java - 在 Java 中更改函数的返回值

Question

我有一个类似以下的功能：

public static double calculateValuationByTransmission(double price, char transmission) {

        if (transmission == 'A') {
            return price * 0.05;
        } else if (transmission == 'M') {
            return price * -0.05;
        } else {
            System.out.println("Transmission must be: 'A' or 'M'");
            return 0;
        }

    }

如您所见，当输入参数transmission未收到预期值时，此函数会显示错误消息。此外，它返回0。我想在另一个函数中使用这个函数，如下所示：

public static double calculateValuation(double price, int mileage, int age, char transmission) {
       
        if (price < 0.0 || mileage < 0 || age < 0) {
            System.out.println("Price, mileage and age must be a positive number");
            return -1;
        }
        //HERE
        double valueTransmission = PAC1Ex2.calculateValuationByTransmission(price, transmission);
        if (valueTransmission == 0.0){
            return -1;
        }

        double preuIntermedi = price - (valueTransmission +
                PAC1Ex2.calculateValuationByMileage(price, mileage));
      
        double preuDevaluacio = PAC1Ex2.calculateValuationByAge(preuIntermedi,age);
       
        System.out.println("Your vehicle is now valued at " + preuDevaluacio +"€&quot;);
        return preuDevaluacio;

    }

但是，当我运行代码时，我可以看到该消息，但我没有返回 -1。我试图将返回值匹配为 0.0，然后将其更改为 -1 但这不起作用（请参阅 //HERE 注释后面的代码）。哪一个是最好的方法？

提前致谢

标签： java

Answer 1

要返回特定状态，您基本上有四个选项：

1. 您可以使用常量：public static final [type] [varname] = [value];. 这用于特定的消息/状态，也可以与类一起使用。

2. 您可以使用可选：Optional<Integer>. 这仅适用于类。返回Optional.empty();以指示不良状态。

3. 要指示无效的返回值，您也可以简单地使用包装类。当您使用double时，包装类将是Double. 因此return null;，每当您检索该值时

    Double valueTransmission = PAC1Ex2.calculateValuationByTransmission(price, transmission);
    if (valueTransmission == null){
        return -1; // or even better, also return null here if applicable
    }
   

4. 正如@Socowi 指出的那样，您还可以使用异常来表示严重错误的输入值。所以我重写了你的代码以使用异常处理：

代码：

public class PAC1Ex2 {

    // define our own exception here
    static public class InvalidTransmissionException extends IllegalArgumentException {
        private static final long serialVersionUID = 1421105202322188571L;
        public InvalidTransmissionException(final String pMessage) {
            super(pMessage);
        }
    }

    public static double calculateValuationByTransmission(final double price, final char transmission) {
        if (transmission == 'A') {
            return price * 0.05;
        } else if (transmission == 'M') {
            return price * -0.05;
        } else {
            throw new InvalidTransmissionException("Transmission [" + transmission + "] invalid! Transmission must be: [A] or [M]!");
            //          System.out.println("Transmission must be: 'A' or 'M'");
            //          return 0;
        }
    }



    public static double calculateValuation(final double price, final int mileage, final int age, final char transmission) {
        if (price < 0.0 || mileage < 0 || age < 0) {
            System.out.println("Price, mileage and age must be a positive number"); // we also could use exceptions here
            return -1;
        }

        //HERE
        try {
            final double valueTransmission = PAC1Ex2.calculateValuationByTransmission(price, transmission);
            final double preuIntermedi = price - (valueTransmission + PAC1Ex2.calculateValuationByMileage(price, mileage));
            final double preuDevaluacio = PAC1Ex2.calculateValuationByAge(preuIntermedi, age);
            System.out.println("Your vehicle is now valued at " + preuDevaluacio + "€&quot;);
            return preuDevaluacio;

        } catch (final InvalidTransmissionException e) {
            return -1;
            // or even pass that exception up forther by a) not catching it, or b) writing "throw e;"
        }

    }



    private static double calculateValuationByAge(final double pPreuIntermedi, final int pAge) {
        // TODO Auto-generated method stub // dummy method
        return 0;
    }

    private static double calculateValuationByMileage(final double pPrice, final int pMileage) {
        // TODO Auto-generated method stub // dummy method
        return 0;
    }


}

警告：浮点数和双精度值的比较总是通过一些容差测试来完成，所以这可能会产生奇怪的结果。还要小心对 Double.NaN 进行测试，因为您不能比较像这样的值，if (x == Double.NaN) ...而是必须使用Double.isNaN(x).

通常你应该首先使用 Wrappers，如果一个异常状态就足够了。如果您需要更多，请使用常量。