GasInTank

instance variable stay long, but local variable only stay short term.

public class CarTester
{
    public static void main(String[] args)
    {
        Car car = new Car();

        // TODO: Add 20 gallons and drive 100 miles
        car.addGas(20);
        car.drive(100);
        
        // TODO: Print actual and expected gas level
        System.out.println("Expected:18");
        System.out.println(car.getGasInTank());
    }
}

public class Car
{
    private double milesDriven;
    private double gasInTank;
    private double milesPerGallon;

    public Car()
    {
        milesDriven = 0;
        gasInTank = 0;
        milesPerGallon = 50;
    }

    /**
       Drives this car by a given distance.
       @param distance the distance to drive
    */
    public void drive(double distance)
    {
        milesDriven = milesDriven + distance;
        double gasConsumed = distance / milesPerGallon;
        gasInTank = gasInTank - gasConsumed;
    }

    public void addGas(double amount)
    {
        gasInTank = gasInTank + amount;
    }
    
    /**
       Gets the current mileage of this car.
       @return the total number of miles driven
    */
    public double getMilesDriven()
    {
        return milesDriven;
    }

    /**
       Gets the current amount of gas in the tank of this car.
       @return the current gas level
    */
    public double getGasInTank()
    {
        return gasInTank;
    }
}