Sunday, 17 February 2013

Difference between Hashtable and HashMap in java

One of the common interview question is "What are differences between Hashtable and HashMap".When I started using them,I used any of them irrespective of their differences.Afterwards i found noticeable differences between them which can affect your performance of application. .Before we actually see differences,let me give you brief introduction of both.

HashMap

HashMap implements Map interface which maps key to value.It is not synchronized and is not thread safe.Duplicate keys are not allowed and null keys as well as value are allowed.
HashMap<Interger,String> employeeHashmap=new HashMap<Integer,String>();
employeeHashmap.put(1,"Arpit");
employeeHashmap.put(2,null);  // will work fine

Hashtable

Hashtable implements Map interface which maps key to value.It is synchronized and thread safe.Duplicate keys are not allowed and null key is not allowed.
Hashtable<Interger,String> employeeHashmap=new Hashtable<Integer,String>();
employeeHashmap.put(1,"Arpit");
employeeHashmap.put(2,null);  //not allowed and will throw NullPointer exception at run time

Hashtable vs HashMap:

Parameter
Hashtable
HashMap
ThreadSafe
Yes
No
Synchronized
Yes
No
Performance
Due to theadSafe and Synchronized,it is often slower than HashMap
In single threaded environment, it is much faster than Hashtable.So if you do not work in multi thread environment ,then hashMap is recommended
Null key
Do not allow
Allows null key as well as values
Fail fast
enumeration in hashtable is not fail fast
Iterator in hashMap is fail fast
Extends
It extends Dictionary class which is quite old
It extends AbstractMap class
Alternative
No alternative
You can use ConcurrentHashMap for multi thread environment

Some important points need to be discussed.
  • Synchonized meaning only one thread can modify one table  at one point of time.When any thread perform update operation on hashtable then it acquires lock on it and other threads have to wait for lock to be released.
  • 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.Structurally modification means inserting or deleting elements that can change structure of map.

Can we synchronize HashMap?

Yes,We can synchonized a HashMap also with the help of Collections.synchonizedMap(hashmap) so HashMap can be synchronized by
Map map=Collections.synchonizedMap(hashmap)
 

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