hashmap的hash算法( 转)

int hash = hash(key.hashCode());  
int i = indexFor(hash, table.length);  

/** 
 * 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. 
 */  
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);  
}  
  
/** 
 * Returns index for hash code h. 
 */  
static int indexFor(int h, int length) {  
    return h & (length-1);  
}  

h ^= (h >>> 20) ^ (h >>> 12);  
return h ^ (h >>> 7) ^ (h >>> 4);