ConcurrentModificationException is a predefined Exception in Java, which occurs while we are using Java Collections, i.e whenever we try to modify an object concurrently without permission ConcurrentModificationException occurs which is present in java.util package.
Procedure: Some steps are required in order to avoid the occurrence of this exception in a single-threaded environment. They are as follows:
- Instead of iterating through collections classes, we can iterate through the arrays. This works perfectly fine for smaller lists what about the bigger list? It’s very basic we know that if the array size is huge then it affects the performance. Hence, this method is only effective for smaller size arrays.
- The next method is the Synchronized block method, Here we actually lock the list in a synchronized block to avoid the exception. Isn’t that cool? but guess what this is also not an effective method to avoid Exception Why? Because the purpose of multithreading is not being used.
- The better way is we have ConcurrentHashMap and CopyOnWriteArrayList Which is the best among the above Methods.
Methods:
Here two ways are proposed of which starting with the naive one and ending up with the optimal approach to reach the goal.
- Using Loops: We used the Iterator remove() method instead of that we can use a for loop to avoid ConcurrentModificationException in a Single-threaded environment. If we add any extra objects then we can ensure that our code takes care of them.
- Using the remove() Method: We can use the remove method to remove the object from the collection. Here there is a problem that is you can only remove the same object and not any other from the list
Example 1:
Java
// Java Program to Avoid// AvoidConcurrentModificationException// Importing Map and List utility classes// from java.util packageimport java.util.List;import java.util.Map;// Importing classes from java.util.concurrent packageimport java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.CopyOnWriteArrayList;// Main classpublic class GFG { // Main driver method public static void main(String[] args) { // Creating an object of List class // Declaring object of string type List<String> marvel = new CopyOnWriteArrayList<String>(); // Adding elements to the above object created // Custom input entries marvel.add("IronMan"); marvel.add("BlackWidow"); marvel.add("Hulk"); marvel.add("DoctorStrange"); marvel.add("SpiderMan"); // Iterating over object created using size() method for (int i = 0; i < marvel.size(); i++) { // Print and display the object elements // using get() method System.out.println("Avenger : " + marvel.get(i)); // Condition check over object elements // If specific element is matched if (marvel.get(i).equals("BlackWidow")) { marvel.remove(i); i--; // Add this specific element marvel.add("CaptianAmerica"); } } // Now getting the final total size by checking // how many elements are there inside object System.out.println("Total Avengers : " + marvel.size()); }} |
Output
Avenger : IronMan Avenger : BlackWidow Avenger : Hulk Avenger : DoctorStrange Avenger : SpiderMan Avenger : CaptianAmerica Total Avengers :5
Note: The Exception can also occur if we try to modify the structure of original list with sublist. An example for the same is below,
Example 2:
Java
// Java Program to Illustrate ConcurrentModificationException// WithArrayListSubList// Importing List and Arraylist classes utility classes// from java.util packageimport java.util.ArrayList;import java.util.List;// Main classpublic class GFG { // Main driver method public static void main(String[] args) { // Creating a List class object // Declaring object of string type List <String> names = new ArrayList <>(); // Adding elements to the object of List class // Custom input entries names.add("Java"); names.add("C++"); names.add("Phython"); names.add("JavaScript"); List < String > first2Names = names.subList(0, 2); System.out.println(names + " , " + first2Names); names.set(1, "Ruby"); // check the output below. 🙂 System.out.println(names + " , " + first2Names); // Now we add another string to // get ConcurrentModificationException names.add("SQL"); // This line throws an exception System.out.println(names + " , " + first2Names); }} |
Output:
Henceforth, we have discussed how to avoid the Exception in both single-threaded and multithreaded environments successfully as depicted from the above outputs.
