Java collections maintaining insertion order

Question

Why do some collection data structures not maintain the order of insertion? What is the special thing achieved compared to maintaining order of insertion? Do we gain someth

Answer 1

When you use a HashSet (or a HashMap) data are stored in "buckets" based on the hash of your object. This way your data is easier to access because you don't have to look for this particular data in the whole Set, you just have to look in the right bucket.

This way you can increase performances on specific points.

Each Collection implementation have its particularity to make it better to use in a certain condition. Each of those particularities have a cost. So if you don't really need it (for example the insertion order) you better use an implementation which doesn't offer it and fits better to your requirements.