Great question—let’s unpack this clearly, since the "unordered" nature of Set can seem confusing at first when paired with iteration.

First, let’s get back to basics: Iterator is a general-purpose interface for traversing elements in any collection that implements the Iterable interface. In Java, the Set interface extends Collection, which in turn extends Iterable. That means every Set implementation (like HashSet, TreeSet, LinkedHashSet) is required to provide an iterator() method—so Iterator support is baked right into the Set contract, no conversion needed.

Now, let’s clarify what "unordered" really means for Set:

• It doesn’t mean the elements have no order at all. Instead, it means the Set does not guarantee that the order of elements will match the order you inserted them, nor does it guarantee the order will stay consistent over time (e.g., if you add/remove elements).
• Each Set implementation has its own internal storage logic that defines a traversal order:
  • HashSet uses a hash table, so its Iterator traverses elements based on their hash codes and the structure of the hash buckets. In your example, the integers 1-5 happen to have hash codes that map to buckets in order, so you see them printed sequentially—but this is coincidence, not a guarantee. If you added strings or custom objects, the order would likely not match insertion order.
  • TreeSet uses a sorted tree structure, so its Iterator traverses elements in natural order (or the order defined by a custom comparator)—this Set is actually ordered, but that’s a specific implementation detail.
  • LinkedHashSet maintains insertion order by using a linked list alongside the hash table, so its Iterator will follow insertion order.

To answer your key question: Iterator does NOT convert Set to a List-like structure. It simply follows the internal traversal logic of the Set implementation you’re using. The Iterator’s job is to provide a way to access each element exactly once, regardless of whether the collection is ordered, unordered, sorted, or not.

Let’s look at your code example to reinforce this:

public class Staticex { 
    public static void main(String[] args) { 
        HashSet set = new HashSet(); 
        set.add(1); 
        set.add(2); 
        set.add(3); 
        set.add(4); 
        set.add(5); 
        Iterator iter = set.iterator(); 
        while (iter.hasNext()) { 
            System.out.println(iter.next()); 
        } 
    } 
}

Your HashSet outputs 1-5 in order here because the hash code of an Integer is the integer itself, so they’re stored in buckets that align with their values. But if you modified the code to add elements like set.add(10); set.add(5);, you might see 5 printed before 10 (depending on the hash table’s bucket layout)—that’s the "unordered" behavior in action.

In short:

• Set supports Iterator because it inherits from Iterable via the Collection hierarchy.
• "Unordered" doesn’t mean no traversal order—it means no guarantee of insertion order.
• Iterator works directly with the Set’s internal storage, no conversion to List required.

内容的提问来源于stack exchange，提问作者Hanseo