ConcurrentHashMap : It was introduced in Java 1.5 to store data using multiple buckets. As the name suggests, it allows concurrent read and writes operations to the map. It only locks a certain portion of the map while doing iteration to provide thread safety so that other readers can still have access to the map without waiting for iteration to complete.
Hashtable : It is a thread-safe legacy class that was introduced in old versions of java to store key or value pairs using a hash table. It does not provide any lock-free read, unlike ConcurrentHashMap. It just locks the entire map while doing iteration.
ConcurrentHashMap and Hashtable, both are thread-safe but ConcurrentHashMap generally avoids read locks and improves performance, unlike Hashtable. ConcurrentHashMap also provides lock-free reads, unlike Hashtable. Therefore, ConcurrentHashMap is considered faster than Hashtable especially when the number of readers is more as compared to the number of writers.
