public static void displayDiameter(Sphere thing) {
  System.out.println("The diameter is " 
                    + thing.diameter() + ".";
}  // end displayDiameter
  

Sphere mySphere = new Sphere(2.0);
Ball myBall = new Ball(5.0, "Volleyball");


displayDiameter(mySphere);  // mySphere's diameter
displayDiameter(myBall);    // myBall's diameter
