绝对值函数使用说明
绝对值函数是JDK中Math.java中的实现方法，其用来得到表达式的绝对值。
其实现非常简单，源码如下：
/**
* Returns the absolute value of an int value.
* If the argument is not negative, the argument is returned.
* If the argument is negative, the negation of the argument is returned.
*
* <p>Note that if the argument is equal to the value of
* Integer#MIN_VALUE, the most negative representable
* int value, the result is that same value, which is
* negative.
*
* @param a the argument whose absolute value is to be determined
* @return the absolute value of the argument.
*/
public static int abs(int a) {
return (a < 0) ? -a : a;
}