public final class Math extends
public static double floor(double a)
public static long round(double a)
public static int round(float a)
public static double ceil(double a)
floor 返回不大于的最大整数;
ceil 则是不小于他的最小整数;
round 它表示“四舍五入”，算法为Math.floor(x+0.5)，即将原来的数字加上0.5后再向下取整;
所以，Math.round(11.5)的结果为12，Math.round(-11.5)的结果为-11;
| Math.floor | Math.round | Math.ceil | |
1.4 | 1.0 | 1 | 2.0 |
1.5 | 1.0 | 2 | 2.0 |
1.6 | 1.0 | 2 | 2.0 |
-1.4 | -2.0 | -1 | -1.0 |
-1.5 | -2.0 | -1 | -1.0 |
-1.6 | -2.0 | -2 | -1.0 |