EC
ConcurrentModificationException
JavaERRORCommonConcurrency

Collection modified during iteration

Quick Answer

Use Iterator.remove() to delete during iteration, or use list.removeIf() to batch-remove elements.

What this means

Thrown by an iterator when the underlying collection is modified structurally during iteration outside of the iterator's own remove() method.

Why it happens
  1. 1Calling list.remove() inside a for-each loop over the same list
  2. 2Two threads iterating and mutating the same collection

Fix

Use removeIf (Java 8+)

Use removeIf (Java 8+)
list.removeIf(s -> s.isEmpty()); // safe — no CME

Why this works

removeIf uses the iterator internally, so structural modifications are tracked correctly.

Code examples
Triggerjava
for (String s : list) {
    if (s.isEmpty()) list.remove(s); // ConcurrentModificationException
}
Safe with Iteratorjava
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    if (it.next().isEmpty()) it.remove();
}
Thread-safe with CopyOnWriteArrayListjava
List<String> safe = new CopyOnWriteArrayList<>(list);
for (String s : safe) { /* safe to remove from safe */ }
Sources
Official documentation ↗

Java SE Documentation

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