如何手动计算字符串的哈希码？

看一下的源代码

java.lang.String

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