HashMaps and Null values

Question

How do you pass in null values into a HashMap  The following code snippet works with options filled in     HashMap lt String  String gt  options   new HashMap lt String  String gt       options put  name    value    Person person   sample searchPerson options     System out println Person getResult   get o  get Id          So the issue is what has to be entered into the options and or method to pass in a null value  I tried the following code without any success       options put null  null     Person person   sample searchPerson null        options put              Person person   sample searchPerson null        options put  name           Person person   sample searchPerson null      options put      Person person   sample searchPerson

User · Answer

HashMap supports both null keys and values  http   docs oracle com javase 6 docs api java util HashMap html         and permits null values and the null key   So your problem is probably not the map itself

User · Answer

You can keep note of below possibilities   1  Values entered in a map can be null   However with multiple null keys and values it will only take a null key value pair once   Map lt String  String gt  codes   new HashMap lt String  String gt      codes put null  null   codes put null null   codes put  C1    Acathan     for String key codes keySet         System out println key       System out println codes get key        output will be    null   key  of the 1st entry null   value of 1st entry C1 Acathan   2  your code will execute null only once  options put null  null     Person person   sample searchPerson null        It depends on the implementation of your searchPerson method  if you want multiple values to be null  you can implement accordingly  Map lt String  String gt  codes   new HashMap lt String  String gt          codes put null  null       codes put  X1  null       codes put  C1    Acathan        codes put  S1  null         for String key codes keySet             System out println key           System out println codes get key            output   null null  X1 null S1 null C1 Acathan

User · Answer

you can probably do it like this   String k   null  String v   null  options put k v

User · Answer

It seems that you are trying to call a method with a Map parameter  So  to call with an empty person name the right approach should be   HashMap lt String  String gt  options   new HashMap lt String  String gt     options put  name   null     Person person   sample searchPerson options     Or you can do it like this  HashMap lt String  String gt  options   new HashMap lt String  String gt     Person person   sample searchPerson options     Using  Person person   sample searchPerson null     Could get you a null pointer exception  It all depends on the implementation of searchPerson   method

User · Answer

Acording to your first code snipet seems ok  but I ve got similar behavior caused by bad programing  Have you checked the  options  variable is not null before the put call   I m using Struts2  2 3 3  webapp and use a HashMap for displaying results  When is executed  in a class initialized by an Action class     if value    null  pdfMap put  date  value toString     else pdfMap put  date         Got this error   Struts Problem Report  Struts has detected an unhandled exception   Messages     File    aoc psisclient samples PDFValidation java Line number     155 Stacktraces  java lang NullPointerException     aoc psisclient samples PDFValidation getRevisionsDetail PDFValidation java 155      aoc action signature PDFUpload execute PDFUpload java 66      sun reflect NativeMethodAccessorImpl invoke0 Native Method      sun reflect NativeMethodAccessorImpl invoke NativeMethodAccessorImpl java 39            Seems the NullPointerException points to the put method  Line number 155   but the problem was that de Map hasn t been initialized before  It compiled ok since the variable is out of the method that set the value

User · Answer

Its a good programming practice to avoid having null values in a Map   If you have an entry with null value  then it is not possible to tell whether an entry is present in the map or has a null value associated with it   You can either define a constant for such cases  Example  String NOT VALID     NA    or you can have another collection storing keys which have null values   Please check this link for more details

[java] HashMaps and Null values?

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