如何在 Java 中为通用转换器扩展这种设计？

Question

这里是一小段货币转换器代码：

 public enum CurrencyType {

        DOLLAR(1),
        POUND(1.2),
        RUPEE(.25);

        private CurrencyType(double factor) {
            this.factor = factor;
        }
        private double factor; 

        public double getFactor() {
            return factor;
        }

    }

    public class Currency {

        public Currency(double value, CurrencyType type) {
            this.value = value;
            this.type = type;
        }

        private CurrencyType type;
        private double value;

        public CurrencyType getCurrencyType() {
            return type;
        }

        public double getCurrencyValue() {
            return value;
        }

        public void setCurrenctyValue(double value){
            this.value = value;
        }

    }

public class CurrencyConversion {


    public static Currency convert(Currency c1, Currency c2)
            throws Exception {
        if (c1 != null && c2 != null) {
            c2.setCurrenctyValue(c1.getCurrencyValue()
                    * c1.getCurrencyType().getFactor()
                    * c2.getCurrencyType().getFactor());
            return c2;
        } else
            throw new Exception();
    }


}

我想改进此代码，使其适用于不同的转换单位，例如：公斤到磅、英里到公里等。看起来像这样：

public class ConversionManager<T extends Convertible> {


    public T convert(T c1, T c2) 
    {
        //return null;
    }
}

感谢您的想法和建议。

Answer 1

在我看来，这是面向对象的方式。请注意，您也可以对数据类型进行模板化，但我在这里只使用了双精度，因为它们可以表示任何标量单位。这提供了一种很好的方式来处理您所要求的不同度量单位之间的转换：距离、货币、质量等。

public interface Metric {
    public String getReferenceUnit();
}

public class Distance implements Metric {
    public String getReferenceUnit() {
        return new String("Meters");
    }
}

public interface Units<T extends Metric> {
    public double getToReferenceFactor();
}

public class Kilometers implements Units<Distance> {
    public double getToReferenceFactor() {
        return 1000;
    }
}

static <T extends Metric> double convert(Units<T> from, Units<T> to) {
    return from.getToReferenceFactor() * (1.0 / to.getToReferenceFactor());
}

Answer 2

不幸的是，Java 中的枚举不支持接口或超类（我想找出正确的原因）。

您可以做的是在您的枚举类型（例如@Convertible）上声明一个注解，并使用反射访问它的因子。这很丑陋，你将无法在泛型中使用“T extends Convertible”表达式。

Answer 3

这是一个使用空心矩阵的通用转换器，它通过传递闭包来组合已知的转换器。在这种情况下，它会转换长度单位：

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Converter {
    private static final Map<String, Map<String, Double>> FACTOR_MAP
            = new HashMap<String, Map<String, Double>>();

    public static double convert(String from, String to, double value) {
        List<Step> stack = new ArrayList<Step>();
        stack.add(new Step(from, value));
        while (!stack.isEmpty()) {
            Step s = stack.remove(0);
            double val = s.value;
            String source = s.unit;
            if (source.equals(to)) {
                return val;
            }
            Map<String, Double> map = FACTOR_MAP.get(source);
            if (map != null) {
                for (Map.Entry<String,Double> entry: map.entrySet()) {
                    stack.add(new Step(entry.getKey(), val*entry.getValue()));
                }
            }
        }
        throw new IllegalArgumentException("Cannot not convert from " + from
                + " to " + to);
    }

    public static void registerFactor(String from, String to, double factor) {
        putFactor(from, to, factor);
        putFactor(to, from, 1.0/factor);
    }

    private static void putFactor(String from, String to, double factor) {
        Map<String, Double> map = FACTOR_MAP.get(from);
        if (map == null) {
            map = new HashMap<String, Double>();
            FACTOR_MAP.put(from, map);
        }
        map.put(to, factor);
    }

    static {
        registerFactor("cm", "mm", 10);
        registerFactor("in", "cm", 2.54);
        registerFactor("in", "pt", 72);
        registerFactor("pc", "pt", 12);
        registerFactor("px", "mm", 0.28);
    }

    private static class Step {
        private String unit;
        private double value;

        Step(String unit, double value) {
            this.unit = unit;
            this.value = value;
        }
    }
}

以下程序：

public class Main {
    private static final String UNITS[] = {"cm", "mm", "in", "pt", "pc", "px"};

    public static void main(String[] args) {
        for (String unit1: UNITS) {
            for (String unit2: UNITS) {
                System.out.println("1" + unit1 + " = "
                        + Converter.convert(unit1, unit2, 1) + unit2);
            }
        }
    }
}

给出以下结果：

1cm = 1.0cm
1cm = 10.0mm
1cm = 0.39370078740157477in
1cm = 28.346456692913385pt
1cm = 2.3622047244094486pc
1cm = 35.71428571428571px
1mm = 0.1cm
1mm = 1.0mm
1mm = 0.03937007874015748in
1mm = 2.8346456692913384pt
1mm = 0.23622047244094485pc
1mm = 3.571428571428571px
1in = 2.54cm
1in = 25.4mm
1in = 1.0in
1in = 72.0pt
1in = 6.0pc
1in = 90.71428571428571px
1pt = 0.035277777777777776cm
1pt = 0.35277777777777775mm
1pt = 0.013888888888888888in
1pt = 1.0pt
1pt = 0.08333333333333333pc
1pt = 1.2599206349206347px
1pc = 0.42333333333333334cm
1pc = 4.233333333333333mm
1pc = 0.16666666666666666in
1pc = 12.0pt
1pc = 1.0pc
1pc = 15.119047619047619px
1px = 0.028000000000000004cm
1px = 0.28mm
1px = 0.011023622047244094in
1px = 0.7937007874015748pt
1px = 0.06614173228346457pc
1px = 1.0px