Question

I’m thinking about filling a collection with a large amount of unique objects.
How is the cost of an insert in a Set (say HashSet) compared to an List (say ArrayList)?

My feeling is that duplicate elimination in sets might cause a slight overhead.

Answer 1

There is no “duplicate elimination” such as comparing to all existing elements. If you insert into hash set, it’s really a dictionary of items by hash code. There’s no duplicate checking unless there already are items with the same hash code. Given a reasonable (well-distributed) hash function, it’s not that bad.

As Will has noted, because of the dictionary structure HashSet is probably a bit slower than an ArrayList (unless you want to insert “between” existing elements). It also is a bit larger. I’m not sure that’s a significant difference though.