Performance: Creating an ArrayList from HashMap.values()

Question

Question is how much it costs to create an ArrayList from a HashMap.values() Collection? Or creating the values Collection alone? Assuming Map.size() > 100k. Objects could also

Answer 1

HashMap.values() doesn't return an ArrayList of values but a Values Collection.

Source:

 public Collection values() {
        Collection vs = values;
        return (vs != null ? vs : (values = new Values()));
    }

Values is an AbstractCollection. The reason for values is just to reference HashMap's iterator.

Your question:

Question is how much it costs to create an ArrayList from a HashMap.values() Collection?

That's a linear complexity (as Bozho said) since

ArrayList valuesList = new ArrayList(hashMap.values());

the ArrayList, valuesList calls the collection hashMap toArray() method which essentially does a for loop from 0..N (size) element in the collection.

Hope this helps.