Why concurrenthashmap doesnt throw concurrentmodificationexception

Iterator; import java. ConcurrentModificationException at java. They use original collection to traverse over the elements of the collection. Note 1 from java-docs : The fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis.

Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs. Note 2 : If you remove an element via Iterator remove method, exception will not be thrown.

However, in case of removing via a particular collection remove method, ConcurrentModificationException will be thrown. ArrayList; import java. These iterators make a copy of the internal collection object array and iterates over the copied collection. Any structural modification done to the iterator affects the copied collection, not original collection. So, original collection remains structurally unchanged. CopyOnWriteArrayList; import java.

For example, in case of ConcurrentHashMap, it does not operate on a separate copy although it is not fail-fast. Instead, it has semantics that is described by the official specification as weakly consistent memory consistency properties in Java. ConcurrentHashMap; import java. HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously.

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. The NoSuchElementException in Java is thrown when one tries to access an iterable beyond its maximum limit. 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.

It contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes a string tokenizer, a random-number generator, and a bit array.

Following are the Important Classes in Java. No iterator is thread-safe. If the underlying collection is changed amidst iteration, a ConcurrentModificationException is thrown. Even iterators of synchronized collections are not thread-safe — you have to synchronize manually. 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 is also known as the universal cursor of Java as it is appropriate for all the classes of the Collection framework.

ConcurrentHashMap does not throw ConcurrentModificationException if the underlying collection is modified during an iteration is in progress.

Iterators may not reflect the exact state of the collection if it is being modified concurrently. An element can be removed from a Collection using the Iterator method remove. This method removes the current element in the Collection. If the remove method is not preceded by the next method, then the exception IllegalStateException is thrown. A program that demonstrates this is given as follows. 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. Now this is new, concurrencyLevel. By default ConcurrentHashMap allows 16 number of concurrent threads. We can change this number using the concurrencyLevl argument. Most of the times 16 should be sufficient and playing with these number may cause undesirable performance issues. Know before you set these arguments.

ConcurrentHashMap does not throw ConcurrentModificationException if the underlying collection is modified during an iteration is in progress. Iterators may not reflect the exact state of the collection if it is being modified concurrently.

It may reflect the state when it was created and at some moment later. The fail-safe property is given a guarantee based on this. ConcurrentHashMap can be considered as an alternative to Hashtable.