What is fail fast and fail-safe iterator in java?

In this blog post we will discuss about one of the very important java interview question What is fail fast and fail-safe iterator in java?

Fail-Fast Iterator

Iterator fails as soon as it realizes that the structure of the underlying data structure has been modified since the iteration has begun.
Structural changes mean adding, removing any element from the collection, merely updating some value in the data structure does not count for the structural modifications.
Fail-Fast Iterator is implemented by keeping a modification count and if iterating thread realizes the changes in modification count then it throws ConcurrentModificationException. Most collections in package util.* are fail-fast by Design.

Fail-Safe Iterator

Fail-safe Iterator is “Weakly Consistent” and does not throw any exception if collection is modified structurally during the iteration.
Such Iterator -may work on clone of collection instead of original collection such as in CopyOnWriteArrayList
Most collections under util.concurrent offer fail-safe Iterators to its users and that’s by Design.
Fail safe collections should be preferred while writing multi-threaded applications to avoid concurrency related issues.
Fail Safe Iterator is guaranteed to list elements as they existed upon construction of Iterator, and may reflect any modifications subsequent to construction (without guarantee).

Let’s understand above points by creating couple of examples:

Example which will throw ConcurrentModificationException known as Fail-Fast Iterators

package com.kkjavatutorials.client;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class CopyOnWriteArrayListTest_1 {

	public static void main(String[] args) {
		List<String> list = new ArrayList<>();
		list.add("A");
		list.add("B");
		list.add("C");
		list.add("A");
		list.add("B");

		Iterator<String> itr = list.iterator();
		while (itr.hasNext()) {
			String element = itr.next();
			if(element.equals("A")){
				list.add("AA");//This will throw ConcurrentModificationException
			}
		}
		
		System.out.println(list);
	}
}

package com.kkjavatutorials.client;

import java.util.ArrayList;

import java.util.Iterator;

import java.util.List;

public class CopyOnWriteArrayListTest_1 {

public static void main(String[] args) {

List<String> list = new ArrayList<>();

list.add("A");

list.add("B");

list.add("C");

list.add("A");

list.add("B");

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

while (itr.hasNext()) {

String element = itr.next();

if(element.equals("A")){

list.add("AA");//This will throw ConcurrentModificationException

}

System.out.println(list);

}

Above program will throw Exception as below:

Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
	at java.util.ArrayList$Itr.next(ArrayList.java:851)
	at com.kkjavatutorials.client.CopyOnWriteArrayListTest_1.main(CopyOnWriteArrayListTest_1.java:19)

Exception in thread "main" java.util.ConcurrentModificationException

at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)

at java.util.ArrayList$Itr.next(ArrayList.java:851)

at com.kkjavatutorials.client.CopyOnWriteArrayListTest_1.main(CopyOnWriteArrayListTest_1.java:19)

Example which will not throw ConcurrentModificationException known as Fail-Safe Iterators

package com.kkjavatutorials.client;

import java.util.Iterator;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;

public class CopyOnWriteArrayListTest_2 {

	public static void main(String[] args) {
		List<String> list = new CopyOnWriteArrayList<>();
		list.add("A");
		list.add("B");
		list.add("C");
		list.add("A");
		list.add("B");

		Iterator<String> itr = list.iterator();
		while (itr.hasNext()) {
			String element = itr.next();
			if(element.equals("A")){
				list.add("AA");//This will not throw ConcurrentModificationException
			}
		}
		
		System.out.println(list);
	}
}

package com.kkjavatutorials.client;

import java.util.Iterator;

import java.util.List;

import java.util.concurrent.CopyOnWriteArrayList;

public class CopyOnWriteArrayListTest_2 {

public static void main(String[] args) {

List<String> list = new CopyOnWriteArrayList<>();

list.add("A");

list.add("B");

list.add("C");

list.add("A");

list.add("B");

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

while (itr.hasNext()) {

String element = itr.next();

if(element.equals("A")){

list.add("AA");//This will not throw ConcurrentModificationException

}

System.out.println(list);

}

Above program will produce below output without any Exception:

[A, B, C, A, B, AA, AA]

1	[A, B, C, A, B, AA, AA]

You may also like:

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What all collections utilizes hashCode() method in java?
Describe CopyOnWriteArrayList? Where is it used in Java Applications?

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What is fail fast and fail-safe iterator in java?

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