Question

As specified in JDK documentation, Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values. Why is this?

Answer 1

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.

Edit

From Hashtable JavaDoc:

To successfully store and retrieve objects from a Hashtable, the
objects used as keys must implement the hashCode method and the equals method.

Since null isn’t an object, you can’t call .equals() or .hashCode() on it, so the Hashtable can’t compute a hash to use it as a key.