public class Car {

  private double myMilesPerGallon;
  private double myGas;

  // Constructs a car with gas gallons in the tank
  //   and gets milesPerGallon miles per gallon
  //  @param gas - amount of gas in tank
  //  @param milesPerGallon - the miles/gal of this car
  public Car(double gas, double milesPerGallon) {
    // Initialize instance variables with
    myGas = gas;
    myMilesPerGallon = milesPerGallon;
  }

  // Adds gas to the tank
  //  @param gas - the amount of gas to be added
  public void addGas( double gas){
      //  Add gas to myGas
    myGas = myGas + gas;
  }

  // Drives the car a certain distance and 
  //   updates the amount of gas in the tank
  //  @param miles - the amount of miles driven
  public void drive(double miles){
    // Determine how much gas was used
    // Subtract this amount from myGas
    myGas = myGas -
       miles/myMilesPerGallon;
  }

  // Gets the amount of gas in the tank
  public double getGas(){
    // Return the number of gallons of gas in the tank
    return myGas;
  }

  // toString method allows printing class instance
  public String toString() {
    return "Gas: " + myGas + 
           ", miles/Gal: " + myMilesPerGallon;
  }
}

public class CarTest {

   public static void main(String[] args) {
      Car car1 = new Car(30, 18);
      System.out.println("Gas in car 1: " +
         car1.getGas());
      car1.drive(200);
      double gasInTank = car1.getGas();
      System.out.println("Gas in car 1: " +
         car1.getGas());
      car1.addGas(10);
      System.out.println("Gas in car 1: " +
         car1.getGas());
      // call toString method implicitly
      System.out.println(car1);
   }
}

Output:
Gas in car 1: 30.0
Gas in car 1: 18.88888888888889
Gas in car 1: 28.88888888888889
Gas: 28.88888888888889, miles/Gal: 18.0