Does a HashMap with a getAndWait() method exist? E.g. a BlockingConcurrentHashMap implementation?

Question

Many threads may populate a HashMap, in some cases I need to wait (block) until an object exists in the HashMap, such as:

BlockingConcurrentHashMap map = new BlockingConcurrentHashMap();
Object x = map.getAndWait(key, 1000);    //(object_to_get, max_delay_ms)

Wondering if such a thing exists already, I hate re-inventing wheels.

Answer 1

You can populate your Hashtable with java.util.concurrent.FutureTask<ObjReturned>s at the start with all the tasks you need to compute. You then use a thread pool to start executing your FutureTasks. You can get your results asynchronously with ObjReturned obj = hashtable.get(key).get(), which will wait if the FutureTask in question is not done yet.

You probably don't want one single thread to retrieve the results since it might wait on the task that will turn out to finish last. You could have multiple retrieval threads, or you could cycle through the keys when you wait too long for one task (there is a method FutureTask.get(waitTime, timeUnit)).