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Hashtable vs ConcurrentHashMap
Hashtable. This class implements a hash table, which maps keys to values. Any non-null object can be used as a key or as a value. To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method. Unlike the new collection implementations, Hashtable is synchronized. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of Hashtable.
ConcurrentHashMap. A hash table supporting full concurrency of retrievals and high expected concurrency for updates. This class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable. However, even though all operations are thread-safe, retrieval operations do not entail locking, and there is not any support for locking the entire table in a way that prevents all access. This class is fully interoperable with Hashtable in programs that rely on its thread safety but not on its synchronization details.
| Properties | Hashtable |
ConcurrentHashMap |
|---|---|---|
Is null key allowed |
No | No |
Is null value allowed |
No | Yes |
| Is Thread Safe | Yes | Yes, Thread safety is ensured by having separate locks for separate buckets, resulting in better performance. Performance is further improved by providing read access concurrently without any blocking. |
| Performance | Slow due to synchronization overhead. | Faster than Hashtable. ConcurrentHashMap is a better choice when there are more reads than writes. |
| Iterator | Hashtable uses enumerator to iterate the values of Hashtable object. Enumerations returned by the Hashtable keys and elements methods are not fail-fast. |
Fail-safe iterator: Iterator provided by the ConcurrentHashMap is fail-safe, which means it will not throw ConcurrentModificationException. |
When you read from a ConcurrentHashMap using get(), there are no locks, contrary to the Hashtable for which all operations are simply synchronized. Hashtable uses single lock for whole data. ConcurrentHashMap uses multiple locks on segment level (16 by default) instead of object level i.e. whole Map. Hashtable is belongs to the Collection framework; ConcurrentHashMap belongs to the Executor framework. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use ConcurrentHashMap in place of Hashtable.
Links
https://www.geeksforgeeks.org/concurrenthashmap-in-java/
https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentHashMap.html
https://docs.oracle.com/javase/8/docs/api/java/util/Hashtable.html
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