There are several notable differences between Hashmap and Hashtable in Java, such as:
1. Hashmap has one sub-type -> LinkedHashMap. It is used if you want insertion order by default, in which case you can easily swap a Hashmap for a LinkedHashMap, which wouldn't be possible in case of a Hashtable.
2. Hashmap allows a single NULL key and any number of NULL values, whereas Hashtable does not allow NULL KEYS OR VALUES.
