空指针异常与静态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?

Answer 1

You need to set hm :

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

before you use it.

Answer 2

您需要将HashMap<String,List<String>>放入您的（最初为空） hm字段中。

Answer 3

采用

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

Answer 4

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

Answer 5

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>>();