java - Hashmap concurrency issue

Question

I have a Hashmap that, for speed reasons, I would like to not require locking on. Will updating it and accessing it at the same time cause any issues, assuming I don't mind stale data?

My accesses are gets, not iterating through it, and deletes are part of the updates.

Answer 1

Yes, it will cause major problems. One example is what could happen when adding a value to the hash map: this can cause a rehash of the table, and if that occurs while another thread is iterating over a collision list (a hash table "bucket"), that thread could erroneously fail to find a key that exists in the map. HashMap is explicitly unsafe for concurrent use.

Use ConcurrentHashMap instead.