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javacollectionssynchronizationhashmaphashtable

Difference between Hashtable and Collections.synchronizedMap(HashMap)

As far as I know, java.util.Hashtable synchronizes each and every method in the java.util.Map interface, while Collections.synchronizedMap(hash_map) returns a wrapper object containing synchronized methods delegating calls to the actual hash_map (correct me if I am wrong).

I have two questions :

  1. What difference does it make to synchronize each and every method and to have a wrapper class? What are the scenarios to choose one over the other?

  2. What happens when we do Collections.synchronizedMap(hash_table)? Will this be equal to simply using a normal java.util.Hashtable?

Solution

  • Here are the answers I've gotten from a bit of (hopefully correct) research:

    1. Both provide the same degree of synchronization. If you were to wrap Hashtable through Collections.synchronized you would have the same degree, but with another redundant layer, of synchronization.

    2. The main difference between Hashtable and Collections.synchronizedMap(HashMap) exist more at the API level. Because Hashtable is part of Java's legacy code, you'll see that the Hashtable API is enhanced to implement the Map interface, to become part of Java's collections framework. This means that if you were to wrap Hashtable through Collections.synchronizedMap(), the API of the wrapped Hashtable would become limited to the Map API. So if the API of Hashtable is encompassed in your definition of behavior, then it is obviously altered/limited.