Q)What is difference between HashMap and HashTable ?

Ans) Both collections implements Map. Both collections store value as key-value pairs. The key differences between the two are

1. Hashmap is not synchronized in nature but hashtable is.
2. Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.
   Fail-safe -if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException?
3. HashMap permits null values and only one null key, while Hashtable doesn't allow key or value as null.