What is the use of adding a null key or value to a HashMap in Java?

Question

HashMap allows one null key and any number of null values. What is the use of it?

Answer 1

The answers so far only consider the worth of have a null key, but the question also asks about any number of null values.

The benefit of storing the value null against a key in a HashMap is the same as in databases, etc - you can record a distinction between having a value that is empty (e.g. string ""), and not having a value at all (null).

Answer 2

One example would be for modeling trees. If you are using a HashMap to represent a tree structure, where the key is the parent and the value is list of children, then the values for the null key would be the root nodes.

Answer 3

I'm not positive what you're asking, but if you're looking for an example of when one would want to use a null key, I use them often in maps to represent the default case (i.e. the value that should be used if a given key isn't present):

Map<A, B> foo;
A search;
B val = foo.containsKey(search) ? foo.get(search) : foo.get(null);

HashMap handles null keys specially (since it can't call .hashCode() on a null object), but null values aren't anything special, they're stored in the map like anything else

Answer 4

A null key can also be helpful when the map stores data for UI selections where the map key represents a bean field.

A corresponding null field value would for example be represented as "(please select)" in the UI selection.

Answer 5

One example of usage for null values is when using a HashMap as a cache for results of an expensive operation (such as a call to an external web service) which may return null.

Putting a null value in the map then allows you to distinguish between the case where the operation has not been performed for a given key (cache.containsKey(someKey) returns false), and where the operation has been performed but returned a null value (cache.containsKey(someKey) returns true, cache.get(someKey) returns null).

Without null values, you would have to either put some special value in the cache to indicate a null response, or simply not cache that response at all and perform the operation every time.

Answer 6

Another example : I use it to group Data by date. But some data don't have date. I can group it with the header "NoDate"