasks;
        this.ctx = ctx;
    }
    // Only call this in onPostExecute! (or add synchronized to method declaration)
    public void taskFinished() {
        if (--runningTasks == 0) {
            LocalBroadcastManager mgr = LocalBroadcastManager.getInstance(this.ctx);
            mgr.sendBroadcast(new Intent("all_tasks_have_finished"));
        }
    }
}

UPDATE 2

According to the comments for this answer, OP is looking for a solution in which he can avoid building a new class. This can be done by sharing an AtomicInteger among the spawned AsyncTasks:

// TODO Update type params according to your needs.
public class MyAsyncTask extends AsyncTask {
    // This instance should be created before creating your async tasks.
    // Its start count should be equal to the number of async tasks that you will spawn.
    // It is important that the same AtomicInteger is supplied to all the spawned async tasks such that they share the same work counter.
    private final AtomicInteger workCounter;

    public MyAsyncTask(AtomicInteger workCounter) {
        this.workCounter = workCounter;
    }

    // TODO implement doInBackground

    @Override
    public void onPostExecute(Void result) {
        // Job is done, decrement the work counter.
        int tasksLeft = this.workCounter.decrementAndGet();
        // If the count has reached zero, all async tasks have finished.
        if (tasksLeft == 0) {
            // Make activity aware by sending a broadcast.
            LocalBroadcastManager mgr = LocalBroadcastManager.getInstance(this.ctx);
            mgr.sendBroadcast(new Intent("all_tasks_have_finished"));    
        }
    }
}