ConcurrentHashMap的get（）与HashTable的get（）

Question

I read that both ConcurrentHashMap 's get() method as well as HashTable 's get() is thread safe, even though former does not use synchronized keyword. Why in HashTable 's get() method implementation synchronized keyword is required to make it thread safe but it is not required in case of ConcurrentHashMap 's get() method.

Answer 1

Broadly because HashTable and ConcurrentHashMap don't have exactly the same behavior and so actual results concerning concurrency. For retrieval operations, the fact that ConcurrentHashMap doesn't lock the whole table makes ConcurrentHashMap to may not reflect the last updated value for the key as get() may overlap with put() / remove() operations :

Retrieval operations (including get) generally do not block, so may overlap with update operations (including put and remove). Retrievals reflect the results of the most recently completed update operations holding upon their onset

And when this delay doesn't matter in many use cases and also you can cope with that with explicit synchronized statements, you prefer to use ConcurrentHashMap over HashTable as it improves the overall Map performance for concurrent access.