java - Why is there a transformation of hashcode to get the hash and is it a good idea for all keys? -


this question has answer here:

i see implementation of hashmap applies kind of transformation hashcode actual hash.
1 please me understand how transformation works and additionally if make difference if object store integer?

as taken javadoc of method hash(object) in openjdk java 8 hashmap implementation (assuming jvm concerned about):

 /**  * computes key.hashcode() , spreads (xors) higher bits of hash  * lower.  because table uses power-of-two masking, sets of  * hashes vary in bits above current mask  * collide. (among known examples sets of float keys  * holding consecutive whole numbers in small tables.)   * apply transform spreads impact of higher bits  * downward. there tradeoff between speed, utility, ,  * quality of bit-spreading. because many common sets of hashes  * reasonably distributed (so don't benefit  * spreading), , because use trees handle large sets of  * collisions in bins, xor shifted bits in  * cheapest possible way reduce systematic lossage,  * incorporate impact of highest bits otherwise  * never used in index calculations because of table bounds.  */ static final int hash(object key) {     int h;     return (key == null) ? 0 : (h = key.hashcode()) ^ (h >>> 16); }

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