Enthuware

I don't understand the question...

It is asking what happens when you call the put method.

It would be a lot helpful if someone would post which classes allow null key/values insertion and which not.

FYI: The function returns the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)

I was doubting.

No Nulls: ArrayDeque, Hashtable, TreeSet, ConcurrentHashMap, ConcurrentSkipListMap (and most other concurrent collections)
No Null Keys: TreeMap

Everything else is OK with null, as much as I was able to gather...

I am confused about the put() method signature.
The signature of put() method in HashMap is V put(K key, V Value);
Where is that signature with objects coming from?

V and K are type parameters. They are relevant only while compilation. After compilation, they are replaced by Object (or their bounds).

Yes that is correct.