Fail-Fast vs Fail-Safe Iterators

import java.util.*;

List<String> list = new ArrayList<>(List.of("a", "b", "c"));

try {
    for (String s : list) {
        list.add("x"); // structural modification — throws!
    }
} catch (ConcurrentModificationException e) {
    System.out.println("ConcurrentModificationException caught!");
}
// Same for HashMap, TreeMap, HashSet, etc.

// modCount is incremented on every structural change
// (add, remove, clear on ArrayList)
// Iterator records modCount on creation
// On each next(), Iterator checks: if current modCount != expected, throw CME

// This detects bugs like:
List<Integer> nums = new ArrayList<>(List.of(1,2,3,4,5));
for (Integer n : nums) {
    if (n == 3) nums.remove(n); // bug caught immediately
}
// ConcurrentModificationException — not silent wrong results

List<Integer> nums = new ArrayList<>(List.of(1,2,3,4,5,6));
Iterator<Integer> it = nums.iterator();

while (it.hasNext()) {
    int n = it.next();
    if (n % 2 == 0) it.remove(); // safe: updates modCount
}

System.out.println(nums); // [1, 3, 5]

List<String> names = new ArrayList<>(List.of("Alice", "Bob", "Ann", "Charlie"));

// Modern: removeIf handles iteration internally
names.removeIf(name -> name.startsWith("A"));
System.out.println(names); // [Bob, Charlie]

// Equivalent but verbose iterator approach:
Iterator<String> it = names.iterator();
while (it.hasNext()) {
    if (it.next().startsWith("A")) it.remove();
}

import java.util.concurrent.CopyOnWriteArrayList;

CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
list.addAll(List.of("a", "b", "c"));

for (String s : list) {
    list.add("x"); // no ConcurrentModificationException!
    System.out.print(s + " "); // prints a, b, c (snapshot)
}
System.out.println();
System.out.println(list); // [a, b, c, x, x, x] — modified copy

import java.util.concurrent.ConcurrentHashMap;

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1); map.put("b", 2); map.put("c", 3);

for (Map.Entry<String, Integer> e : map.entrySet()) {
    map.put("d", 4); // no CME — weakly consistent
    System.out.println(e.getKey() + "=" + e.getValue());
}
// May or may not print "d=4" — weakly consistent

import java.util.*;

List<String> syncList = Collections.synchronizedList(new ArrayList<>());
syncList.addAll(List.of("a", "b", "c"));

// Must synchronize during iteration!
synchronized (syncList) {
    for (String s : syncList) {
        System.out.println(s);
    }
}
// Without the synchronized block, CME is still possible from another thread

import java.util.*;

List<String> original = new ArrayList<>(List.of("a", "b", "c"));

// Take a snapshot copy before iterating
List<String> snapshot = List.copyOf(original);

for (String s : snapshot) {
    // Safe to modify original during snapshot iteration
    original.remove(s);
    System.out.println("Removed: " + s);
}
System.out.println(original); // []

import java.util.*;
import java.util.concurrent.*;

class SessionManager {
    private final CopyOnWriteArrayList<Session> sessions = new CopyOnWriteArrayList<>();

    record Session(String id, long expiresAt) {
        boolean isExpired() { return System.currentTimeMillis() > expiresAt; }
    }

    void add(Session s) { sessions.add(s); }

    // Safe to call from multiple threads while iterating
    void purgeExpired() {
        sessions.removeIf(Session::isExpired);
    }

    List<Session> active() {
        return sessions.stream().filter(s -> !s.isExpired()).toList();
    }
}