Runnable Version of the Program

Here is the full version of the program with the new methods. It would be worth your time to study how things fit together. Notice how the big object (a Fleet) is made up of smaller objects (the Cars) and how the methods of the big object are made up from the methods of the smaller objects.

class Fleet
{
  // data
  private Car town;
  private Car suv;

  // constructor
  Fleet( int start1, int end1, double gal1, 
         int start2, int end2, double gal2 )
  {
    town = new Car( start1, end1, gal1) ;
    suv  = new Car( start2, end2, gal2) ;
  }

  // method
  double calculateMPG()
  {
    double sumMPG; 
    sumMPG = town.calculateMPG() + suv.calculateMPG() ;
    return sumMPG/2.0;
  }

  void fillUp( int townOdo, double townGal, int suvOdo, double suvGal)
  {

    town.fillUp( townOdo, townGal );

    suv .fillUp( suvOdo, suvGal );
  }
}

class Car
{
  // data
  private int startMiles;   // Stating odometer reading
  private int endMiles;     // Ending odometer reading
  private double gallons;   // Gallons of gas used between the readings

  // constructor
  Car(  int first, int last, double gals  )
  {
    startMiles = first ;
    endMiles   = last ;
    gallons    = gals ;
  }

  // methods
  double calculateMPG()
  {
    return (endMiles - startMiles)/gallons ;
  }

  void fillUp(int newOdo, double fillUpGals )
  {
     startMiles  =  endMiles ;
     endMiles    =  newOdo ;
     gallons     =  fillUpGals ;
  }
}

class FleetTester
{
  public static void main ( String[] args)
  {
    Fleet myCars = new Fleet( 1000, 1234, 10, 777, 999, 20 );
    System.out.println("Fleet average MPG= " + myCars.calculateMPG() );

    myCars.fillUp( 1434, 10, 1099, 10 );
    System.out.println("new average MPG= " + myCars.calculateMPG() );
  }
}