Java.Collections.How will an instantiated iterator for collection behave if collection.remove() is called?

👉 Short Answer:

If the collection is structurally modified outside the iterator (like calling collection.remove() directly), the iterator becomes invalid, and a ConcurrentModificationException will be thrown on the next iterator operation.

🧪 Example:

List<String> list = new ArrayList<>(List.of("A", "B", "C"));
Iterator<String> it = list.iterator();

list.remove("B"); // ❌ Structural modification outside the iterator

while (it.hasNext()) {
    System.out.println(it.next()); // 💥 ConcurrentModificationException
}

⚠️ Why?

• The iterator captured the list’s modCount when it was created.
• When you call list.remove("B"), modCount is changed behind the iterator’s back.
• When it.next() is called, it checks modCount and sees that it has changed → 💥 fail-fast exception.

✅ What’s the correct way to remove during iteration?

Use the iterator’s own remove() method:

Iterator<String> it = list.iterator();
while (it.hasNext()) {
    if (it.next().equals("B")) {
        it.remove(); // ✅ Safe
    }
}

This way:

• The iterator is aware of the removal.
• The internal modCount is updated properly by the iterator itself.
• No exception is thrown.

🧠 Summary

Operation	Result
collection.remove() during iteration	❌ Unsafe — may cause ConcurrentModificationException
iterator.remove()	✅ Safe — iterator handles it correctly

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💬 Analogy

Imagine you’re reading a book (iterator) while someone comes in and tears out a page (collection.remove()) without telling you.

When you turn to that page → crash!
If you remove the page yourself (iterator.remove()), you’re in control, so all is well.