java - 使用 Jscience 计算 n 个单位的价格(提供单价和数量)

Question

我正在寻找一种更好的方法来使用 Jscience 进行计算，但通用模式很难获得清晰的解决方案。如果定义了单位数量的单价，我需要计算 n 个单位的价格。就像，

    Measure<Double, ? extends Quantity> unitQuantity = Measure.valueOf(1.0,
            Unit.valueOf("kg"));
    Amount<Money> unitPrice = Amount.valueOf(150.0, USD);
    Measure<Double, ? extends Quantity> quantity = Measure.valueOf(500.0,
            MILLI(Unit.valueOf("kg")));
    Amount<Money> amount = unitPrice.times(quantity.to(unitQuantity.getUnit())
                       .getValue() / unitQuantity.getValue());

对于上面的代码，我收到此错误:

The method to(Unit<capture#7-of ? extends Quantity>) in the type 
Measure<Double,capture#7-of ? extends Quantity> is not applicable for 
the arguments (Unit<capture#8-of ? extends Quantity>)

我在 Java Tutorials 阅读了有关泛型的内容并尝试了以下方法，但仍然没有明确的解决方案:

    Amount<Money> amount = compute(unitPrice,unitQuantity,quantity)

    private <T extends Quantity> Amount<Money> compute(Amount<Money> unitPrice,
        Measure<Double, T> unitQuantity, Measure<Double, T> quantity) {
    return unitPrice.times(quantity.to(unitQuantity.getUnit()).getValue()
            / unitQuantity.getValue());
}

现在收到此错误:

   The method compute(Amount<Money>, Measure<Double,T>, Measure<Double,T>) 
   in the type JscienceEx is not applicable for the arguments (Amount<Money>, 
   Measure<Double,capture#7-of ? extends Quantity>, Measure<Double,capture#8-of ?    
   extends Quantity>)

Answer 1

Unit.valueOf("kg")工厂方法在某些情况下可能很方便。但在这种情况下，它没有保留足够的信息。它仅返回“某些”数量的单位 - 因此 ? extends Quantity在返回类型中。但你不知道这是否是数量，例如Mass或Power 。而只要你不知道这一点，你就无法判断是否调用了quantity.to(unitQuantity.getUnit())有效。

在这种情况下，您可以使用 new BaseUnit<Mass>("kg") 来解决此问题而不是 Unit.valueOf("kg") :

import static javax.measure.unit.SI.MILLI;

import javax.measure.Measure;
import javax.measure.quantity.Mass;
import javax.measure.unit.BaseUnit;

import org.jscience.economics.money.Currency;
import org.jscience.economics.money.Money;
import org.jscience.physics.amount.Amount;

public class JScienceUnits
{
    public static void main(String[] args)
    {
        BaseUnit<Mass> kg = new BaseUnit<Mass>("kg");
        Measure<Double, Mass> unitQuantity = Measure.valueOf(1.0, kg);
        Amount<Money> unitPrice = Amount.valueOf(150.0, Currency.USD);
        Measure<Double, Mass> quantity = Measure.valueOf(500.0, MILLI(kg));
        Amount<Money> amount = unitPrice.times(
            quantity.to(unitQuantity.getUnit()).getValue() / 
            unitQuantity.getValue());

        System.out.println(
            "Money for "+quantity+
            " with unit price "+unitPrice+
            " is "+amount);
    }
}