Why is iterator next throws a ConcurrentModificationException?

It’s because you’ve modified the backing list between getting the Iterator via iterator() and calling next() . Make sure to to perform additions to the Collection outside of the loop. You are defining the Iterator upon instantiation of your object, IteratorTest . You then add some data to the ArrayList , arr .

How iterator knows about ConcurrentModificationException?

Iterator fail-fast property checks for any modification in the structure of the underlying collection everytime we try to get the next element. If there are any modifications found, it throws ConcurrentModificationException.

How do I get around ConcurrentModificationException?

To Avoid ConcurrentModificationException in multi-threaded environment. You can convert the list to an array and then iterate on the array. This approach works well for small or medium size list but if the list is large then it will affect the performance a lot.

How do I fix ConcurrentModificationException in Java?

How do you fix Java’s ConcurrentModificationException? There are two basic approaches: Do not make any changes to a collection while an Iterator loops through it. If you can’t stop the underlying collection from being modified during iteration, create a clone of the target data structure and iterate through the clone.

What causes Java Util ConcurrentModificationException?

util. concurrentmodificationexception is an error in Java. The error occurs when the iterator is traversing a list, and a command is used to change an element’s value during that traversal.

How can you avoid ConcurrentModificationException while iterating a collection?

How to avoid ConcurrentModificationException in a multi-threaded environment?

We can iterate over the array instead of iterating over the collection class.
Locking the list by putting it in the synchronized block is another way to avoid the concurrent modification exception.

How does ConcurrentHashMap Iterator work?

Despite javadoc, ConcurrentHashMap Iterator does NOT return a snapshot, it operates on live concurrent hash table handling all concurrency cases optimistically, in the same way all lock-free data structures do. Basically, it contains reference to current hashtable, bin index in that hashtable and last returned Node.

What is Java ConcurrentModificationException?

The ConcurrentModificationException occurs when an object is tried to be modified concurrently when it is not permissible. This exception usually comes when one is working with Java Collection classes. For Example – It is not permissible for a thread to modify a Collection when some other thread is iterating over it.