Thread: Trying to test some basic hash functions using generic LinkedList and I'm having null pointers

Here I have an array of LinkedLists of the string type. I'm trying to store the string in the hash code index of my array but I'm getting a Null Pointer exception when i try to add it. Does this have to do with my casting or am I implementing this whole thing incorrectly. Any help would be greatly appreciated. Here is the code:

public class Hash {
 
	public static void main (String[] args){
		//Scanner input=new Scanner(System.in);
 
		//System.out.println("Enter your string: ");
		String s="aaba1xxfsf";
		int h=Math.abs(s.hashCode())%100000;
		System.out.println("hash code is: "+h);
		@SuppressWarnings("unchecked")
		LinkedList<String>[] list=(LinkedList<String>[])new LinkedList[10000];
		list[h].add(s);//Null pointer exception happens here
		for(LinkedList<String> i: list){
		    if(i!=null)
			System.out.println("Your string is at index "+i);
 
		}
	}
 
}

It all has to do with your creating an array of a reference type but adding no objects to the array, having essentially an array of null. Think of reference arrays as parking spaces in a parking lot. You can't use any cars until you add them them to the lot first.

I thought I was adding something to the array using list[h].add(s). I figured this would add the first element to that LinkedList and thus the array wouldn't be null at that value in the array. So I have to add a list to the array before I can add an element to the list?? Update: I added this line: list[h]=new LinkedList<String>(); before my add statement and it ran correctly. Thanks for the help. I guess I'm still having trouble with the theory of why it creates a blank array rather than an array of LinkedLists.

You are creating an array of object, and it doesn't matter what that object is, nothing will be held by that array except null until you put something in. Doing this, list[h].add(s) doesn't add anything to the array. All it does is add to a linked list that doesn't yet exist, that is not yet held by the array and hence your null.

On a side note, using a hashCode as an index for an array seems madness to me. Are you sure that you wouldn't rather use a Map instead?

It's for a data structures course and we're using the hash code to place various strings and sub strings into an array of linked lists so we can manipulate the output. It might not be the best example of how to use hash codes, but we have to write the hash function and table ourselves and for whatever reason this is what we have to do. I appreciate the help though.