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Showing content from https://www.geeksforgeeks.org/java/concurrentmodificationexception-in-java-with-examples/ below:

ConcurrentModificationException in Java with Examples

Last Updated : 12 Jul, 2025

ConcurrentModificationException in Multi threaded environment

multi threaded environment

, if during the detection of the resource, any method finds that there is a concurrent modification of that object which is not permissible, then this ConcurrentModificationException might be thrown.

If this exception is detected, then the results of the iteration are undefined.
Generally, some iterator implementations choose to throw this exception as soon as it is encountered, called fail-fast iterators.

For example:

If we are trying to modify any collection in the code using a thread, but some another thread is already using that collection, then this will not be allowed.

ConcurrentModificationException in Single threaded environment

Since, there is no guarantee that whenever this exception is raised, an object is been under concurrent modification by some different thread, this exception is also thrown in the

single-threaded environment

. If we invoke a sequence of methods on an object that violates its contract, then the object throws ConcurrentModificationException.

For example:

if while iterating over the collection, we directly try to modify that collection, then the given

fail-fast iterator

will throw this ConcurrentModificationException.

Example:

In the following code, an ArrayList is implemented. Then few values are added to it and few modifications are made over it while traversing,

Java


 // Java program to show
// ConcurrentModificationException

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

public class modificationError {
    public static void main(String args[])
    {

        // Creating an object of ArrayList Object
        ArrayList<String> arr
            = new ArrayList<String>();

        arr.add("One");
        arr.add("Two");
        arr.add("Three");
        arr.add("Four");

        try {
            // Printing the elements
            System.out.println(
                "ArrayList: ");
            Iterator<String> iter
                = arr.iterator();

            while (iter.hasNext()) {

                System.out.print(iter.next()
                                 + ", ");

                // ConcurrentModificationException
                // is raised here as an element
                // is added during the iteration
                System.out.println(
                    "\n\nTrying to add"
                    + " an element in "
                    + "between iteration\n");
                arr.add("Five");
            }
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

Output:

ArrayList: 
One, 

Trying to add an element in between iteration

java.util.ConcurrentModificationException

How to avoid ConcurrentModificationException?

To avoid this exception,

Simply we can do the modifications once the iteration is done, or
Implement the concept of the synchronized block or method

Example:

Let’s see how to resolve this exception by simply changing the place of modification.

Java


 // Java program to show
// ConcurrentModificationException

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

public class modificationError {
    public static void main(String args[])
    {

        // Creating an object of ArrayList Object
        ArrayList<String> arr
            = new ArrayList<String>();

        arr.add("One");
        arr.add("Two");
        arr.add("Three");
        arr.add("Four");

        try {
            // Printing the elements
            System.out.println(
                "ArrayList: ");
            Iterator<String> iter
                = arr.iterator();

            while (iter.hasNext()) {
                System.out.print(iter.next()
                                 + ", ");
            }

            // No exception is raised as
            // a modification is done
            // after the iteration
            System.out.println(
                "\n\nTrying to add"
                + " an element in "
                + "between iteration: "
                + arr.add("Five"));

            // Printing the elements
            System.out.println(
                "\nUpdated ArrayList: ");
            iter = arr.iterator();

            while (iter.hasNext()) {
                System.out.print(iter.next()
                                 + ", ");
            }
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

Output:

ArrayList: 
One, Two, Three, Four, 

Trying to add an element in between iteration: true

Updated ArrayList: 
One, Two, Three, Four, Five,

From the output, it can be seen clearly that with minimal changes in the code, ConcurrentModificationException can be eliminated.

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