finding a hash function for long integer array

Question

I am looking for a hash function for an integer array containing about 17 integers each. There are about 1000 items in the HashMap and I want the computation to be as fast as possible.

I am now kind of confused by so many hash functions to choose and I notice that most of them are designed for strings with different characters. So is there a hash function designed for strings with only numbers and quick to run?

Thanks for your patience!

Answer 1

You did not specify any requirements (except speed of calculation), but take a look at java.util.Arrays#hashCode. It should be fast, too, just iterating once over the array and combining the elements in an int calculation.

Returns a hash code based on the contents of the specified array. For any two non-null int arrays a and b such that Arrays.equals(a, b), it is also the case that Arrays.hashCode(a) == Arrays.hashCode(b).

The value returned by this method is the same value that would be obtained by invoking the hashCode method on a List containing a sequence of Integer instances representing the elements of a in the same order. If a is null, this method returns 0.

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And the hashmap accepts an array of integer as the key.

Actually, no!

You could technically use int[] as a key in a HashMap in Java (you can use any kind of Object), but that won't work well, as arrays don't define a useful hashCode method (or a useful equals method). So the key will use object identity. Two arrays with identical content will be considered to be different from each-other.

You could use List<Integer>, which does implement hashCode and equals. But keep in mind that you must not mutate the list after setting it as a key. That would break the hashtable.

Answer 2

hashmap functions can be found in https://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html

Creating a hashmap is easy.. it goes as

HashMap<Object, Integer> map = new HashMap<Object, Integer>();