Passing variables in the same class between methods?

Question

I havent touched java in awhile. I need some help, what I am trying to do with this program is get the car class to calculate the carbonfootprint inside of the getcarbonfootprint() method. HOWEVER, like all the videos that I have been going through, I DON'T want to pass it back to the main class through a return value. Instead, I want to use the same variables in the class car through methods. I tried using the this value, however, that doesn't work either. If you can link me to the right location on this question, that will work as well.

MAIN CLASS:

package carbonfootprinttest;

public class CarbonFootprinttest {

    public static void main(String[] args) {
        building house = new building();
        car fusion = new car();
        bike trek = new bike();
        double mytest = fusion.returncarbonfootprint();
        System.out.print(mytest);
    }
}

CAR CLASS:

public class car implements carbonfootprint {
    private double mpg = 25;
    public double yearly = 500;
    public double carbonfootprint;

    public void setmpg(){
        System.out.println("The mpg of a ford fusion is " + mpg + " MPG.");
    }

    @Override
    public void getcarbonfootprint() {
        carbonfootprint = (yearly * mpg)/9;
    }
    public double returncarbonfootprint(){
        System.out.println(carbonfootprint);
        return carbonfootprint;
    }
}

Answer 1

package carbonfootprinttest;

public class CarbonFootprinttest {
    public static void main(String[] args) {
    building house = new building();
    car fusion = new car();
    bike trek = new bike();
    fusion.getcarbonfootprint();
    double mytest = fusion.returncarbonfootprint();
    System.out.print(mytest);
}

Need to call your method getcarbonfootprint() before returncarbonfootprint()

Answer 2

You should call getcarbonfootprint() first, it will generate and set value to variable carbonfootprint and then call returncarbonfootprint() to get updated value.

fusion.getcarbonfootprint();//Add this line of code in main method
    double mytest = fusion.returncarbonfootprint();