hashmap的hash算法( 转)

HashMap 中hash table 定位算法：

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

其中indexFor和hash源码如下：

/**  * 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);  }  

现在分析一下hash算法：

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