Java String中的hashCode函数

String 类中的hash函数如下：

public int hashCode() {
  int h = hash;
  if (h == 0) {
      int off = offset;
      char val[] = value;
      int len = count;            for (int i = 0; i < len; i++) {
                 h = 31*h + val[off++];
             }
             hash = h;
         }
         return h;
     }

 

 

具体的计算公式是：s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

 

 

上面的代码可见简洁和高效。JDK的源代码。

 

这个hash算法的名称叫：BKDR Hash Function

 

This hash function comes from Brian Kernighan and Dennis Ritchie's book "The C Programming Language". It is a simple hash function using a strange set of possible seeds which all constitute a pattern of 31....31...31 etc, it seems to be very similar to the DJB hash function.

 

算法来自于Brian Kernighan and Dennis Ritchie合著的C语言编程一书。函数用一个特殊的种子集合构建哈希函数

 

该算法代码：

public long BKDRHash(String str)
    {
       long seed = 131; // 31 131 1313 13131 131313 etc..
       long hash = 0;      for(int i = 0; i < str.length(); i++)
       {
          hash = (hash * seed) + str.charAt(i);
       }      return hash;
    }
    /* End Of BKDR Hash Function */

 

参考路径：http://www.partow.net/programming/hashfunctions/index.html