Answer / eknath

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))

Answer / naveen

There is null check in the put method implementation of
hashtable, so it does not support null values and null keys.


public Object put(Object key, Object value) {
// Make sure the value is not null
if (value == null) throw new NullPointerException();
}

above is HashTable put method logic implemented by Sun.

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