How are segments created in ConcurrentHashMap?

A ConcurrentHashMap is divided into number of segments, and the example which I am explaining here used default as 32 on initialization. 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.

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.

How many segments are there in HashMap?

Depending upon the level of concurrency required the concurrent HashMap is internally divided into segments. If the level of concurrency required is not specified then it is takes 16 as the default value. So internally the ConcurrentHashMap will be divided into 16 segments. Each Segment behaves independently.

Can 2 threads perform write operations on same ConcurrentHashMap object 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 do you initialize ConcurrentHashMap?

A good approach can be having initialization like this: ConcurrentHashMap instance = new ConcurrentHashMap( 16 , 0 . 9f, 1 ); An initial capacity of 16 ensures a reasonably good number of elements before resizing happens.

Where can I use ConcurrentHashMap?

ConcurrentHashMap

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

How does ConcurrentHashMap works in Java?

ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications. In ConcurrentHashMap, the Object is divided into a number of segments according to the concurrency level. The default concurrency-level of ConcurrentHashMap is 16.

Why ConcurrentHashMap is weakly consistent?

The default iterator for the ConcurrentHashMap is weakly consistent. This means that this Iterator can tolerate concurrent modification, traverses elements as they existed when Iterator was constructed and may (but isn’t guaranteed to) reflect modifications to the Collection after the construction of the Iterator.

What data structure is used in ConcurrentHashMap?

Key points of ConcurrentHashMap: The underlined data structure for ConcurrentHashMap is Hashtable. ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications.

How do I sort ConcurrentHashMap by key?

you can’t sort a ConcurrentHashMap at all. you would have to use an alternate data structure if you want to sort (by key or by value).

What is the default number of partitions segments in ConcurrentHashMap?

In ConcurrentHashMap, the Object is divided into a number of segments according to the concurrency level. The default concurrency-level of ConcurrentHashMap is 16.

How does ConcurrentHashMap work in multithreading?

ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications. In ConcurrentHashMap, at a time any number of threads can perform retrieval operation but for updated in the object, the thread must lock the particular segment in which the thread wants to operate.

What is segment locking in ConcurrentHashMap?

In ConcurrentHashMap, at a time any number of threads can perform retrieval operation but for updation in object, thread must lock the particular segment in which thread want to operate.This type of locking mechanism is known as Segment locking or bucket locking .Hence at a time 16 updation operations can be performed by threads.

What is the use of ConcurrentHashMap?

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.

What is the use of multi level lock in concurrentconcurrenthashmap?

ConcurrentHashMap uses segments instead of buckets and when new record get insert lock will get acquire only on segment not the complete list of segments. So here idea makes its clear that multi level lock will get acquire on the same. As no concurrency level has been set explictity, the ConcurrentHashMap gets divided into 16 segments.

What is the difference between tostring() and replace() method of ConcurrentHashMap class?

The replace () method of ConcurrentHashMap class replaces the entry for a key only if currently mapped to some value. This is equivalent to, for this map. 24. The toString () method of ConcurrentHashMap class returns a string representation of this map.