Why ConcurrentHashMap doesn't allow null keys and null values ?

Reason #1. Purpose of null key
 If map.get(key) returns null, you can't detect whether the key explicitly maps to null or the key isn't mapped.

Reason #2. Ambiguities
In the ConcurrentHashMap implementation, map can be changed in between.

If it supports null key, in below code key k may be deleted in between get and containsKey calls and code will return null instead of KeyNotPresentException (expected)

if (map.containsKey(k)) {

     return map.get(k);

} else {

     throw new KeyNotPresentException();

}