Whats the benefit of allowing multiple null keys in hashmap?

Question

Just for experimenting, I added multiple null keys in a Hashmap instance. And it didn't complain. What's the benefit of doing that?

The code is,

Map hmap = new HashMap();
hmap.put("sushil","sushil11" );
hmap.put(null,null);
hmap.put(null,"king");
hmap.put(null,"nagasaki");
hmap.put(null,null);

How many keys are there in the map?

Answer 1

I would guess you haven't added multiple null-keys. You just overwrote the same nullkey multiple times.

Answer 2

A normal hashmap will have unique keys, so you're overwriting the entry for the null key repeatedly. You won't have multiple identical keys (for this you need a MultiMap or similar)

Answer 3

It is used to get switch:case:default behavior.

Example:

Problem Definition: Coffee shop in CS Department building. They provide coffee to CS Student for $1.00, to IT department students $1.25 and others for $1.50.

Then Map will be:

Key -> Value

IT -> 1.25

CS -> 1.00

null -> 1.50

if(map.containsKey(dept))

price = map.get(dept);

else

price = map.get(null);

P.S. - I am not "Department-ist" if that's a word. :)

Answer 4

There's an API call for this:

size: Returns the number of key-value mappings in this map.

hmap.size();

As noted you're just overwriting the key/value pair with a new value.