Question

Why the capacity must be a multiple or 2?
Why use “&” in the indexFor functions?
Why recompute the hash in the hash function instead of directly using the key’s hash code?

I think there are some important differences between the this implementation and the description on the “Introduction to Algorithm”.

What does “>>>” mean?

static int hash(int h) {
        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
}

Can anyone give me some guide ? I appreciate If some one can explain the hash algorithm.
Thanks a lot!

Answer 1

This is a performance optimization. The usual way to map a hash code to a table index is

table_index = hash_code % table_length;

The % operator is expensive. If table_length is a power of 2, then the calculation:

table_index = hash_code & (table_length - 1);

is equivalent to the (much) more expensive modulo operation.