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

User · Accepted Answer

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 lt A  B gt  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Here s my only-somewhat-contrived example of a case where the null key can be useful   public class Timer       private static final Logger LOG   Logger getLogger Timer class       private static final Map lt String  Long gt  START TIMES   new HashMap lt String  Long gt          public static synchronized void start             long now   System currentTimeMillis            if  START TIMES containsKey null                 LOG warn  Anonymous timer was started twice without being stopped  previous timer has run for      now - START TIMES get null  longValue      ms                       START TIMES put null  now              public static synchronized long stop             if    START TIMES containsKey null                 return 0                     return printTimer  Anonymous   START TIMES remove null   System currentTimeMillis                public static synchronized void start String name            long now   System currentTimeMillis            if  START TIMES containsKey name                 LOG warn name     timer was started twice without being stopped  previous timer has run for      now - START TIMES get name  longValue      ms                       START TIMES put name  now              public static synchronized long stop String name            if    START TIMES containsKey name                 return 0                     return printTimer name  START TIMES remove name   System currentTimeMillis                private static long printTimer String name  long start  long end            LOG info name     timer ran for      end - start     ms            return end - start

User · Answer

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

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

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