Collection doubt from Dan's mock test

[Suman Nagelia]

Hi
I have one problem with the following question answer:

a. Stores key/value pairs.
b. Allows null elements, keys, and values.
c. Duplicate entries replace old entries.
d. Entries are sorted using a comparator or the
Comparable interface.

Which of these classes provides the specified features?
a. LinkedList
b. TreeMap
c. TreeSet
d. HashMap
e. HashSet
f. Hashtable
g. None of the above

Answer is TreeMap
But according to my understanding answer should be g. None of the above as TreeMap doesn't satisfies the second condition "b. Allows null elements, keys, and values." Treemap allows null values but doesn't allow null key .
Can anyone give more thought on this and explain to me is anywhere I am wrong in understanding the question or what?
Thanks

[Karthik Veeramani]

Correct me if im wrong, but cant a TreeMap take 1 null key (and many null values)... I guess the question means only this, by 'accepting null'.

[Suman Nagelia]

As far my understanding of TreeMap it'll not accept any null key.

[Rory French]

try the following code. It compiles and runs. So I guess TreeMap does accept a null key and null values.

import java.util.TreeMap;
class TreeMaps {
public static void main(String[] s) {
TreeMap tm = new TreeMap();
tm.put(null, null);
}
}