Java.Collections.Delve in fail fast in collections

In Java, many collections (like ArrayList, HashMap, etc.) have fail-fast iterators. This means that if the collection is structurally modified after the iterator is created, the iterator will throw a:

java.util.ConcurrentModificationException

✅ What counts as a structural modification?

• Add or remove elements directly via the collection itself.
• Clear the collection.
• Any change that alters the size or structure of the collection.

🧠 Note: Changes through the iterator itself (e.g., iterator.remove()) do not cause failure — they’re considered safe.

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🔥 Why does it fail fast?

• To catch concurrent modifications early.
• To avoid unpredictable behavior if a collection changes while it’s being iterated over.
• To make bugs easier to diagnose by failing at the moment of misuse.

🧪 Example: ArrayList Fail-Fast Behavior

List<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");
list.add("Go");

Iterator<String> iterator = list.iterator();

list.add("Rust"); // structural modification after iterator created

while (iterator.hasNext()) {
    System.out.println(iterator.next()); // throws ConcurrentModificationException
}

✅ Safe way using iterator’s own method

List<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");
list.add("Go");

Iterator<String> iterator = list.iterator();

while (iterator.hasNext()) {
    String value = iterator.next();
    if (value.equals("Python")) {
        iterator.remove(); // Safe way to remove during iteration
    }
}
System.out.println(list); // [Java, Go]

🧠 Under the Hood: modCount

Java collections use an internal variable called modCount (modification count):

• When you structurally modify the collection, modCount increases.
• The iterator captures the modCount at the time it was created.
• If modCount has changed unexpectedly during iteration, it throws a ConcurrentModificationException.

💬 Common Mistakes to Watch For

• Modifying a collection inside a for-each loop: javaCopyEdit

for (String item : list) {
    list.remove(item); // ❌ throws ConcurrentModificationException
}

Fix: Use an explicit Iterator with remove().

🛡️ How to handle it?

• Copy the collection before iterating (if modification is needed).
• Use CopyOnWriteArrayList or other concurrent collections in multithreaded environments.
• Or use iterator’s remove() method for safe structural changes.