When to use ConcurrentHashMap in Java | Key summary of ConcurrentHashMap | Java collection interview Question

ConcurrentHashMap is best suited when you have multiple readers and few writers. If writers outnumber reader, or writer is equal to reader, than performance of ConcurrentHashMap effectively reduces to synchronized map or Hashtable. Performance of CHM drops, because you got to lock all portion of Map, and effectively each reader will wait for another writer, operating on that portion of Map. ConcurrentHashMap is a good choice for caches, which can be initialized during application start up and later accessed my many request processing threads. As javadoc states, CHM is also a good replacement of Hashtable and should be used whenever possible, keeping in mind, that CHM provides slightly weeker form of synchronization than Hashtable.


Summary
Now we know What is ConcurrentHashMap in Java and when to use ConcurrentHashMap, it’s time to know and revise some important points about CHM in Java.

1. ConcurrentHashMap allows concurrent read and thread-safe update operation.

2. During update operation, ConcurrentHashMap only lock a portion of Map instead of whole Map.

3. Concurrent update is achieved by internally dividing Map into small portion which is defined by concurrency level.

4. Choose concurrency level carefully as a significant higher number can be waste of time and space and lower number may introduce thread contention in case writers overnumber concurrency level.

5. All operations of ConcurrentHashMap are thread-safe.

6. Since ConcurrentHashMap implementation doesn't lock whole Map, there is chance of read overlapping with update operations like put() and remove(). In that case result returned by get() method will reflect most recently completed operation from there start.

7. Iterator returned by ConcurrentHashMap is weekly consistent, fail safe and never throw ConcurrentModificationException. In Java.

8. ConcurrentHashMap doesn't allow null as key or value.

9. You can use ConcurrentHashMap in place of Hashtable but with caution as CHM doesn't lock whole Map.

10. During putAll() and clear() operations, concurrent read may only reflect insertion or deletion of some entries.

That’s all on What is ConcurrentHashMap in Java and when to use it. We have also seen little bit about internal working of ConcurrentHashMap and how it achieves it’s thread-safety and better performance over Hashtable and synchronized Map. Use ConcurrentHashMap in Java program, when there will be more reader than writers and it’s a good choice for creating cache in Java as well.