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<String, String> options = new HashMap<String, String>();  
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();

What does "without any success" imply? What's the problem? Do you get an error message? — Till Helge, Feb 26 '13 at 14:09
Learn how to use a Map separately, then integrate it into your program. http://docs.oracle.com/javase/6/docs/api/java/util/Map.html — Christophe Roussy, Feb 26 '13 at 14:22
why we are not getting null pointer exception after adding null as key in the map? — Amit, Jan 25 '16 at 13:08
`" "` in not a `null` string, it is a string containing a single space character. So to search for that use `" "`. `""` is also not null, it in an empty string. Only `null` is the correct pointer to null. So `put.("name", null);` — Sheldon, Jul 18 '17 at 12:24
Ugly, but perhaps create a `Map`. When I see `HashMap` on the left hand side of a variable declaration my inclination is to remove the "Hash". — Sridhar Sarnobat, Aug 02 '17 at 21:24

score 139 · Answer 1 · edited Jul 20 '17 at 09:57

139

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.

edited Jul 20 '17 at 09:57

Andrew Tobilko

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answered Feb 26 '13 at 14:24

kenor

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86

Just adding that ConcurrentHashMap does not allow null to be used as a key or value. – alfonx Jul 22 '14 at 14:14

score 14 · Answer 2 · edited Aug 29 '16 at 14:35

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<String, String> codes = new HashMap<String, String>();

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<String, String> codes = new HashMap<String, String>();

    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

score 4 · Answer 3 · answered Nov 28 '14 at 06:09

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<String, String> options = new HashMap<String, String>();
options.put("name", null);  
Person person = sample.searchPerson(options);

Or you can do it like this

HashMap<String, String> options = new HashMap<String, String>();
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.

score 0 · Answer 4 · answered Jun 28 '17 at 07:24

0

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.

answered Jun 28 '17 at 07:24

Pritesh Mhatre

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Yes, though when refactoring bad code and wanting to preserve null behavior you may still need to insert null into a Map. – Sridhar Sarnobat Aug 02 '17 at 21:22
9

`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.`: what about `Map.containsKey`? Or iterating over all entries of map and checking their `Map.Entry.getKey()`? – izogfif Mar 15 '18 at 11:56

Mahesh Shivaiah · Answer 5 · 2022-04-12T10:10:08.450

0

i was getting hashmap with null values like this

{fieldCode1=F0001, fieldId10=null, fieldId11=null }

i used the method from using Guava library using below link https://www.techiedelight.com/remove-null-values-map-java/

Iterables.removeIf(inputParams.values(), Predicates.isNull());

inputParams is the hashmap

Hashmap value after using the method {fieldCode1=F0001}

Use the below import packages import com.google.common.collect.Iterables; import com.google.common.base.Predicates;

edited Apr 12 '22 at 10:10

answered Apr 10 '22 at 12:24

Mahesh Shivaiah

1
2

Hey, the question already has an answer and you answer does not seem to add any more information, especially because the OP doesn't seem to use Guava as you suggested. You might have a look at https://stackoverflow.com/help/how-to-answer – Moritz Apr 10 '22 at 14:29

score -2 · Answer 6 · answered Feb 14 '14 at 12:18

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.

The question does not seem to be related to Struts. – Eric Tobias May 24 '16 at 09:29 — Eric Tobias, May 24 '16 at 09:29

score -7 · Answer 7 · answered May 13 '14 at 17:54

-7

you can probably do it like this:

String k = null;
String v = null;
options.put(k,v);

answered May 13 '14 at 17:54

user2337320

1

HashMaps and Null values?

7 Answers7

1. Values entered in a map can be `null`.

2. your code will execute `null` only once

Linked

HashMaps and Null values?

7 Answers7

1. Values entered in a map can be null.

2. your code will execute null only once

Linked

1. Values entered in a map can be `null`.

2. your code will execute `null` only once