From the 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.
In a nutshell, 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.
HashMap
is newer, and has more advanced capabilities, which are basically just an improvement on the Hashtable
functionality. As such, when HashMap
was created, it was specifically designed to handle null
values as keys and handles them as a special case.
Specifically, the use of null
as a key is handled like this when issuing a .get(key)
:
(key==null ? k==null : key.equals(k))
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