ConcurrentHashMap and Hashtable in Java [duplicate]

by

ConcurrentHashMap and Hashtable locking mechanism

  • Hashtable is belongs to the Collection framework; ConcurrentHashMap belongs to the Executor framework.
  • 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.
  • ConcurrentHashMap locking is applied only for updates. In case of retrievals, it allows full concurrency, retrievals reflect the results of the most recently completed update operations. So reads can happen very fast while writes are done with a lock.
  • ConcurrentHashMap doesn’t throw a ConcurrentModificationException if one thread tries to modify it while another is iterating over it and does not allow null values.
  • ConcurrentHashMap returns Iterator, which fails-safe (i.e. iterator will make a copy of the internal data structure) on concurrent modification.
  • ConcurrentHashMap uses a database shards logic (Segment<K, V>[] segments) is known as Concurrency-Level, i.e. divides the data into shards(segments) then puts locks on each shard (segment) instead of putting a single lock for whole data (Map). The default value is 16.

To understand the ConcurrentHashMap more technically please look at this link

The following analogy helps you get understand the concept only(not logic)

  • Assume Hashtable and ConcurrentHashMap are two types of Homes.
  • Hashtable locks home’s main door.
  • ConcurrentHashMap locks specific room door instead of main door.

Which is more efficient for threaded applications?

ConcurrentHashMap is more efficient for threaded applications.

More Related Contents:

  1. What are the differences between a HashMap and a Hashtable in Java?
  2. What happens when a duplicate key is put into a HashMap?
  3. How does Java order items in a HashMap or a HashTable?
  4. Why does HashMap require that the initial capacity be a power of two?
  5. Why Hashtable does not allow null keys or values?
  6. How to put HashMap into different elements with same key
  7. Get ArrayList entry of a HashMap Entry [closed]
  8. How to loop through a HashMap in JSP?
  9. How can I convert JSON to a HashMap using Gson?
  10. How to sort a HashMap in Java [duplicate]
  11. How do HashTables deal with collisions?
  12. Is a HashMap thread-safe for different keys?
  13. What is the significance of load factor in HashMap?
  14. Why there is no ConcurrentHashSet against ConcurrentHashMap
  15. Can an array be used as a HashMap key?
  16. Sorting hashmap based on keys
  17. What is the optimal capacity and load factor for a fixed-size HashMap?
  18. HashMap implementation in Java. How does the bucket index calculation work?
  19. HashMap to return default value for non-found keys?
  20. Why does LinkedHashSet extend HashSet and implement Set
  21. HashMap and int as key
  22. Java: how to convert HashMap to array
  23. Android – Get value from HashMap
  24. Hashtable key within integer interval
  25. JDK 8 – “The type java.util.Map$Entry cannot be resolved” [duplicate]
  26. How does ConcurrentHashMap work internally?
  27. Iterating over and deleting from Hashtable in Java
  28. Why are immutable objects in hashmaps so effective?
  29. How to convert a string to a HashMap? [duplicate]
  30. Explain the timing causing HashMap.put() to execute an infinite loop

Leave a Comment