Why do we need ConcurrentHashMap in Java?

You should use ConcurrentHashMap when you need very high concurrency in your project. It is thread safe without synchronizing the whole map . Reads can happen very fast while write is done with a lock. There is no locking at the object level.

Why do we use ConcurrentHashMap in Java?

ConcurrentHashMap: It allows concurrent access to the map. Part of the map called Segment (internal data structure) is only getting locked while adding or updating the map. So ConcurrentHashMap allows concurrent threads to read the value without locking at all. This data structure was introduced to improve performance.

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.

What is 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.

What are benefits of using ConcurrentHashMap over synchronized HashMap?

ConcurrentHashMap allows performing concurrent read and write operation. Hence, performance is relatively better than the Synchronized Map. In Synchronized HashMap, multiple threads can not access the map concurrently. Hence, the performance is relatively less than the ConcurrentHashMap.

ConcurrentHashMap internal working in java | ConcurrentHashMap internal implementation in java

Where we can use ConcurrentHashMap?

Use ConcurrentHashMap only when the class where it's being used is thread safe; otherwise the use of a thread safe data structure in a non thread safe class adds to the confusion of the next person looking at the code.

How ConcurrentHashMap is fail safe?

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.

Why is ConcurrentHashMap faster?

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

Does ConcurrentHashMap allow null values?

The main reason that nulls aren't allowed in ConcurrentMaps (ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that may be just barely tolerable in non-concurrent maps can't be accommodated. The main one is that if map.

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.

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.

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.

Can ConcurrentHashMap throws ConcurrentModificationException?

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.

Can we start a thread twice?

No. After starting a thread, it can never be started again. If you does so, an IllegalThreadStateException is thrown. In such case, thread will run once but for second time, it will throw exception.

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.

How do you create a concurrent list in Java?

Why HashTable doesn't allow null and HashMap does?

Now you must be wondering why HashTable doesn't allow null and HashMap do? The answer is simple. In order to successfully store and retrieve objects from a HashTable, the objects used as keys must implement the hashCode method and the equals method. Since null is not an object, it can't implement these methods.

What is difference between synchronizedMap and ConcurrentHashMap?

synchronizedMap() requires each thread to acquire a lock on the entire object for both read/write operations. By comparison, the ConcurrentHashMap allows threads to acquire locks on separate segments of the collection, and make modifications at the same time.

What is the difference between LinkedHashMap and HashMap?

The key difference between HashMap and LinkedHashMap is order. Elements of a HashMap are not in order, totally random, whereas elements of LinkedHashMap are ordered. The entries of a LinkedHashMap are in key insertion order, which is the order in which the keys are inserted in the Map.

Why is HashMap faster than Hashtable?

HashMap is not synchronized, therefore it's faster and uses less memory than Hashtable. Generally, unsynchronized objects are faster than synchronized ones in a single threaded application.

What is difference between fail-fast and fail-safe?

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.

Can we create a HashMap key Singleton?

It's now clear that you can't use the Singleton class in a HashMap as a key. Even If you override the HashCode Object as a key is stored as a reference in Map. So If you change its implementation it's get reflected in the Map while executing the hashcode method of Singleton Class.

Is ArrayList thread-safe?

Vectors are synchronized. Any method that touches the Vector 's contents is thread safe. ArrayList , on the other hand, is unsynchronized, making them, therefore, not thread safe.

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.

Does ConcurrentHashMap maintain insertion order?

ConcurrentHashMap and HashTable do not preserve the insertion order of mappings in the map. Also, the addition and removal of any element might change its iteration order. On the other hand, Collections. SynchronizedMap() is backed by the specified map and retains the insertion order of the map.