Question

I have data stored in a HashMap, which I want to access via multiple threads simultaneously, to split the work done on the items.

Normally (with a List for example) I would just give each thread an index to start with and could easily split the work like this:

for(int i = startIndex; i < startIndex+batchSize && i < list.size(); i++)
{
    Item a = list.get(i);
    // do stuff with the Item
}

Of course this doesnt work with a HashMap, because I can’t access it via an index.

Is there an easy way to iterate only over a part of the map? Should I rather use another data structure for this case?

I read about SortedMap, but it has too much overhead I dont need (sorting the items). I have a lot of data and performance is crucial.

Any tips would be highly appreciated.

Answer 1

If you only do the traversal a few times, or if the map doesn’t change you could get a Set of keys, and then send that to an array. From there its pretty much your normal method. But obviously if the HashMap changed then you would have to do those two operations over again which could get very costly.