Quick Answer

Handle collection modification during iteration.

Understanding the Issue

This exception occurs when a collection is modified while being iterated, except through the iterator's own methods.

The Problem

This code demonstrates the issue:

Java Error 
List<String> list = new ArrayList<>();
list.add("a"); list.add("b");
for (String s : list) {
    if (s.equals("a")) list.remove(s); // Throws
}

The Solution

Here's the corrected code:

Java Fixed 
// Solution 1: Use Iterator.remove()
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    if (it.next().equals("a")) it.remove();
}

// Solution 2: Java 8 removeIf()
list.removeIf(s -> s.equals("a"));

// Solution 3: Concurrent collections
List<String> safeList = new CopyOnWriteArrayList<>(list);

// Solution 4: Create copy for iteration
for (String s : new ArrayList<>(list)) {
    if (s.equals("a")) list.remove(s);
}

Key Takeaways

Never modify collections directly during iteration.