ConcurrentModificationException

Why This Happens

Java collections use a fail-fast mechanism that detects structural modification during iteration. If you add or remove elements from a list while a for-each loop is running, the iterator detects the change and throws this exception. This can happen in single-threaded code too.

The Problem

List<String> names = new ArrayList<>(List.of("Alice", "Bob", "Charlie"));
for (String name : names) {
    if (name.startsWith("B")) {
        names.remove(name); // ConcurrentModificationException
    }
}

The Fix

List<String> names = new ArrayList<>(List.of("Alice", "Bob", "Charlie"));
Iterator<String> it = names.iterator();
while (it.hasNext()) {
    if (it.next().startsWith("B")) {
        it.remove(); // Safe removal via iterator
    }
}
// Or: names.removeIf(name -> name.startsWith("B"));

Step-by-Step Fix

1. 1

   Identify the modification during iteration

   Find the loop that iterates over the collection and the line inside it that modifies the same collection.

2. 2

   Determine the modification type

   Check if you are adding, removing, or replacing elements during iteration.

3. 3

   Use safe modification patterns

   Use Iterator.remove(), Collection.removeIf(), or collect items to remove in a separate list and remove after iteration.