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!
>>> is unsigned right shift. Regular >> in Java will preserve and propagate the sign bit and leave a negative number negative. >>> will fill the sign bit with zeros as shifting occurs - Hot Licks 2012-04-04 16:10
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.
hash_code % table_length could be negative, resulting in an ArrayOutOfBoundsException. You need to use Math.abs(hash_code % table_length) to get a usable (ie safe) inde - Bohemian 2012-04-03 23:20
Pay no attention to the man behind the curtain.
The actual algorithm is no doubt a combination of "what feels good" to the developer, fixes for some odd degenerate cases, and simple tradition (for which users often develop obscure dependencies).
And note this:
* Applies a supplemental hash function to a given hashCode, which * defends against poor quality hash functions. This is critical * because HashMap uses power-of-two length hash tables, that * otherwise encounter collisions for hashCodes that do not differ * in lower bits. Note: Null keys always map to hash 0, thus index 0.
Net: So long as it works and the performance is good, you don't care.