Java collection insertion: Set vs. List

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)?

Answer 1

If the goal is the uniqueness of the elements, you should use an implementation of the java.util.Set interface. The class java.util.HashSet and java.util.LinkedHashSet have O(alpha) (close to O(1) in the best case) complexity for insert, delete and contains check.

ArrayList have O(n) for object (not index) contains check (you have to scroll through the whole list) and insertion (if the insertion is not in tail of the list, you have to shift the whole underline array).

You can use LinkedHashSet that preserve the order of insertion and have the same potentiality of HashSet (takes up only a bit more of memory).