Purpose of iterators

Question

Why are iterators used if you can access a collection's value with an "extended for" cycle?

Answer 1

An iterator is useful if used in a while loop (your loop will continue until the collector will have no more elements), otherwise the "extended for" cycle sometimes will be not correct.
e.g. I wrote a code to calculate prime numbers: those numbers would be calcolated only when necessary, calling the hasNext() method.
So, if you use an iterator cycle you would have no limits, in other situation to do a for cycle you should first determine the number of elements in the list and this would stop the loop immediately!
I hope you would understand sorry for my english..