HashMap assignment and null pointer exception

Here is some simple piece of code:

Code:

---

import java.io.IOException;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.HashMap;

public class InvertedIndex implements Serializable {

        private HashMap<String, ArrayList<Integer>> index;
        private static final long serialVersionUID = 1L;

        /**
        * @param token
        * @param id
        */
        public void add(String token, int id) {
                ArrayList<Integer> al = new ArrayList<Integer>();
                al.add(id);
                System.out.println(al.toString());
                System.out.println(token);
                index.put(token, al);
        }
       
        public void print(){
                System.out.println(index.toString());
        }

        /**
        * @param args
        * @throws IOException
        */
        public static void main(String[] args) {
                String[] words = new String[4];
                words[0] = "one";
                words[1] = "two";
                words[2] = "other";
                words[3] = "two";
                InvertedIndex ii = new InvertedIndex();
                for(int i = 0; i< words.length; i++){
                        ii.add(words[i],i);
                }
        }

}

---

When I execute it, I get a null pointer exception (which is by far the least informative exception I've seen in Java :p ) and I can't understand why. The exception is triggered when the index.put(token, al); is executed.

Can you please guide me?

Well, I found the problem .. my HashMap was not initialized. But I wish there was a day when Java would return "object not initialized" exception instead of "null pointer exception".

Ah, but a NullPointerException doesn't necessarily arise out of forgetting to initialize a reference. A method can return null, and a NPE will be thrown anywhere you attempt to dereference the returned value.

db