Question

Java 8在这里。根据谷歌的说法：

将磅转换为千克：multiply by 0.453592
将公斤转换为磅：multiply by 2.20462
将英寸转换为米：multiply by 0.0254
将米转换为英寸：multiply by 39.3701

所以我掀起了这个小Java代码：

public class MeasurementConverter {
    private static final int SCALE = 2;

    public static void main(String[] args) {
        new MeasurementConverter().run();
    }

    private void run() {
        BigDecimal hundredLbs = new BigDecimal(100.00);
        BigDecimal hundredInches = new BigDecimal(100.00);

        System.out.println(hundredLbs + " pounds is " + poundsToKilos(hundredLbs) + " kilos and converts back to " + kilosToPounds(poundsToKilos(hundredLbs)) + " pounds.");
        System.out.println(hundredInches + " inches is " + inchesToMeters(hundredInches) + " meters and converts back to " + metersToInches(inchesToMeters(hundredInches)) + " inches.");
    }

    private BigDecimal metersToInches(BigDecimal meters) {
        return meters.multiply(new BigDecimal("39.3701").setScale(SCALE, BigDecimal.ROUND_HALF_UP));
    }

    private BigDecimal inchesToMeters(BigDecimal inches) {
        return inches.multiply(new BigDecimal("0.0254").setScale(SCALE, BigDecimal.ROUND_HALF_UP));
    }

    private BigDecimal kilosToPounds(BigDecimal kilos) {
        return kilos.multiply(new BigDecimal("2.20462").setScale(SCALE, BigDecimal.ROUND_HALF_UP));
    }

    private BigDecimal poundsToKilos(BigDecimal pounds) {
        return pounds.multiply(new BigDecimal("0.45359").setScale(SCALE, BigDecimal.ROUND_HALF_UP));
    }
}

运行时打印出来：

100 pounds is 45.00 kilos and converts back to 99.0000 pounds.
100 inches is 3.00 meters and converts back to 118.1100 inches.

虽然我期待打印出来：

100.00 pounds is 45.00 kilos and converts back to 100.00 pounds.
100.00 inches is 3.00 meters and converts back to 100.00 inches.

我所关心的是，Imperial＆lt; - ＆gt; Metric转换精确到2位小数，并且最终输出始终精确到2位小数。 任何人都可以看到我出错的地方以及解决方法是什么？

Answer 1

你的括号搞砸了。考虑一下：

kilos.multiply(new BigDecimal("2.20462").setScale(SCALE, BigDecimal.ROUND_HALF_UP));

首先，您在2.20462上设置比例，结果为2.20，然后相乘。

现在考虑反向转换：

pounds.multiply(new BigDecimal("0.45359").setScale(SCALE, BigDecimal.ROUND_HALF_UP));

在这里，您有效地乘以0.45。 2.2*0.45=0.99解释了结果。

您已在乘法结果上设置了比例，而不是在乘数上。基本上最后一个括号是在错误的地方。它应该是这样的：

pounds.multiply(new BigDecimal("0.45359")).setScale(SCALE, BigDecimal.ROUND_HALF_UP);

这是corrected code。

Answer 2

不是SCALE和HALF_UP参数的乘法？你只将它添加到BigDecimal。

我想你想要：

inches.multiply(new BigDecimal("0.0254"), new MathContext(SCALE, RoundingMode.HALF_UP));

将SCALE转换为8并将代码与MathContext一起使用导致：

100 pounds is 45.35900 kilos and converts back to 99.999359 pounds.
100 inches is 2.5400 meters and converts back to 100.00005 inches.

Answer 3

您的转化因素已关闭。您将转换因子四舍五入到小数点后两位：

private static final int SCALE = 2;
[...]    
new BigDecimal("0.0254").setScale(SCALE, BigDecimal.ROUND_HALF_UP)

因此，＆＃34; 0,0254＆＃34; 被四舍五入到 0.03 ，这不适合你的＆＃34;反向＆＃34 ;转换因子 39.37 。这就是为什么你的逆转不等于原始价值的原因。

Answer 4

您正在转换过程中进行四舍五入。因此，反向操作的输入值已经关闭。

如果要将值显示为：

100.00 pounds is 45.00 kilos and converts back to 100.00 pounds.
100.00 inches is 3.00 meters and converts back to 100.00 inches.

从您的所有私有方法中删除setScale。例如：

private BigDecimal metersToInches(BigDecimal meters) {
    return meters.multiply(new BigDecimal("39.3701"));
}

然后您可以使用所需格式简单地格式化输出：

private BigDecimal formatOutput(BigDecimal value) {
    return value.setScale(0, BigDecimal.ROUND_HALF_UP).setScale(SCALE);
}

最后：

System.out.println(hundredLbs + " pounds is " + formatOutput(poundsToKilos(hundredLbs)) + " kilos and converts back to " + formatOutput(kilosToPounds(poundsToKilos(hundredLbs))) + " pounds.");
System.out.println(hundredInches + " inches is " + formatOutput(inchesToMeters(hundredInches)) + " meters and converts back to " + formatOutput(metersToInches(inchesToMeters(hundredInches))) + " inches.");

给出：

100.00 pounds is 45.00 kilos and converts back to 100.00 pounds.
100.00 inches is 3.00 meters and converts back to 100.00 inches.

或者，如果要显示带小数位的值，只需将formatOutput方法更改为：

private BigDecimal formatOutput(BigDecimal value) {
    return value.setScale(SCALE, BigDecimal.ROUND_HALF_UP);
}

将以下列方式打印出值（我会说它更好更正确！）

100.00 pounds is 45.36 kilos and converts back to 100.00 pounds.
100.00 inches is 2.54 meters and converts back to 100.00 inches.

Java 8 BigDecimal乘法方法在被反转时失去精度

4 个答案: