Which is faster HashMap or ConcurrentHashMap?

HashMap allows key and value to be null. ConcurrentHashMap does not allow null key/value. It will throw NullPointerException. HashMap is faster.

Why ConcurrentHashMap is faster than HashMap?

HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously. But ConcurrentHashMap performance is low sometimes because sometimes Threads are required to wait on ConcurrentHashMap.

Why ConcurrentHashMap is faster than Hashtable in Java?

ConcurrentHashMap divides the whole map into different segments and locks only a particular segment during the update operation, instead of Hashtable, which locks whole Map. The ConcurrentHashMap also provides lock-free read, which is not possible in Hashtable.

Are you aware of HashMap ConcurrentHashMap SynchronizedMap which one is faster?

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.

What is difference between ConcurrentHashMap and synchronized HashMap?

SynchronizedMap acquires lock on the entire Map instance , while ConcurrentHashMap divides the Map instance into multiple segments and locking is done on those.

#12 - Difference b/w ConcurrentHashMap

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 convert HashMap to ConcurrentHashMap?

1. Synchronize HashMap – ConcurrentHashMap. Our first choice should always be using the ConcurrentHashMap class if we wish to use a Map in concurrent environment. ConcurrentHashMap support concurrent access to it's key-value pairs by design.

When should we use ConcurrentHashMap?

What is the advantage of using HashMap?

Advantages of HashMap

Allows insertion of key value pair. HashMap is non synchronized. HashMap cannot be shared between multiple threads without proper synchronization. HashMap is a fail-fast iterator.

Why is HashMap faster?

The reason that HashMap is faster than HashSet is that the HashMap uses the unique keys to access the values. It stores each value with a corresponding key and we can retrieve these values faster using keys during iteration. While HashSet is completely based on objects and therefore retrieval of values is slower.

How ConcurrentHashMap is fail-safe?

Iterator of ConcurrentHashMap is fail-safe, it means that it doesn't throw ConcurrentModificationException even if underlying ConcurrentHashMap is modified once Iteration begins.

Is HashMap thread-safe for read only?

As long as no thread is modifying the map (adding or removing a key/value pair, or mutating an existing value) while other threads are reading then it is thread-safe.

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.

What happens happens when we iterate over ConcurrentHashMap and HashMap?

HashMap iterator is fail-fast and ArrayList throws ConcurrentModificationException if concurrent modification happens during iteration. ConcurrentHashMap is fail-safe and it will never throw ConcurrentModificationException during iteration. HashMap allows key and value to be null.

Is ConcurrentHashMap volatile?

No, you don't. volatile means that the variable cannot be cached in a register, and so will always be "write-through" to memory. This means that one thread's change to a variable will be visible to other threads.

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.

Does HashMap allow duplicate keys?

HashMap stores key, value pairs and it does not allow duplicate keys. If the key is duplicate then the old key is replaced with the new value.

Is ConcurrentHashMap put thread-safe?

ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications.

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.

Why is HashMap not synchronized?

hashmap doesn't uses hashtable in any way. hashmap is not synchronized because it was never meant to be, it's simple implementation of maintaining a bucket of hashcode and a linkedlist assigned to that hashcode with key-value pair as nodes.

Can 2 threads on same ConcurrentHashMap object access it concurrently?

Having two threads that change the map at the very same point time is not possible. Because the code within that ConcurrentHashMap will not allow two threads to manipulate things in parallel!

How do you avoid ConcurrentModificationException while iterating a HashMap?

Is ConcurrentHashMap keySet thread-safe?

keySet() is thread safe. However, it may act in very strange ways, as pointed out in the quote you included. As a Set , entries may appear and/or disappear at random. I.e. if you call contains twice on the same object, the two results may differ.

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.