What is fail-fast, fail-safe and ConcurrentModificationException in java?

 Iterator returned by few Collection framework Classes are fail-fast,  means any structural  modification made to these classes during iteration will throw ConcurrentModificationException in java.

Iterator returned by few Collection framework Classes are fail-safe, means any structural modification made to these classes during iteration won’t throw any Exception in java.

package javaresearch;

import java.util.List;

import java.util.ArrayList;

import java.util.Collections;

import java.util.Enumeration;

import java.util.Iterator;

import java.util.ListIterator;

public class ArrayListExample {

           

              public static void main(String args[]) {

                         

          // creates array with initial capacity of 10.

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

        arrayList.add("Govind");

        arrayList.add("Ballabh");

          // fail-fast

     Iterator<String> iterator = arrayList.iterator();

          while (iterator.hasNext()) {

                 arrayList.add("Khan"); // unComment to avoid ConcurrentModificationException

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

          }

          // fail-fast

          ListIterator<String> listIterator = arrayList.listIterator();

          listIterator.next();

          listIterator.previous();

          while (listIterator.hasNext()) {

                 arrayList.add("Java"); // unComment to avoid ConcurrentModificationException

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

          }

          // fail-fast

          Enumeration<String> listEnum = Collections.enumeration(arrayList);

          while (listEnum.hasMoreElements()) {

                 arrayList.add("Technology"); // unComment to avoid ConcurrentModificationException

                 System.out.println(listEnum.nextElement());

          }

          //enhanced for loop is fail-fast

          for (String string : arrayList) {

                 arrayList.add("Madhubani-Bihar"); // unComment to avoid ConcurrentModificationException

                 System.out.println(string);

          }

   }

}

package javaresearch;

import java.util.List;

import java.util.Collections;

import java.util.Enumeration;

import java.util.Iterator;

import java.util.ListIterator;

import java.util.concurrent.CopyOnWriteArrayList;

public class CopyOnWriteArrayListExample {

           

            public static void main(String args[]) {

                         

        // creates array with initial capacity of 10.

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

        copyOnWriteArrayList.add("Govind");

        copyOnWriteArrayList.add("Ballabh");

 

        // fail-safe

        Iterator<String> iterator = copyOnWriteArrayList.iterator();

        while (iterator.hasNext()) {

              copyOnWriteArrayList.add("Khan");

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

        }

       

        System.out.println("-------------------------------------------------");

        for (String string : copyOnWriteArrayList) {

               System.out.println(string);

        }

       

        System.out.println("------------------------------------------");

        // fail-safe

        ListIterator<String> listIterator = copyOnWriteArrayList.listIterator();

       // listIterator.next();

        //listIterator.previous();

        while (listIterator.hasNext()) {

                    copyOnWriteArrayList.add("Java");

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

        }

        System.out.println("------------------------------------------");

        // fail-safe

        Enumeration<String> listEnum = Collections

                     .enumeration(copyOnWriteArrayList);

        while (listEnum.hasMoreElements()) {

                   copyOnWriteArrayList.add("Technology");

               System.out.println(listEnum.nextElement());

        }

        System.out.println("------------------------------------------");

        //enhanced for loop is fail-safe

        for (String string : copyOnWriteArrayList) {

               copyOnWriteArrayList.add("Madhubani-Bihar");

               System.out.println(string);

        }

 }

}

o/p view:-----------------------

Govind

Ballabh

-------------------------------------------------

Govind

Ballabh

Khan

Khan

------------------------------------------

Govind

Ballabh

Khan

Khan

------------------------------------------

Govind

Ballabh

Khan

Khan

Java

Java

Java

Java

------------------------------------------

Govind

Ballabh

Khan

Khan

Java

Java

Java

Java

Technology

Technology

Technology

Technology

Technology

Technology

Technology

Technology