We want a hash function to be easy to compute and to avoid clustering , the hashing of many keys to the same value.
One hash function that is usually good is to treat the key (e.g., a string of characters) as an integer and find the remainder modulo a prime p:
int hash = key % p;Since this produces an integer from 0 to p - 1, we make the array size p.
The Java .hashCode() for String is roughly: [from Wikipedia.]
public static int hashCode(String input) {
int h = 0;
int len = input.length();
for (int i = 0; i < len; i++) {
h = 31 * h + input.charAt(i);
}
return h; }