Why concurrent modification exception occurs in java?

Last Update: May 30, 2022

This is a question our experts keep getting from time to time. Now, we have got the complete detailed explanation and answer for everyone, who is interested!

Asked by: Prof. Curtis Ebert
Score: 4.2/5 (50 votes)

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.

How do I fix concurrent modification exception?

We can also avoid the Concurrent Modification Exception in a single threaded environment. We can use the remove() method of Iterator to remove the object from the underlying collection object. But in this case, you can remove only the same object and not any other object from the list.

How do you prevent concurrent modification exception on a map?

use ConcurrentHashMap. keep on using simple HashMap, but build a new map on each modification and switch maps behind the scenes (synchronizing the switch operation or using AtomicReference)

Which method of iterator throws concurrent modification exception?

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.

What is the concurrent modification exception in Java stack overflow?

If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception. For example, if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception.

How to fix ConcurrentModificationException In Java | Remove an Object from ArrayList while iterating

29 related questions found

What is iterator in Java?

Iterator in Java. In Java, an Iterator is one of the Java cursors. Java Iterator is an interface that is practiced in order to iterate over a collection of Java object components entirety one by one. ... The Java Iterator also helps in the operations like READ and REMOVE.

What is CopyOnWriteArrayList in Java?

CopyOnWriteArrayList is a thread-safe variant of ArrayList where operations which can change the ArrayList (add, update, set methods) creates a clone of the underlying array. CopyOnWriteArrayList is to be used in a Thread based environment where read operations are very frequent and update operations are rare.

What is the difference between HashMap and ConcurrentHashMap?

HashMap is non-Synchronized in nature i.e. HashMap is not Thread-safe whereas ConcurrentHashMap is Thread-safe in nature. HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously.

Which is faster and uses less memory?

Sqldatareader is fast as compare to Dataset. Because it stored data in forward only and also stores only one record at a time. And dataset stores all records at the same time. This is the reason, SqlDataReader is faster than Dataset.

How does ArrayList increase in size?

The ArrayList size increases dynamically because whenever the ArrayList class requires to resize then it will create a new array of bigger size and copies all the elements from the old array to the new array. And now it is using the new array's reference for its internal usage.

How does ConcurrentHashMap works in Java?

ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications. At a time any number of threads are applicable for a read operation without locking the ConcurrentHashMap object which is not there in HashMap. ... The default concurrency-level of ConcurrentHashMap is 16.

What is exception explain its keyword with example?

An exception is an unwanted or unexpected event, which occurs during the execution of a program i.e at run time, that disrupts the normal flow of the program's instructions. Error vs Exception. Error: An Error indicates serious problem that a reasonable application should not try to catch.

How can we avoid ConcurrentModificationException in a multi threaded environment?

To Avoid ConcurrentModificationException in multi-threaded environment
  1. You can convert the list to an array and then iterate on the array. ...
  2. You can lock the list while iterating by putting it in a synchronized block. ...
  3. If you are using JDK1.

Can we modify list while iterating?

At the end the whole list will have the letter "D" as its content. It's not a good idea to use an enhanced for loop in this case, you're not using the iteration variable for anything, and besides you can't modify the list's contents using the iteration variable.

What is fail safe and fail fast in Java?

Fail-Safe. 1. Exception. Any changes in the collection, such as adding, removing and updating collection during a thread are iterating collection then Fail fast throw concurrent modification exception. The fail-safe collection doesn't throw exception.

Is ArrayList thread safe?

ArrayList , on the other hand, is unsynchronized, making them, therefore, not thread safe. With that difference in mind, using synchronization will incur a performance hit. So if you don't need a thread-safe collection, use the ArrayList .

Which browser uses least RAM?

For this reason, Opera lands the first place as the browser that uses the least amount of PC memory while UR takes the second place. Just a few MB less of system resources used can have a big impact.

Does Chrome now use less RAM?

Google says it's seeing up to a 65% improvement on the Apple Energy Impact score in the macOS Activity Monitor for background tabs. The latest version of Chrome for Android also brings a 5% improvement in memory usage, 7.5% faster startup times, and up to 2% faster page loads.

Does Chrome actually use more RAM?

How Does Google Chrome Manage RAM? Browsers like Chrome manage RAM this way to offer better stability and faster speeds. But Chrome still uses a lot of RAM. ... If every tab and extension was run in the same process, you might have to restart the whole browser instead of a single tab.

Can we convert HashMap to ConcurrentHashMap?

Synchronize HashMap – Collections.

Synchronized HashMap also works very similar to ConcurrentHashMap, with few differences. ... Iterators from both classes should be used inside synchronized block but the iterator from SynchronizedHashMap is fail-fast. ConcurrentHashMap iterators are not fail-fast.

What is the advantage of ConcurrentHashMap?

The advantages of using ConcurrentHashMap are as follows: It provides very high concurrency in a multi-threaded environment. The read operation can be very fast when the write operation is done with a lock. It provides No object-level Locking.

Which is faster HashMap or ConcurrentHashMap?

If you choose a single thread access use HashMap , it is simply faster. For add method it is even as much as 3x more efficient. Only get is faster on ConcurrentHashMap , but not much. When operating on ConcurrentHashMap with many threads it is similarly effective to operating on separate HashMaps for each thread.

What is WeakHashMap in Java?

WeakHashMap is an implementation of the Map interface that stores only weak references to its keys. Storing only weak references allows a key-value pair to be garbage-collected when its key is no longer referenced outside of the WeakHashMap. ... Usually, it will be freed the next time the garbage collector runs.

What is difference between Hashtable and ConcurrentHashMap in Java?

Hashtable uses single lock for whole data. ConcurrentHashMap uses multiple locks on segment level (16 by default) instead of object level i.e. whole Map . ConcurrentHashMap locking is applied only for updates. ... So reads can happen very fast while writes are done with a lock.