What are the differences between ConcurrentHashMap and HashMap, and when should each be used?

Question

Accepted Answer

HashMap is not thread-safe; ConcurrentHashMap is designed for concurrent access with better performance in multi-threaded environments.

HashMap allows one null key and multiple null values; ConcurrentHashMap does not allow null keys or values.
HashMap is not synchronized, making it unsafe for concurrent access; ConcurrentHashMap uses segmentation to allow concurrent reads and writes.
HashMap can lead to data inconsistency in multi-threaded scenarios; ConcurrentHashMap provides thread safety without locking the entire map.
Example: In a multi-threaded application, using ConcurrentHashMap prevents issues like lost updates and stale data.

Accepted Answer

Hash map uses object level lock...

Concurrent hashmap has 16 buckets and each bocker has seperate lock..

So at a time 16 locks can be obtained...16 updates can be done

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