Java - Creating a method which returns an iterator

I'm currently creating a class called ArraySet that implements the Set interface. I'm supposed to create an iterator method that returns the values in natural order, where the time complexities are iterator(): O(1); hasNext(): O(1); next(): O(1); required space: O(1). The method is supposed to return an Iterator that does this. I'm confused by the way this method works and what is exactly wanted from me. Because it's a method I shouldn't be able to create hasNext(), or next() methods inside of it, and what Iterator am I trying to return? I tried just returning an Iterator but it's abstract and cannot be instantiated. I can understand making a new iterator class, but am having trouble understanding how this works in method form. Basically, what the method looks like at the moment is this, but like I've said I don't know what to even put inside of it. Also, if it helps there are two fields in the ArraySet class, int size (the number of elements in the Array), and T[] elements (the array where the elements are stored, strictly in natural order though I'm not even sure how to enforce natural order)

public Iterator<T> iterator() {
  return null;
}