How to avoid ConcurrentModificationException while iterating a collection?

Answer 1

Java Collection classes are fail-fast which means that if the Collection will be changed while some thread is traversing over it using iterator, the iterator.next() will throw ConcurrentModificationException. This situation can come in case of multithreaded as well as single threaded environment.

Lets see the scenario with an example:

import java.util.*;

public class IteratorExample {

    public static void main(String args[]){
        List<String> myList = new ArrayList<String>();

        myList.add("1");
        myList.add("2");
        myList.add("3");
        myList.add("4");
        myList.add("5");

        Iterator<String> it = myList.iterator();
        while(it.hasNext()){
            String value = it.next();
            System.out.println("List Value:"+value);
            if(value.equals("3")) myList.remove(value);
        }

        Map<String,String> myMap = new HashMap<String,String>();
        myMap.put("1", "1");
        myMap.put("2", "2");
        myMap.put("3", "3");

        Iterator<String> it1 = myMap.keySet().iterator();
        while(it1.hasNext()){
            String key = it1.next();
            System.out.println("Map Value:"+myMap.get(key));
            if(key.equals("2")){
                myMap.put("1","4");
                //myMap.put("4", "4");
            }
        }

    }
}

Output is:

List Value:1
List Value:2
List Value:3
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at com.journaldev.java.IteratorExample.main(IteratorExample.java:27)
From the output stack trace, its clear that the exception is coming when we call iterator next() function. If you are wondering how Iterator checks for the modification, its implementation is present in AbstractList class where an int variable modCount is defined that provides the number of times list size has been changed. This value is used in every next() call to check for any modifications in a function checkForComodification().

Now comment the list part and run the program again.

Output will be:

Map Value:3
Map Value:2
Map Value:4

Since we are updating the existing key value in the myMap, its size has not been changed and we are not getting ConcurrentModificationException. Note that the output may differ in your system because HashMap keyset is not ordered like list. If you will uncomment the statement where I am adding a new key-value in the HashMap, it will cause ConcurrentModificationException.