Interview question

What is the difference between fail-fast and fail-safe iterators in Java? Java में fail-fast और fail-safe iterators में क्या अंतर है?

Answer

Type	Behavior on modification during iteration	Examples
Fail-fast	Throws ConcurrentModificationException immediately	ArrayList, HashMap, HashSet iterators
Fail-safe	Works on a copy, no exception, but may not reflect the latest changes	CopyOnWriteArrayList, ConcurrentHashMap

// Fail-fast example - throws exception
List<String> list = new ArrayList<>(Arrays.asList('A', 'B', 'C'));
for (String item : list) {
    if (item.equals('B')) {
        list.remove(item);  // ConcurrentModificationException!
    }
}
// Fail-fast iterators use a 'modCount' internally - if it changes
// unexpectedly during iteration, the exception is thrown

// SAFE alternative using Iterator.remove()
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    String item = it.next();
    if (item.equals('B')) {
        it.remove();  // safe - the iterator itself tracks this modification
    }
}

// Fail-safe example - no exception, but may miss recent updates
List<String> safeList = new CopyOnWriteArrayList<>(Arrays.asList('A', 'B', 'C'));
for (String item : safeList) {
    if (item.equals('B')) {
        safeList.remove(item);  // no exception - operates on a snapshot copy
    }
}
System.out.println(safeList);  // [A, C]

// ConcurrentHashMap - fail-safe for concurrent read/write scenarios
Map<String, Integer> concurrentMap = new ConcurrentHashMap<>();
concurrentMap.put('a', 1);
// safe to modify while another thread iterates, unlike HashMap

Type	Iteration के दौरान modification	उदाहरण
Fail-fast	तुरंत ConcurrentModificationException	ArrayList, HashMap iterators
Fail-safe	Copy पर काम, exception नहीं	CopyOnWriteArrayList, ConcurrentHashMap

List<String> list = new ArrayList<>(Arrays.asList('A', 'B', 'C'));
for (String item : list) {
    if (item.equals('B')) {
        list.remove(item);  // ConcurrentModificationException!
    }
}

// सुरक्षित alternative
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    String item = it.next();
    if (item.equals('B')) {
        it.remove();  // सुरक्षित
    }
}

// Fail-safe उदाहरण
List<String> safeList = new CopyOnWriteArrayList<>(Arrays.asList('A', 'B', 'C'));
for (String item : safeList) {
    if (item.equals('B')) {
        safeList.remove(item);  // exception नहीं
    }
}
System.out.println(safeList);  // [A, C]

Map<String, Integer> concurrentMap = new ConcurrentHashMap<>();
concurrentMap.put('a', 1);

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