Saturday, October 2, 2010

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

HashMap vs Hashtable in Java
Difference between HashMap and Hashtable in Java question oftenly asked in core Java interviews to check whether candidate understand correct usage of collection classes and aware of alternative solutions available. Along with How HashMap internally works in Java and ArrayList vs Vector, this  is one of the oldest question from Collection framework in Java. Hashtable is a legacy Collection class and it's there in Java API from long time but it got refactored to implement Map interface in Java 4 and from there Hashtable became part of Java Collection framework. Hashtable vs HashMap in Java is so popular a question that it can top any list of Java Collection interview Question. You just can't afford not to prepare HashMap vs Hashtable before going to any Java programming interview. In this Java article we will not only see some important differences between HashMap and Hashtable but also some similarities between these two collection classes. Let's first see How different they are :

Difference between HashMap and Hashtable in Java

Both HashMap and Hashtable implements Map interface but there are some significant difference between them which is important to remember before deciding whether to use HashMap or Hashtable in Java. Some of them is thread-safety, synchronization and speed. here are those differences :

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. One of the major differences between HashMap and Hashtable is that HashMap is non synchronized whereas Hashtable is synchronized, which means Hashtable is thread-safe and can be shared between multiple threads but HashMap can not be shared between multiple threads without proper synchronization. Java 5 introduces ConcurrentHashMap which is an alternative of Hashtable and provides better scalability than Hashtable in Java.

3. Another significant difference between HashMap vs Hashtable is that Iterator in the HashMap is  a fail-fast iterator  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. This is also an important difference between Enumeration and Iterator in Java.

4. One more notable difference between Hashtable and HashMap is that because of thread-safety and synchronization Hashtable is much slower than HashMap if used in Single threaded environment. So if you don't need synchronization and HashMap is only used by one thread, it out perform Hashtable in Java.

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

Difference between HashMap and Hashtable in Java | HashMap vs Hashtable




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 or ListIterator has been created on a collection object and some other thread tries to modify the collection object "structurally", a concurrent modification exception will 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);

In Summary there are significant differences between Hashtable and HashMap in Java e.g. thread-safety and speed and based upon that only use Hashtable if you absolutely need thread-safety, if you are running Java 5 consider using ConcurrentHashMap in Java.

19 comments :

ferdhie said...

what about ConcurrentHashMap? What is the difference?

Javin @ FIX Protocol Tutorial said...

Hi Fedhie, ConcurrentHashMap is new addition in Java collections suite and best suited for situations where number of readers outnumber number of writers in a big way. Since concurrent read is allowed it improves performance significantly.

Thanks
Javin

Javin @ FIX Protocol Tutorials said...

Hi Anonymous thanks you liked it you can also see How HashMap works in Java , to know more about HashMap interview questions.

Anand said...

Synchronizing a hashmap is possible but rather a pain because that is what the hashtable is there... But, synchronizing manually can be risky and at times lead to the most dreaded deadlocks. You can know more about "Synchronization & Deadlocks" here .

Anonymous said...

Such a nice tutorial ....

Anonymous said...

can you care to explain difference bet fail-fast and fail-safe iterator on hashtable and hashmap , which one uses what, also hashmap is better or hashtable what is your opinion

Javin said...

Hi Anonymous fail-fast iterator means if one thread is iterating over hashmap and other thread trying to modify hashmap structurally it will throw ConcurrentModification Exception and fail immediately while in fail-safe iterator Iterations is done on copy of collection object instead of original e.g. in case of CopyOnWriteArraylist. iterator of hashmap is fail-fast and hashtable doesn't have any iterator that is another difference between hashmap and hashtable.

Anonymous said...

here are some more differences between hashtable and hashmap

1) hashtable extends Dictionary interface which is quite old while hashmap extends Map interface.
2) hashtalbe doesn't have counterpart like ConcurrentHashMap.
3) another important difference between hashtable and hashmap is , hashtable is less secure than hashmap because of Enumeration it uses. while hashmap uses iterator which prevents Concurrent Modification of HashMap, which is not possible in case of hashtable.
4) stay out of hashtable use hashmap instead.

Anonymous said...

wow Anonymous got more differences between hashtable and hashmap :)

Neerja said...

I think key difference between hashmap and hastable is what you mentioned about Iterator and Enumeration , as Iterator being fail-fast beccomes natural choice over enumeration.

Anonymous said...

correct, though both hashtable and hashmap are older than new concurrenthashmap iterator and enumeration is key point.I would suggest to leave behind both hashmap and hashtable and move forward to concurrenthashMap.

Javin @ spring interview questions answers said...

Thanks Nagesh for your comment and good to know that you like this hashtable vs hashmap tutorial. you may also like Difference between ArrayList and Vector in java

Anonymous said...

iterator can be implemented for hashtable also

Anonymous said...

from Java4 I guest hashtable is also part of collection framework along with Vector. with the age of ConcurrentHashMap, does Hashtable still holds value ?

Anonymous said...

So it was it Hashtable() instead of HashTable() or something...

Anonymous said...

Can you please put a feature and performance comparison between
1. HashMap vs ConcurrentHashMap
2. HashMap vs Hashtable
3. HashMap vs LinkedHashMap
4. Hashtable vs ConcurrentHashMap
5. Hashtable vs LinkedHashMap and
6. ConcurrentHashMap vs LinkedHashMap
that would be simply great. Also if you could share when to use Hashtable, HashMap, ConcurrentHashMap and LinkedHashMap, that would be amazing, looking forward to see these comparison.

Anonymous said...

Hi

We can synchronize Map object using Collections.synchronizedMap(mapObj)...

Then

1) what is difference for map object(after synchronize using synchronizedMap method) and hashtable object?

2) Will be there any difference in synchronization using the method and by default the class as synchronized..

Anonymous said...

Hi

For getting the synchronized map the method name is collections.synchronizedMap() not synchronizeMap()

Anonymous said...

Hi,

what is the difference between synchronized hash-map and hash table?

I don't see any difference here but interviewer asked this quest ion.Can some one please explain me?

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