Difference between Fail Fast and Fail Safe iterator in Java

Fail-fast iterators in Java throw ConcurrentModificationException exception if the collection is modified while iterating over it. Fail-safe iterators do not throw any exception even if the collection is modified while iterating over it.

Default iterators for Collections from java.util package such as ArrayList, HashMap, etc. are Fail-Fast.

Sample Java code to demonstrate Fail-fast iterators

public class FailFastExample {
    static public void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<Integer>();
        numbers.add(1);
        numbers.add(2);
 
        Iterator<Integer> iterator = numbers.iterator();
        while (iterator.hasNext()) {
            Integer number = iterator.next();
            numbers.add(50);
        }
    }
}
 
Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
	at java.util.ArrayList$Itr.next(ArrayList.java:859)
	at FailFastExample.main(FailFastExample.java:12)
 

Fail Safe Iterator makes copy of the internal data structure (object array) and iterates over the copied data structure. Any structural modification done to the iterator affects the copied data structure. So original data structure remains structurally unchanged. Hence no ConcurrentModificationException throws by the fail safe iterator.

Iterators on Collections from java.util.concurrent package such as ConcurrentHashMap, CopyOnWriteArrayList, etc. are Fail-Safe in nature.

Sample Java code to demonstrate Fail-safe iterators

public class FailSafeExample {
    static public void main(String[] args) {
        ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
        map.put("Dhoni", 7);
        map.put("Virat", 18);
        map.put("Chris", 333);
        map.put("Sachin", 10);
        Iterator<String> iterator = map.keySet().iterator();
        while (iterator.hasNext()) {
            String key = iterator.next();
            map.put("Bhuvneshvar", 1);
        }
        iterator = map.keySet().iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}

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