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java.util.ConcurrentModificationException is a commonly encountered runtime exception in Java, especially when dealing with collections. It occurs when an object is modified concurrently while being iterated by a thread. In this blog post, we will discuss the causes of the ConcurrentModificationException, how to prevent it, and some best practices to avoid it in your Java applications.

Causes of java.util.ConcurrentModificationException:

The most common cause of this exception is modifying a collection while iterating over it. Here are a few examples that could trigger this exception:

1. Removing an element while iterating a collection: When a collection is iterated over, if an element is removed by another thread or the same thread outside the iterator, it would cause a ConcurrentModificationException.
2. Adding an element while iterating a collection: If an element is added to a collection during iteration, it would also cause a ConcurrentModificationException.
3. Updating an element while iterating a collection: When the contents of a collection are updated during iteration, it can lead to a ConcurrentModificationException.

Preventing java.util.ConcurrentModificationException:

Here are some best practices to prevent java.util.ConcurrentModificationException:

1. Use synchronized collection: The easiest way to avoid this exception is to use a synchronized collection. Collections.synchronizedList(), Collections.synchronizedSet(), and Collections.synchronizedMap() are some of the synchronized collections available in Java.
2. Use an Iterator: Instead of using a for-each loop to iterate over a collection, it is recommended to use an iterator. An iterator has a remove() method that can be used to remove elements from the collection while iterating.
3. Use a concurrent collection: Concurrent collections are specifically designed for concurrent access. They can be used to prevent this exception. ConcurrentHashMap, ConcurrentLinkedQueue, and ConcurrentSkipListSet are some of the concurrent collections available in Java.
4. Use CopyOnWriteArrayList: CopyOnWriteArrayList is another option that can be used to prevent this exception. It is a thread-safe variant of ArrayList, which creates a new copy of the underlying array whenever a modification is made.

Best practices to avoid java.util.ConcurrentModificationException:

Here are some best practices to avoid java.util.ConcurrentModificationException:

1. Avoid modifying the collection while iterating: As mentioned earlier, modifying a collection while iterating is the most common cause of this exception. Therefore, it is best to avoid modifying the collection during iteration.
2. Use synchronized blocks: Synchronized blocks can be used to prevent multiple threads from modifying the same collection at the same time. Synchronized blocks should be used when accessing synchronized collections.
3. Use immutable collections: Immutable collections can be used to avoid this exception entirely. Since they cannot be modified, it is safe to iterate over them without worrying about concurrent modification.
4. Use the right collection for the job: Choosing the right collection for the job is important. Concurrent collections should be used when multiple threads need to access the same collection concurrently. Otherwise, a synchronized or immutable collection should be used.

Conclusion:

In conclusion, java.util.ConcurrentModificationException is a common exception that occurs when a collection is modified concurrently while being iterated over. To prevent this exception, it is recommended to use synchronized collections, iterators, concurrent collections, or CopyOnWriteArrayList. Best practices to avoid this exception include avoiding modifying the collection while iterating, using synchronized blocks, using immutable collections, and using the right collection for the job. By following these best practices, you can ensure that your Java application is free from this exception.