ConcurrentModificationException while using Iterator in Java

Last Updated : 21 Apr, 2021

ConcurrentModificationException has thrown by methods that have detected concurrent modification of an object when such modification is not permissible. If a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this ConcurrentModificationException. Here we will be understanding this exception with an example of why it occurs and how changes are made simultaneously which is the root cause for this exception. In the later part, we will understand how to fix it up.

Example 1: ConcurrentModificationException

Java

// Java Program to ConcurrentModificationException while 
// using Iterator 
  
// Importing ArrayList and Iterator classes from java.util 
// package 
import java.util.ArrayList; 
import java.util.Iterator; 
  
// Main class 
public class GFG { 
  
    // Main driver method 
    public static void main(String[] args) 
    { 
        // Creating an object of ArrayList class 
        // Declaring object of Integer type 
        ArrayList<Integer> list = new ArrayList<>(); 
  
        // Adding custom integer elements to the object 
        list.add(1); 
        list.add(2); 
        list.add(3); 
        list.add(4); 
        list.add(5); 
  
        // Iterating over object elements using iterator 
        Iterator<Integer> iterator = list.iterator(); 
  
        // Condition check 
        // It holds true till there is single element 
        // remainin in the List 
        while (iterator.hasNext()) { 
  
            // Rolling over to next element using next() 
            // method 
            Integer value = iterator.next(); 
  
            // Print the element value 
            System.out.println("value: " + value); 
  
            // If element equals certain value 
            if (value.equals(2)) { 
  
                // Display command for better readability 
                System.out.println( 
                    "========================"); 
  
                // Removing entered value in object 
                System.out.println("removing value: "
                                   + value); 
  
                // Making changes simultaneously 
                System.out.println( 
                    "========================"); 
                list.remove(value); 
            } 
        } 
    } 
}

Output:

Output Explanation:

ConcurrentModificationException is thrown when the next() method is called as the iterator is iterating the List, and we are making modifications in it simultaneously. Now in order to avoid this exception so let us do discuss a way out of using iterator directly which is as follows:

Example 2: Resolving ConcurrentModificationException

Java

// Java Program to Avoid ConcurrentModificationException by 
// directly using Iterator 
  
// Importing ArrayList and Iterator classes 
// from java.util package 
import java.util.ArrayList; 
import java.util.Iterator; 
  
// Main class 
public class Main { 
  
    // Mai driver method 
    public static void main(String[] args) 
    { 
        // Creating an ArrayList object of integer type 
        ArrayList<Integer> list = new ArrayList<>(); 
  
        // Custom integer elements are added 
        list.add(1); 
        list.add(2); 
        list.add(3); 
        list.add(4); 
        list.add(5); 
  
        // Iterating directly over elements of object 
        Iterator<Integer> iterator = list.iterator(); 
  
        // Condition check 
        // It holds true till there is single element 
        // remaining in the List using hasNext() method 
        while (iterator.hasNext()) { 
  
            // Rolling over elements using next() method 
            Integer value = iterator.next(); 
  
            // print the values 
            System.out.println("value: " + value); 
  
            // If value equals certain integer element 
            // entered Say it be 2 
            if (value.equals(2)) { 
  
                // Display command only 
                System.out.println( 
                    "========================"); 
  
                // Removing the entered value  
                System.out.println("removing value: "
                                   + value); 
                 
                // Display command only 
                System.out.println( 
                    "========================"); 
                
                // Removing current value in Collection 
                // using remove() method  
                iterator.remove(); 
            } 
        } 
    } 
}