Question

我正在寻找使用Jscience进行计算的更好方法，但通用模式更难以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。等，

    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阅读了关于泛型的内容并尝试了以下内容，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

在某些情况下，Unit.valueOf("kg")工厂方法可能很方便。但在这种情况下，它没有保留足够的信息。它只返回＆＃34; some＆＃34;的单位。数量 - 因此返回类型中为? extends Quantity。但是，您不知道这是否是数量，例如Mass或Power。只要您不知道这一点，就无法说出对quantity.to(unitQuantity.getUnit())的通话是否有效。

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

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);
    }
}

Answer 2

找到两种方法。转换失败时抛出ConversionException。

@SuppressWarnings("unchecked")
public static <U extends Quantity, V extends Quantity> Amount<Money> computeAmount(
        Measure<Double, U> unitQuantity, Amount<Money> unitRate,
        Measure<Double, V> netQuantity) {
    return unitRate.times(netQuantity.to((Unit<V>) unitQuantity.getUnit())
            .getValue() / unitQuantity.getValue());
}

-

public static <U extends Quantity, V extends Quantity> Amount<Money> computeAmount(
        Measure<Double, U> unitQuantity, Amount<Money> unitRate,
        Measure<Double, V> netQuantity) {
    UnitConverter toUnitQuantityUnit = netQuantity.getUnit()
            .getConverterTo(unitQuantity.getUnit());
    return unitRate
            .times(toUnitQuantityUnit.convert(netQuantity.getValue())
                    / unitQuantity.getValue());
}

使用Jscience计算n个单位的价格，提供单价和数量

2 个答案: