Question

What is the main advantage of using Hashtable comparing with HashMap. Because, Hashtable main advantage is synchronization.
Now map can also synchronized using synchronizedMap().

Map m = Collections.synchronizedMap(hashMap);

Answer 1

Hashtable existed before the collections framework, so it is retained mainly for backward compatibility. Use ConcurrentHashMap instead.

Note that there is a slight semantic difference – Hashtable does not allow nulls, while HashMap allows null values and a null key.