Filling in the Definition

Here is the program with some of the Car definition filled in:

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

  // constructor


  // methods

}

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() );
  }
}

An instance variable is a variable that holds part of the state of an object. Each object (each "instance" of the class) contains its own instance variables. Instance variables hold on to their values as long as the object exists. An assignment statements can change the value in an instance variable (see the next chapter), but otherwise it holds its value for the lifetime of the object.