Difference between Iterator and ListIterator.

Iterator is a one-way cursor for any Collection. ListIterator is a richer cursor available only for List — bidirectional, index-aware, and able to insert and replace elements during traversal.

API comparison

Why the asymmetry?

Iterator is the lowest-common-denominator contract for any collection — Set, Queue, custom iterables. Sets and queues have no notion of "previous" element or "index of position N", so those operations would be meaningless. ListIterator lives on List because lists are positional by definition.

Code

List<String> names = new ArrayList<>(List.of("Ada", "Bob", "Cy"));

// Iterator: forward + remove
Iterator<String> it = names.iterator();
while (it.hasNext()) {
    if (it.next().startsWith("B")) it.remove();
}

// ListIterator: replace in place + insert
ListIterator<String> lit = names.listIterator();
while (lit.hasNext()) {
    int idx = lit.nextIndex();
    String s = lit.next();
    lit.set(s.toUpperCase());          // replace
    if (idx == 0) lit.add("ZERO");     // inserts AFTER cursor
}

// Start from end
ListIterator<String> back = names.listIterator(names.size());
while (back.hasPrevious()) {
    System.out.println(back.previous());
}

Pitfalls

• set(E) and remove() operate on the element returned by the most recent next() or previous(). Call them twice in a row and you get IllegalStateException.
• add(E) inserts at the cursor position — the new element is not returned by a subsequent next(), but previous() will see it.
• On a LinkedList, ListIterator is the only efficient way to mutate mid-list — list.get(i) is O(n) each call.