向上取整：Math.ceil(double a) 
向下取整：Math.floor(double a) 
四舍五入取整：Math.round(double a) 

1、向输出上取整

    public static void mathCeil() {
        Double number = 5.3;
        // Double 向上取整
        double ceil = Math.ceil(number);
        System.out.println("number:" + number);
        System.out.println("ceil:" + ceil);
        int ceilRes = new Double(ceil).intValue();
        System.out.println("ceilRes:" + ceilRes);
    }

输出：

number:5.3
ceil:6.0
ceilRes:6

2、向下取整

    public static void mathFloor () {
        Double number = 5.9;
        // Double 向下取整
        double floor = Math.floor(number);
        System.out.println("number:" + number);
        System.out.println("floor:" + floor);
        int floorRes = new Double(floor).intValue();
        System.out.println("floorRes:" + floorRes);
    }

输出：

number:5.9
floor:5.0
floorRes:5

3、四舍五入

    public static void mathRound() {
        Double number1 = 5.9;
        Double number2 = 5.3;
        // Double number1 四舍五入
        double round1 = Math.round(number1);
        System.out.println("number1:" + number1);
        System.out.println("round1:" + round1);
        int round1Res = new Double(round1).intValue();
        System.out.println("round1Res:" + round1Res);
        // Double number2 四舍五入
        double round2 = Math.round(number2);
        System.out.println("number1:" + number2);
        System.out.println("round2:" + round2);
        int round2Res = new Double(round2).intValue();
        System.out.println("round2Res:" + round2Res);
    }

输出：

number1:5.9
round1:6.0
round1Res:6

number1:5.3
round2:5.0
round2Res:5