What is fail-fast property?

Fail-fast is a property of software with the help of which it can stop the normal execution of the program if a failure situation occurs. A fail-fast system is designed to immediately report any failure or condition that is likely to lead to failure. Fail fast property makes our software more robust. We can easily find the bugs and fix them.

In Java, iterator is considered as fail fast. If an iterator has been created on a collection of objects and some other thread in the program tries to modify the same collection, a concurrent modification exception will be thrown. If after the creation of the iterator on a collection, the collection is modified by any method other than the iterators remove or add method, it will always throw the concurrent modification exception. Suppose we have created an iterator over Array List in java and while traversing the list in while loop we try to remove element with iterator.remove() method, it will give the “java.lang.IllegalStateException” exception. We can use the remove method of ListIterator in the while loop.

To illustrate, Java iterator is fail fast see the below example

 import java.util.*;

public class demoIterator

{

    public static void main(String s[])

    {

        ArrayList al  = new ArrayList();

        //Adding Elements to the Arraylist

        al.add("fffff");

        al.add("bbbbb");

        al.add("ddddd");

        al.add("ccccc");

        al.add("aaaaa");

        //Printing all the element of ArrayList

        System.out.println("All Elements in the ArrayList"+al);

        //Traverse the elements in ArrayList one by one with the help of Iterator

        System.out.println("Start traversing elements of list one by one");

        Iterator it = al.iterator();

        while(it.hasNext())

        {

            //In Iterator, List can be only be traversed in forward with Iterator.next();

            it.remove();

            System.out.println(it.next());

        }

    }

}

Output of the program:

Exception in thread "main" java.lang.IllegalStateException

All Elements in the ArrayList[fffff, bbbbb, ddddd, ccccc, aaaaa]

Start traversing elements of list one by one

        at java.util.AbstractList$Itr.remove(AbstractList.java:356)

        at demoIterator.main(demoIterator.java:22)