HashMap
1) HashMap is non synchronized. It is not-thread safe and can't be shared between many threads without proper synchronization code.
2) HashMap allows one null key and multiple null values.
3) HashMap is a new class introduced in JDK 1.2.
4) HashMap is fast.
5) We can make the HashMap as synchronized by calling this code
Map m = Collections.synchronizedMap(hashMap);
6) HashMap is traversed by Iterator.
7) Iterator in HashMap is fail-fast.
8) HashMap inherits AbstractMap class.
Hashtable
1) Hashtable is synchronized. It is thread-safe and can be shared with many threads.
2) Hashtable doesn't allow any null key or value.
3) Hashtable is a legacy class.
4) Hashtable is slow.
5) Hashtable is internally synchronized and can't be unsynchronized.
6) Hashtable is traversed by Enumerator and Iterator.
7) Enumerator in Hashtable is not fail-fast.
8) Hashtable inherits Dictionary class.