java的几种保留小数位数的方法

java的几种保留小数位数的方法

法一：DecimalFormat

public class DecimalFormatDemo {
    public static void main(String[] args) {
        double decimal=2.15545422;
        DecimalFormat df=new DecimalFormat("#.0000");
        String format = df.format(decimal);
        double v = Double.parseDouble(format);
        System.out.println(v);
    }
}

法二：String.format

public class DecimalFormatDemo {
    public static void main(String[] args) {
        double decimal=2.15555422;
        String format = String.format("%.3f", decimal);
        double v = Double.parseDouble(format);
        System.out.println(v);
    }
}

法三：NumberFormat

public class DecimalFormatDemo {
    public static void main(String[] args) {
        double decimal=2.15555422;
        /*DecimalFormat df=new DecimalFormat("#.0000");
        String format = df.format(decimal);
        double v = Double.parseDouble(format);
        System.out.println(v);*/
        /*String format = String.format("%.3f", decimal);
        double v = Double.parseDouble(format);
        System.out.println(v);*/
        NumberFormat nf=NumberFormat.getNumberInstance();
        nf.setMaximumFractionDigits(3);
        String format = nf.format(decimal);
        double v = Double.parseDouble(format);
        System.out.println(v);
    }
}

法四：BigDecimal

ROUND_UP 只要保留位数之后有一位为0就进位，且与正负无关

public class DecimalFormatDemo {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal(3.6666666666);
        BigDecimal b = new BigDecimal(-3.650000001);
        System.out.println(a.setScale(2,BigDecimal.ROUND_UP));
        System.out.println(b.setScale(2,BigDecimal.ROUND_UP));
    }
}
 3.660000001——>3.67
 3.666666666——>3.67
 -3.65000001——>-3.66
 -3.65000000——>3.65

ROUND_DOWN 向下保留，不管保留位数后面是多少，统统舍去

public class DecimalFormatDemo {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal(3.66666666);
        BigDecimal b = new BigDecimal(-3.66666666);
        System.out.println(a.setScale(2,BigDecimal.ROUND_DOWN));
        System.out.println(b.setScale(2,BigDecimal.ROUND_DOWN));
    }
}
3.66
-3.66

ROUND_CEILING 向正无穷方向舍入

public class DecimalFormatDemo {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal(3.66666666);
        BigDecimal b = new BigDecimal(-3.66666666);
        System.out.println(a.setScale(2,BigDecimal.ROUND_CEILING));
        System.out.println(b.setScale(2,BigDecimal.ROUND_CEILING));
    }
}
3.67
-3.66

ROUND_FLOOR 向负无穷方向舍入

public class DecimalFormatDemo {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal(3.66666666);
        BigDecimal b = new BigDecimal(-3.66666666);
        System.out.println(a.setScale(2,BigDecimal.ROUND_FLOOR));
        System.out.println(b.setScale(2,BigDecimal.ROUND_FLOOR));
    }
}
3.66
-3.67

ROUND_HELF_UP：数字保留法（四舍五入）

public class DecimalFormatDemo {
    public static void main(String[] args) {
        BigDecimal a = new BigDecimal(3.66666666);
        BigDecimal b = new BigDecimal(-3.66666666);
        System.out.println(a.setScale(2,BigDecimal.ROUND_HALF_DOWN));
        System.out.println(b.setScale(2,BigDecimal.ROUND_HALF_DOWN));
    }
}
3.67
-3.67