Question

I’ve used an array as hash table for hashing alogrithm with values:

int[] arr={4 , 5 , 64 ,432 };

and keys with consective integers in array as:

int keys[]={ 1 , 2 , 3 ,4};

Could anyone please tell me, what would be the good approach in mapping those integers keys with those arrays location? Is the following a short and better approach with little or no collision (or something larger values)?

 keys[i] % arrlength  // where i is for different element of an array

Thanks in advance.

Answer 1

I assume you’re trying to implement some kind of hash table as an exercise. Otherwise, you should just use a java.util.HashMap or java.util.HashTree or similar.

For a small set of values, as you have given above, your solution is fine. The real question will come when your data grows much bigger.

You have identified that collisions are undesirable – that is true. Sometimes, some knowledge of the likely keys can help you design a good hash function. Sometimes, you can assume that the key class will have a good hash() method. Since hash() is a method defined by Object, every class implements it. It would be neatest for you to be able to utilise the hash() method of your key, rather than have to build a new algorithm specially for your map.

If all integer keys are equally likely, then a mod function will spread them out evenly amongst the different buckets, minimising collisions. However, if you know that the keys are going to be numbered consecutively, it might be better to use a List than a HashMap – this will guarantee no collisions.