Clone an Iterator in Java?

Question

In a game I have a list of players, let's say like this:

LinkedList<String> players = new LinkedList<String>();

I want to let each player interact with each of the other players, so I write two nested loops:

Iterator<String> i1 = players.iterator();
while (i1.hasNext()) {
    String p1 = i1.next();
    Iterator<String> i2 = players.iterator();
    // But I want to do this: Iterator<String> i2 = i1.clone();
    while (i2.hasNext()) {
        String p2 = i2.next();
        System.out.println("Interact: " + p1 + ", " + p2);
    }
}

Since I only want each pair of players to interact once, I want to start the inner loop with the player after the outer loop's current player. So I want to clone the iterator, but that doesn't compile.

So, what should I do instead?

Answer 1

The following will do it:

ListIterator<String> i1 = players.listIterator(0);
while (i1.hasNext()) {
    String p1 = i1.next();
    ListIterator<String> i2 = players.listIterator(i1.nextIndex());
    while (i2.hasNext()) {
        String p2 = i2.next();
        System.out.println("Interact: " + p1 + ", " + p2);
    }
}

It relies on the ListIterator's ability to start from the given position and to also know its current position.

Answer 2

In addition to aix answer, I'd like to point out that however you create an iterator starting at a specific index, it's bound to be a linear operation. If it wasn't, you would be able to do arbitrary access to the list in constant time using

elementN = createIterator(linkedList, N).next();

which would be contradictory.

In your situation I therefore believe that the most efficient solution would actually be to do

List<String> tmp = new ArrayList<String>(players);
for (int p1 = 0; p1 < tmp.size(); p1++)
    for (int p2 = p1+1; p2 < tmp.size(); p2++)
        System.out.println("Interact: " + tmp.get(p1) + ", " + tmp.get(p2));

Note however, that it is still the same complexity as the solution by aix; O(n²) but probably with a smaller constant factor.