Differences between Hashtable and HashMap

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 employeeHashmap=new HashMap();  
employeeHashmap.put(1,"Harit");  
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 employeeHashmap=new Hashtable();  
employeeHashmap.put(1,"Harit");  
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)