Question

Here is an implementation of HashMap.

It provides this code for getting index of the bin:

private int getIndex(K key)
{
    int hash = key.hashCode() % nodes.length;
    if (hash < 0)
        hash += nodes.length;
    return hash;
}

To make sure the hash value is not bigger than the size of the table,
the result of the user provided hash function is used modulo the
length of the table. We need the index to be non-negative, but the
modulus operator (%) will return a negative number if the left operand
(the hash value) is negative, so we have to test for it and make it
non-negative.

If hash turns out to be very big negative value, the additions hash += nodes.length in the cycle may take a lot of processing.

I think there should be O(1) algorithm for it (independent of hash value).

If so, how can it be achieved?

Answer 1

It can’t be a very big negative number.

The result of anything % nodes.length is always less that nodes.length in absolute value, so you need a single if, not a loop. This is exactly what the code does:

if (hash < 0) /* `if', not `while' */
    hash += nodes.length;