data structures - Why doesn't java.util.Set have get(int index)?

Question

I'm sure there's a good reason, but could someone please explain why the java.util.Set interface lacks get(int Index), or any similar get() method?

It seems that sets are great for putting things into, but I can't find an elegant way of retrieving a single item from it.

If I know I want the first item, I can use set.iterator().next(), but otherwise it seems I have to cast to an Array to retrieve an item at a specific index?

What are the appropriate ways of retrieving data from a set? (other than using an iterator)

I'm sure the fact that it's excluded from the API means there's a good reason for not doing this -- could someone please enlighten me?

EDIT: Some extremely great answers here, and a few saying "more context". The specific scenario was a dbUnit test, where I could reasonably assert that the returned set from a query had only 1 item, and I was trying to access that item.

However, the question is more valid without the scenario, as it remains more focussed:

What's the difference between set and list.

Thanks to all for the fantastic answers below.

Answer 1

Because sets have no ordering. Some implementations do (particularly those implementing the java.util.SortedSet interface), but that is not a general property of sets.

If you're trying to use sets this way, you should consider using a list instead.