math类是数学相关的工具类，里面提供了大量的静态方法，
java.util.math
public static double abs(double num), 获取绝对值。有很多种重载
public static double ceil(double num), 向上取整.
publkic static double floor(double num), 向下取整，
public static long round(double num);四舍五入.
!!!Math.PI代表近似的圆周率常亮（double）.
