• 四舍五入保留整数

通过使用+0.5后强制类型转换来实现

(int)(num1 + 0.5)

public class Main {
    static int sum = 0;
    public static void main(String[] args) {
        double num1 = 6.56468;
        double num2 = 2.185658;
        double num3 = 1.8232;
        double num4 = 1.9986778;
        //四舍五入且保留整数
        System.out.println((int)(num1 + 0.5));
        System.out.println((int)(num2 + 0.5));
        System.out.println((int)(num3 + 0.5));
        System.out.println((int)(num4 + 0.5));
    }

}

7
2
2
2

•  四舍五入保留整数

同时使用Math.round()函数来实现

public class Main {
    static int sum = 0;
    public static void main(String[] args) {
        double num1 = 6.56468;
        double num2 = 2.185658;
        double num3 = 1.8232;
        double num4 = 1.9986778;
        System.out.println(Math.round(num1));
        System.out.println(Math.round(num2));
        System.out.println(Math.round(num3));
        System.out.println(Math.round(num4));

    }

}

7
2
2
2

• 四舍五入保留任意位数

DecimalFormat df=new DecimalFormat("0.000");

构建此类，来确定保留位数

public class Main {
    static int sum = 0;
    public static void main(String[] args) {
        double num1 = 6.56468;
        double num2 = 2.185658;
        double num3 = 1.8232;
        double num4 = 1.9986778;
        
        DecimalFormat df=new DecimalFormat("0.000");//表示位数
        System.out.println(df.format(num1));
        System.out.println(df.format(num2));
        System.out.println(df.format(num3));
        System.out.println(df.format(num4));
    }

}

6.565
2.186
1.823
1.999