Question

~~~Update: Solved! Thanks everyone!~~~

I’m working on a project from the Blue Pelican Java book, lesson 16 project Gas Mileage. It asks to create two classes, one is Automobile which holds the methods I will work with. The other class, Tester, is the main class. Every time I run the Tester class, it returns a value of -Infinity. I can’t figure out why, other than that I’ve singled out the problem is in the Automobile class at line 14 in the takeTrip method. When I leave that method out of the Tester class, it returns the correct values.
This is the Automobile class:

public class Automobile
{
    public Automobile(double m) // Accepts value m to the double mpg. Also declares
    {
        double mpg = m;
        double gallons;
    }
    public void fillUp(double f) // Adds fuel to the tank
    {
        gallons += f;
    }
    public void takeTrip(double t) // Takes away fuel from the tank depending upon how many miles are driven
    {
        gallons -= t / mpg; // Not sure how to do this line. For some reason, when I reference mpg, the output of Tester is "-infinity". Shouldn't it do gallons = gallons - (miles driven / mpg)?
    }
    public double reportFuel() // Returns value of how much fuel is left in tank
    {
        double r = gallons;
        return r;
    }
    public double mpg;
    public double gallons;
}

And this is the Tester class:

public class Tester
{
    public static void main(String args[])
    {
        Automobile myBmw = new Automobile(24); // Passes constructor argument of 24 mpg
        myBmw.fillUp(20); // Adds 20 gallons to fillUp method
        myBmw.takeTrip(100); // Takes away the fuel used for 100 miles using the takeTrip method
        double fuel_left = myBmw.reportFuel(); // Initializes fuel_left to the method reportFuel
        System.out.println(fuel_left);
    }
}

Any help is appreciated, thanks!
-AJ

Answer 1

You constructor doesn’t need the ‘double’ identifier. Here you are creating a new variable also called mpg, which is forgotten after the constructor completes. Instead use this:

public Automobile(double m) // Accepts value m to the double mpg. Also declares
{
    mpg = m;
}