Order of elements in a set in java

Question

If i create 2 lists from the same set, can I be sure that I get the same ordering in both the lists? (I do not care about the ordering as long as both the lists have the same order and I am not performing any operations on the sets between creating the two lists.)

List l = new ArrayList(set);

List l1 = new ArrayList(set);

I understand that there are guaranteed ways of creating these lists and getting the same order and that there isn't a good reason for me to create two lists this way, but I would like to know why the ordering of elements in a set would change if no modify operations are performed on it.

Edit: The set is an unordered HashSet

Answer 1

You will propably get the same ordering in the lists l and l1. But since most Sets are unordered, you have no guarantee that there will be the same order.

Technically you could write an implementation of the Set interface which changes its order everytime any method is called. This would still fulfil the interface.

Since in the constructor new ArrayList(Collection) the toArray method of the collection is called, we can have a look at the Javadoc of Set#toArray():

Returns an array containing all of the elements in this set. If this set makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

While the Javadoc of Set#iterator() says there is no general guarantee:

Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee).

Given this, I would strongly advise you not to rely on the ordering of the lists.