Changing return value of a function in Java

Question

I have a function like the following one:

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;
        }

    }

As you can see this function shows the an error message when the input parameter transmission does not receive the expected values. Furthermore, it returns 0. I want to use this function in another function as follows:

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 +"€");
        return preuDevaluacio;

    }

However, when I run the code I can see the message but I'm not returning -1. I have tried to match the return as 0.0 and then change it to -1 but this does not work (see code following the //HERE comment). Which one would be the best way to do it?

Thanks in advance

Answer 1

To return specific states, you basically have FOUR options:

1. You can use constants: public static final [type] [varname] = [value];. This is used for specific messages/states, and can also be used with classes.

2. You can use Optional: Optional<Integer>. This only works with classes. Return Optional.empty(); to indicate bad state.

3. To indicate invalid return values, you can also simply use a wrapper class. As you're using double, the wrapper class would be Double. So return null;, and whenever you retrieve the value

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

4. As @Socowi pointed out, you can also use Exceptions to signal critically bad input values. So I rewrote your code to use Exception Handling:

Code:

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 + "€");
            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;
    }


}

Warning: comparing values of floats and doubles is always done via some tolerance tests, so this might bear strange results. Also be careful with testing agains Double.NaN, because you cannot compare a value like if (x == Double.NaN) ... but must instead use Double.isNaN(x).

Usually you should use Wrappers first, if one exception state suffices. If you need more, use constants.