Why ConcurrentHashMap is fail safe?

This is because, they operate on the clone of the collection, not on the original collection and that's why they are called fail-safe iterators. Iterator on CopyOnWriteArrayList, ConcurrentHashMap classes are examples of fail-safe Iterator.

Is ConcurrentHashMap fail-fast?

concurrent package such as ConcurrentHashMap, CopyOnWriteArrayList, etc. are Fail-Safe in nature. In the code snippet above, we're using Fail-Safe Iterator. Hence, even though a new element is added to the Collection during the iteration, it doesn't throw an exception.

What is the difference between Failfast and failsafe?

The Major difference between Fail Fast and Fail Safe iterator is that the Fail Safe does not throw any ConcurrentModificationException in modifying the object during the iteration process, contrary to fail fast, which throws an exception in such scenarios.

Is ConcurrentHashMap thread-safe?

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.

What is failsafe and fail-fast with example?

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. 2. Type of collection. ArrayList and hashmap collection are the examples of fail-fast ...

What are fail fast and fail safe Iterator ? || Java Collection Interview Question

How does fail-safe work?

Fail-safe iterators allow modifications of a collection while iterating over it. These iterators don't throw any Exception if a collection is modified while iterating over it. They use copy of original collection to traverse over the elements of the collection.

What is difference between ConcurrentHashMap and HashMap?

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.

How does ConcurrentHashMap achieve thread-safety?

ConcurrentHashMap class achieves thread-safety by dividing the map into segments, the lock is required not for the entire object but for one segment, i.e one thread requires a lock of one segment. In ConcurrenHashap the read operation doesn't require any lock.

Is ConcurrentHashMap remove thread-safe?

A ConcurrentHashMap is a thread-safe version of a HashMap that allows concurrent read and thread-safe update operation.

Is ConcurrentHashMap slower than HashMap?

Only modifying operations on ConcurrentHashMap are synchronized. Hence, add or remove operations on ConcurrentHashMap are slower than on HashMap . The read operations on both, ConcurrentHashMap and HashMap , give same performance as read operations on both maps are not synchronized.

Why is there no concurrent modification exception in ConcurrentHashMap?

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.

What is difference between Hashtable and ConcurrentHashMap in Java?

Hashtable is belongs to the Collection framework; ConcurrentHashMap belongs to the Executor framework. 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.

Is Hashtable fail-safe?

Iterator in the Hashtable is fail-safe because enumerator for the Hashtable is not throw ConcurrentModificationException if any other Thread modifies the map structurally by adding or removing any element except Iterator's own remove() method.

When should we use ConcurrentHashMap?

How many threads access ConcurrentHashMap?

A ConcurrentHashMap has internal final class called Segment so we can say that ConcurrentHashMap is internally divided in segments of size 32, so at max 32 threads can work at a time.

Is ConcurrentHashMap immutable?

Read only immutable ConcurrentHashMap <-- this definitely doesn't make sense. Any Map instance that's safely published (eg a plain old HashMap ), can be safely read by multiple threads. The only danger here is guaranteeing that the Map is only read from and not written to.

Does ConcurrentHashMap allow duplicate keys?

Summary of ConcurrentHashMap features

– It does not allow duplicate keys. – It does not allow null to be used as a key or value.

Can ConcurrentHashMap have null key?

The main reason that null is not allowed in ConcurrentMaps such as ConcurrentHashMaps, ConcurrentSkipListMaps is to avoid ambiguities. If map. get(key) returns null, you cannot detect whether the key explicitly maps to null or the key itself is not mapped. In a non-concurrent map, you may check this using map.

Can two threads update the ConcurrentHashMap simultaneously?

There can't be two threads changing things at the same time. The whole point of using such data structures is that they prevent more than one thread updating that "core internal data" at the same time. Having two threads that change the map at the very same point time is not possible.

How does ConcurrentHashMap achieve scalability?

Unlike Hashtable which achieves its thread-safety by compromising the scalability, ConcurrentHashMap uses advanced techniques e.g. dividing the map into segments to remain thread-safe and scalable at the same time.

Is ConcurrentHashMap keySet thread-safe?

ConcurrentHashMap keySet() is thread safe so there might be no need to synchronize or take a copy. "iterator will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator" – this is enough for something to be 'thread safe'.

Why is ConcurrentHashMap faster?

ConcurrentHashMap uses multiple buckets to store data. This avoids read locks and greatly improves performance over a HashTable.

Can we replace Hashtable with ConcurrentHashMap?

Is it safe to replace the Hashtable instances with ConcurrentHashmap instances for performance gain? In most cases it should be safe and yield better performance. The effort on changing depends on whether you used the Map interface or Hashtable directly.

Can we convert HashMap to ConcurrentHashMap?

Java HashMap is not synchronized by default. If we add/remove key-value pairs from a HashMap in a concurrent application where multiple threads are adding and removing pairs, we may end up having inconsistent state of the map. Learn to synchronize hashmap and ConcurrentHashMap in Java.