Java 8 BigDecimal multiply 方法在倒数时丢失精度

Question

Java 8 在这里。根据 Google:

将磅换算成千克：multiply by 0.453592
将千克换算成磅：multiply by 2.20462
将英寸换算为米：multiply by 0.0254
将米换算成英寸：multiply by 39.3701

所以我编写了这个 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：

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.

虽然我期待它打印出来：

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.

我只关心英制<->公制转换精确到小数点后两位，最终输出始终精确到小数点后两位。 谁能看出我哪里出错了，解决方法是什么？

Answer 1

乘法的 SCALE 和 HALF_UP 参数不是吗？您只将它添加到 BigDecimal。

我想你想要：

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

将 SCALE 更改为 8 并将您的代码与 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 2

您的转换系数已关闭。您将转换系数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，"0,0254" 向上舍入为 0.03，这不适合您的 "reverse" 转换39.37 的因数。这就是为什么您的逆转不等于原始值的原因。

Answer 3

你把括号弄乱了。考虑一下：

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

在这里，您首先在 2.20462 上设置比例，结果是 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));

在这里你有效地乘以 0.45。 2.2*0.45=0.99 解释了结果。

您设置了乘法结果的比例，而不是乘法器。基本上最后一个括号在错误的地方。它应该是这样的：

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

这是 corrected code。

Answer 4

您正在转换过程中进行舍入。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"));
}

然后您可以简单地将输出格式化为您想要的格式：

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，如果您想显示带小数位的值，您只需将 formatOutput 方法更改为：

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

这将按以下方式打印出值（我认为它更好更正确！）

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.

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