Null pointer exception with static hashMap

Question

 public class arraylst
 {
   static HashMap<String,List<String>>hm;
   public static void main(String[] args)
   {
       hm.put("2",Arrays.asList("a","b","c"));
   }
 }

I don't understand why this causes NullPointerException.

Can someone please help me out?

Because hm is null -- you never assign an object to it. Now, let's please close this question. — user166390
– user166390, Commented Dec 27, 2011 at 4:08

Ted Hopp · Accepted Answer · 2011-12-27 04:09:06Z

4

You need to set hm:

hm = new HashMap<String, List<String>>();

before you use it.

answered Dec 27, 2011 at 4:09

Ted Hopp

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Comments

SLaks · Accepted Answer · 2011-12-27 04:08:28Z

2

You need to put a HashMap<String,List<String>> into your (initially empty) hm field.

answered Dec 27, 2011 at 4:08

SLaks

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jaychapani · Accepted Answer · 2011-12-27 04:10:55Z

2

Use

static HashMap<String, List<String>> hm = new HashMap<String, List<String>>();

answered Dec 27, 2011 at 4:10

jaychapani

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Comments

Aravind Yarram · Accepted Answer · 2011-12-27 04:10:54Z

1

Unlike primitive variables, the Classes should be explicitly initialized. So create an instance of HasMap.

answered Dec 27, 2011 at 4:10

Aravind Yarram

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Aruna Karunarathna · Accepted Answer · 2011-12-27 05:06:40Z

0

You are pointing to a reference which don't have a Object. So you are trying to refer a Object which haven't yet created.

So using the "new" key word create an Object to overcome the exception

static HashMap<String,List<String>> hm = new HashMap<String,List<String>>();

answered Dec 27, 2011 at 5:06

Aruna Karunarathna

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