HashMap assignment and null pointer exception

[merik]

Here is some simple piece of 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?

[merik]

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".

[DarrylBurke]

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