Hashtable is the older class, and its use is generally discouraged. Perhaps they saw the need for a null key, and more importantly – null values, and added it in the HashMap implementation.
HashMap is newer, and has more advanced capabilities, which are basically just an improvement on the Hashtable functionality. When HashMap was created, it was specifically designed to handle null values as keys and handles them as a special case.
To successfully store and retrieve objects from a Hashtable, the
objects used as keys must implement the hashCode method and the equals method.
null isn’t an object, you can’t call
.hashCode() on it, so the
Hashtable can’t compute a hash to use it as a key.