How do you remove elements from a list while iterating?

Use iterator.remove() inside an explicit Iterator loop, or list.removeIf(predicate). Calling list.remove() inside a for-each loop throws ConcurrentModificationException.

Stage 5: Collections compared, lesson 7 of 8

Iterator vs ListIterator

Q: Iterator vs ListIterator?

Iterator works on any collection and only moves forward, with remove(). ListIterator works on lists, moves both ways, knows the index and can also set() and add() elements.

Intermediate3 min read@since 8Code runs on your Java 25

Explain it forThe essentials plus production detail and pitfalls.

Both walk through elements and let you change the collection safely while iterating.

Iterator works with every collection. Methods: hasNext(), next(), remove() and (Java 8) forEachRemaining(). Forward only.
ListIterator extends Iterator and works only with lists (list.listIterator(), or listIterator(index) to start in the middle). It adds:
both directions: hasPrevious() and previous();
positions: nextIndex() and previousIndex();
changes: set(e) replaces the last returned element, add(e) inserts at the current position.

Use a for-each loop to just read, an Iterator to remove while looping (or simply removeIf), and a ListIterator to replace, insert or walk backwards.

Side by side

	Iterator	ListIterator
Works with	Any Collection	Lists only
Direction	Forward	Forward and backward
Start position	The beginning	Anywhere: `listIterator(index)`
Remove	`remove()`	`remove()`
Replace	No	`set(e)`
Insert	No	`add(e)`
Index	No	`nextIndex()`, `previousIndex()`

Example

Java

List<String> names = new ArrayList<>(List.of("asha", "ravi", "meera"));

Iterator<String> it = names.iterator();
while (it.hasNext()) {
    if (it.next().startsWith("r")) it.remove();     // safe removal while iterating
}

ListIterator<String> li = names.listIterator();
while (li.hasNext()) {
    String n = li.next();
    li.set(n.substring(0, 1).toUpperCase() + n.substring(1));   // replace in place: Asha, Meera
    if (n.equals("asha")) li.add("Kiran");                      // insert after Asha
}
System.out.println(names);                          // [Asha, Kiran, Meera]

while (li.hasPrevious()) System.out.print(li.previous() + " ");   // Meera Kiran Asha

Common mistake

Calling list.remove(x) inside a for-each loop over the same list. Use iterator.remove() or list.removeIf(...).

Under the hood

remove() and set() act on the element most recently returned by next() or previous(); calling them before next(), twice in a row, or set() after add() throws IllegalStateException. Changing the list directly (not through the iterator) during iteration throws ConcurrentModificationException on the next step (see "Fail-fast vs fail-safe").

Check yourself

Which method is available on ListIterator but not on Iterator?

How this connects

Know these first

Every List, Set and Queue: ArrayList to BlockingQueue

Where this leads

You've reached the end of this thread. Try a learning path for what's next.

Part of Java 8 and collections, practically.

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