How many threads can 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.

Can multiple threads read ConcurrentHashMap at the same time?

ConcurrentHashMap is divided into different segments based on concurrency level. So different threads can access different segments concurrently in java. Can threads read the segment of ConcurrentHashMap locked by some other thread in java? Yes.

Is ConcurrentHashMap put thread-safe?

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

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.

What is the concurrency level of ConcurrentHashMap in Java?

Default concurrency level is 16, and accordingly Map is divided into 16 part and each part is governed with different lock. This means, 16 thread can operate on Map simultaneously, until they are operating on different part of Map. This makes ConcurrentHashMap high performance despite keeping thread-safety intact.

#12 - Difference b/w ConcurrentHashMap

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.

What is default size of ConcurrentHashMap?

Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).

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.

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 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.

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.

What is the maximum thread priority in Java?

All Java threads have a priority in the range 1-10. priority ie. priority by default is 5. Whenever a new Java thread is created it has the same priority as the thread which created it.

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.

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.

Can 2 threads perform write operations on same ConcurrentHashMap object simultaneously Mcq?

Can 2 threads on same ConcurrentHashMap object access it concurrently in java? ConcurrentHashMap is divided into different segments based on concurrency level. So different threads can access different segments concurrently in java.

How does ConcurrentHashMap works internally?

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.

Why do we need ConcurrentHashMap?

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.

How do you avoid ConcurrentModificationException while iterating a HashMap?

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.

Is ArrayList fail-fast?

Iterator of ArrayList is fail fast , so while you are iterating over the ArrayList using the Iterator if underlying ArrayList is modified by any method other than add and remove provided by Iterator itself it will throw ConcurrentModificationException and will bail out.

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.

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.

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.

Is ConcurrentHashMap clear thread-safe?

clear() method in Java? A ConcurrentHashMap is a thread-safe version of a HashMap that allows concurrent read and thread-safe update operation.