1、public static double abs(double num):获取绝对值，有多种重载

2、public static double ceil(double num):向上取整

3、public static double floor(double num):向下取整

4、public static long round(double num):四舍五入

Math.PI ---近似圆周率

public class DemoMath {
    public static void main(String[] args) {

        System.out.println(Math.abs(1.1));
        System.out.println(Math.abs(-1.1));
        System.out.println("======== 1 ========");
        System.out.println(Math.ceil(1.1));
        System.out.println(Math.ceil(1.8));
        System.out.println(Math.ceil(-1.1));
        System.out.println(Math.ceil(-1.8));
        System.out.println("======== 2 ========");
        System.out.println(Math.floor(1.1));
        System.out.println(Math.floor(1.8));
        System.out.println(Math.floor(-1.1));
        System.out.println(Math.floor(-1.8));
        System.out.println("======== 3 ========");
        System.out.println(Math.round(1.1));
        System.out.println(Math.round(1.8));
        System.out.println(Math.round(-1.1));
        System.out.println(Math.round(-1.8));
        System.out.println("======== 3 ========");
        System.out.println(Math.PI);
    }
}

输出结果：

1.1
1.1
======== 1 ========
2.0
2.0
-1.0
-1.0
======== 2 ========
1.0
1.0
-2.0
-2.0
======== 3 ========
1
2
-1
-2
======== 3 ========
3.141592653589793