最为典型的就是java.lang.String的hashcode实现
/**
* Returns a hash code for this string. The hash code for a
* {@code String} object is computed as
* <blockquote><pre>
* s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
* </pre></blockquote>
* using {@code int} arithmetic, where {@code s[i]} is the
* <i>i</i>th character of the string, {@code n} is the length of
* the string, and {@code ^} indicates exponentiation.
* (The hash value of the empty string is zero.)
*
* @return  a hash code value for this object.
*/
public int hashCode() {
   int h = hash;
   if (h == 0 && value.length > 0) {
       char val[] = value;

       for (int i = 0; i < value.length; i++) {
           h = 31 * h + val[i];
       }
       hash = h;
   }
   return h;
} 

参考注释,hash计算公式可以计为s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1],  
n为字符串长度,s[]即为String源码中其内部维护的char类型的数组val[],  
推导如下:如果字符串长度 n = 3
i = 0 -> h = 31 * 0 + val[0] = val[0];
i = 1 -> h = 31 * (31 * 0 + val[0]) + val[1] =  31 * val[0] + val[1];
i = 2 -> h = 31 * (31 * (31 * 0 + val[0]) + val[1]) + val[2] = 31^2 * val[0] +31 * val[1] + val[2];