HashSet vs ArrayList contains performance

Question

When processing large amounts of data I often find myself doing the following:

HashSet<String> set = new HashSet<String> (); //Adding elements to the set ArrayList<String> list = new ArrayList<String> (set); 

Something like "dumping" the contents of the set in the list. I usually do this since the elements I add often contain duplicates I want to remove, and this seems like an easy way to remove them.

With only that objective in mind (avoiding duplicates) I could also write:

ArrayList<String> list = new ArrayList<String> (); // Processing here if (! list.contains(element)) list.add(element); //More processing here 

And thus no need for "dumping" the set into the list. However, I'd be doing a small check before inserting each element (which I'm assuming HashSet does as well)

Is any of the two possibilities clearly more efficient?

Answer 1

The set will give much better performance (O(n) vs O(n^2) for the list), and that's normal because set membership (the contains operation) is the very purpose of a set.

Contains for a HashSet is O(1) compared to O(n) for a list, therefore you should never use a list if you often need to run contains.

Answer 2

The ArrayList uses an array for storing the data. The ArrayList.contains will be of O(n) complexity. So essentially searching in array again and again will have O(n^2) complexity.

While HashSet uses hashing mechanism for storing the elements into their respective buckets. The operation of HashSet will be faster for long list of values. It will reach the element in O(1).