Difference between HashMap and HashTable? Can we make hashmap synchronized?

This question oftenly asked in interview to check whether candidate understand correct usage of collection classes and aware of alternative solutions available.

1. The HashMap class is roughly equivalent to Hashtable, except that it is non synchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn't allow nulls).

2. HashMap does not guarantee that the order of the map will remain constant over time.

3. HashMap is non synchronized whereas Hashtable is synchronized.

4. Iterator in the HashMap is  fail-fast  while the enumerator for the Hashtable is not and throw ConcurrentModificationException if any other Thread modifies the map structurally  by adding or removing any element except Iterator's own remove()  method. But this is not a guaranteed behavior and will be done by JVM on best effort.

Note on Some Important Terms

1)Synchronized means only one thread can modify a hash table at one point of time. Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.

2)Fail-safe is relevant from the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object "structurally", a concurrent modification exception wjavascript:void(0)ill be thrown. It is possible for other threads though to invoke "set" method since it doesn't modify the collection "structurally". However, if prior to calling "set", the collection has been modified structurally, "IllegalArgumentException" will be thrown.

3)Structurally modification means deleting or inserting element which could effectively change the structure of map.

HashMap can be synchronized by

Map m = Collections.synchronizeMap(hashMap);

Hope this will be useful.

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