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 options = new HashMap         


        

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

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.