String的 哈希码 计算如下：

String's hash code is computed as:

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

使用 int 算术，其中 s [i] 是字符串的 i -th字符， n 是字符串的长度， ^ 表示 exponentiation 。 （空字符串的哈希值为零。）

using int arithmetic, where s[i] is the i-th character of the string, n is the length of the string, and ^ indicates exponentiation. (The hash value of the empty string is zero.)

因此，这个整数计算的溢出很容易发生，根据 Java语言规范-15.8.2 ：

Hence, the overflow of this integer computation can easily occur, resulting in negative according to Java Language specification-15.8.2:

如果整数加法溢出，则结果是数学的低阶
位以一些足够大的
二进制补码格式表示的总和。如果发生溢出，那么
结果的符号与两个
操作数值的数学和的符号不同。

If an integer addition overflows, then the result is the low-order bits of the mathematical sum as represented in some sufficiently large two's-complement format. If overflow occurs, then the sign of the result is not the same as the sign of the mathematical sum of the two operand values.