Complete Constructor

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

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

  // methods
  double calculateMPG()
  {
    return  ;
  }

}

class MilesPerGallon
{
  public static void main( String[] args ) 
  {
    Car car = new Car( 32456, 32810, 10.6 );
    System.out.println( "Miles per gallon is " + car.calculateMPG() );
  }
}

Constructors can get complicated; but for most programs all you need to do is copy values from the parameters to the instance variables. You might wonder why you need to do this. Why not just leave the data in the parameters? There are two reasons:

1. The parameters of the constructor can be "seen" only by statements inside the body of the constructor (the statements between the two braces.) A method such as calculateMPG() cannot see the parameters of the constructor.
2. Data in parameters is temporary. Parameters are used to communicate data, not to hold data.

Think of a parameter as a scrap of paper containing information handed to the constructor. The constructor has to copy the information to someplace permanent that can be seen by the other methods.