Java.Collections.

🛡️ Fail-Safe Iterators — Real Examples

1. ✅ CopyOnWriteArrayList.iterator()

• Part of the java.util.concurrent package.
• Iterator works on a snapshot copy of the array.
• Safe to modify the list while iterating — changes do not affect the iterator.

import java.util.concurrent.CopyOnWriteArrayList;

CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
list.add("Java");
list.add("Python");

for (String lang : list) {
    list.remove(lang); // ✅ No ConcurrentModificationException
}
System.out.println(list); // []

🔧 Internals:

• On every add or remove, it creates a new copy of the internal array.
• Iterators never throw ConcurrentModificationException.
• Tradeoff: Very safe, but expensive for write-heavy use cases.

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2. ✅ ConcurrentHashMap.keySet().iterator()

• Also part of java.util.concurrent.
• Its iterator is weakly consistent:
  • Doesn’t throw ConcurrentModificationException
  • Reflects some or all updates during iteration

import java.util.concurrent.ConcurrentHashMap;

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("A", 1);
map.put("B", 2);

for (String key : map.keySet()) {
    map.put("C", 3); // ✅ Safe — no exception
    System.out.println(key);
}

🔧 Internals:

• Uses lock striping or CAS (compare-and-swap) for concurrency.
• Provides a weakly consistent view of the map during iteration.
• Perfect for high-performance concurrent read/write access.

3. ✅ ConcurrentSkipListSet.iterator() / ConcurrentSkipListMap.iterator()

• These are sorted, thread-safe collections.
• Iterators are fail-safe and weakly consistent like ConcurrentHashMap.

import java.util.concurrent.ConcurrentSkipListSet;

ConcurrentSkipListSet<String> set = new ConcurrentSkipListSet<>();
set.add("Alpha");
set.add("Beta");

for (String item : set) {
    set.add("Gamma"); // ✅ Safe, no crash
    System.out.println(item);
}

🔄 Key Traits of Fail-Safe Iterators

Property	Description
🛡️ No Exceptions	They don’t throw ConcurrentModificationException
👀 Snapshot or Weak View	They either work on a copy (snapshot) or see some concurrent changes
🧵 Thread-Safe	Designed to be safe in multithreaded environments
⚠️ Tradeoff	May be slower for writing, or may not show all updates during iteration