New to Java

Hi All,
In an interview they ask me this questions : "Difference between HashMap and HashTable"

I told them as:-

Hashtable :-Hashtable is synchronized. so it is slow. & not allow null values.
Hash Map:- HashMap is not synchronized & is fast & allow null values.

they dont satisfied with this:

then any one reply back to me with clear & clevar differences ?

Thanks in advance
Nagendra