详细说明：
Math.abs()的实现源码
通过三目运算符判断a是否小于0来实现
/**
* Returns the absolute value of an {code int} value.
* If the argument is not negative, the argument is returned.
* If the argument is negative, the negation of the argument is returned.
*
* Note that if the argument is equal to the value of
*
