Java beginner exercise with methods

Question

I am real beginner in Java and I have one simple exercise where I need to convert m/h into km/h using a method and a return from it.

I have to define 2 situations: if km/h < 0 return -1 (error) and if km/h > 0 return km/h * 1.609 (value in m/h).

I tried everything I could think of but I either get a no return statement error or no output when I try to run it.

I can't understand why even if I gave it more than one return option it just doesn't work whatever the value is. I could use System.outprintln or String but the exercise specify I must use a return method.

here is my code, written in IntelliJ:

package EXERCISE;

public class Main {

    public static void main(String[] args) {
        toMilesPerHour(0);
    }

    public static double toMilesPerHour(double kilometersPerHour) {

        if (kilometersPerHour < 0) {
            return -1;
        }
        else if (kilometersPerHour > 0) {
            return kilometersPerHour * 1.609d;
        }
        else if (kilometersPerHour == 0) {
            return 0;
        }

        return kilometersPerHour * 1.609;

        // if I don't write return here it gives me no return statement error,
        // if I write it, it gives me no output with value > or < 0 but no error.
    }

}

Answer 1

public static double toMilesPerHour(double kilometersPerHour) {

    if (kilometersPerHour < 0) {
        return -1;
    }
    else {
        return kilometersPerHour * 1.609;
    }
}

Answer 2

Try it like this:

public static double toMilesPerHour(double kilometersPerHour) {
    return (kilometersPerHour > 0 ? kilometersPerHour*1.609 : -1; 
}

You could also throw an exception if speed is negative:

public static double toMilesPerHour(double kilometersPerHour) {
    if (kilometersPerHour < 0) throw new IllegalArgumentException("speed cannot be negative");
    return kilometersPerHour*1.609; 
}

Answer 3

Even if you use a method, you have to print the returned value:

package EXERCISE;

public class Main {

    public static void main(String[] args) {

        System.out.println(toMilesPerHour(0));

    }

    public static double toMilesPerHour(double kilometersPerHour) {

        if (kilometersPerHour < 0) {
            return -1;
        }
        else if (kilometersPerHour > 0) {
            return kilometersPerHour * 1.609d;
        }
        else if (kilometersPerHour == 0) {
            return 0;
        }
        return kilometersPerHour * 1.609;
        //if I don't write return here it gives me no return statement error,
        //if I write it, it gives me no output with value > or < 0 but no error.
    }
}

Furthermore, you can get rid of the return statement at the end:

    public static double toMilesPerHour(double kilometersPerHour) {
        if (kilometersPerHour < 0) {
            return -1;
        }
        else {
            // you don't need to check if kilometersPerHour is 0, since every number multiplied with 0 is 0
            return kilometersPerHour * 1.609;
        }
    }

Answer 4

If you use if()... else you have exactly two options. Either you go into the if clause and do whats there, or you go into the else clause. If if-clause and else-clause both have a return-statement, everything goes fine.

But you don't have if-else-clauses! You have if-else-if-clauses! If the if-clause doesn't hold, you go into the else-clause, which again has an if-clause and so on. Finally you don't have a default else clause.... if you don't get into the first if-clause, you check in the else-clause, if the condition holds. If not, you check the condition in the next else clause. The last return-statement finally is the default else clause. We all, as humans, see, that every condition (<, == >) is covered, but the compiler doesn't see that!

Answer 5

The reason compiler gives you "no return statement" error is because you didn't cover all possible cases with your ifs : there is Double.NaN , which is not equal to 0 (or any other value, for that matter, including itself) and neither greater nor lesser than 0 .

Also, compiler doesn't analyze your code deep enough to check if you covered all possible variants anyway: if you replace double with long , the result is the same - the compiler will see the reachable branch that doesn't return anything, and so it produces an error. To fix an error, you need to have all branches return something:

if (kilometersPerHour < 0) {
    return -1;
}
else if (kilometersPerHour == 0) {
    return 0;
}

// Note: I don't have explicit check for NaN,
// because any arithmetic on NaN produces NaN,
// which would be the correct result for this function.
// So I let the "*" operator to check for NaN instead.
return kilometersPerHour * 1.609;