What are the differences between HashMap and Hashtable?

Both provide key-value access to data. The Hashtable is one of the original collection classes in Java. HashMap is part of the new Collections Framework, added with Java 2, v1.2.
The key difference between the two is that access to the Hashtable is synchronized on the table while access to the HashMap isn't. You can add it, but it isn't there by default.
Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't. If you change the map while iterating, you'll know.
And, a third difference is that HashMap permits null values in it, while Hashtable doesn't.

For new code, I would tend to always use HashMap. 

	Difference between HashMap and HashTable?
A:	The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

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);