Java 8 BigDecimal 乘法方法在反转时丢失精度

Question

这里是 Java 8。据谷歌称：

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

所以我编写了这段 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));
    }
}

当它运行时，会打印出：

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.

而我期待它打印出来：

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.

我所关心的是英制<->公制转换精确到小数点后两位，并且最终输出始终精确到小数点后两位。谁能看到我哪里出了问题以及解决方法是什么？

Answer 1

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，您首先设置2.20462结果的比例2.20，然后相乘。

现在考虑逆向转换：

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

在这里你有效地乘以0.45。这2.2*0.45=0.99解释了结果。

您已在乘法结果上设置了比例，而不是在乘法器上设置了比例。基本上最后一个支架放错了地方。它应该是这样的：

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

这是更正后的代码。