Rice University - Comp 212 - Intermediate Programming

Spring 2006

Lecture #2 - The Best Little Pizza House in Texas (part 2)

Today's menu:

  1. Discussion of the syllabus.
  2. Back to the Pizza deal!

An Object-Oriented Programming (OOP) solution

A pizza has a price and a shape.  We want to compute the price/area ratio.  It is the shape that intrinsically "knows" how to compute its area.  A shape is an abstract concept.  The area of a shape is an abstract concept.  A rectangle is a concrete and specific shape.  A rectangle knows its own width and height and therefore can compute its area.  A circle is a concrete and specific shape.  A circle knows its own diameter and therefore can compute its area.  In the table shown below, we describe how this conceptual view of a pizza is expressed in Java as an object-oriented (OO) model and compare it with an equivalent functional representation in Scheme.  This OO pizza model involves the following fundamental concepts in OOP.
OO Design in Java Data-directed Design in Scheme
/**
 * A shape is an abstract entity that intrinsically knows how to compute its area.
 */
public interface IShape {
  public double getArea();
}		 ;; A Shape is either
;;   (make-Rectangle w h) where
;;   w and h are numbers representing
;;   the width and the height
;; or
;;   (make-Circle r) where
;;   r is a number representing the radius
;; contract: getArea: Shape -> number
;; purpose: 
;; (getArea s) returns the area of s.
/**
 * A rectangle is a shape.
 * It has a width and a height and
 * a way of computing its area using its width and height.
 */
public class Rectangle implements IShape {
  private double _width;
  private double _height;
  // Below is a very special construct in Java
  // called the constructor.  It is used to
  // initialize an instance of a Rectangle to
  // a given width and height.
  public Rectangle(double w, double h) {
      _width = w;
      _height = h;
 }
  public double getArea() {
    // code may involve _width and _height
  }
}		 (define-struct Rectangle(width height))
;; make-Rectangle, Rectangle-width,
;; Rectangle-height
;; are automatically generated
/**
 * A circle is a shape.
 * It has a radius and a way of computing its area using its radius.
 */
public class Circle implements IShape {
  private double _radius;
  public Circle(double r) {
    _radius = r;
  }
  public double getArea() {
    ... _radius ...
  }
}		 (define-struct Circle( radius))
;; make-Circle, Circle-radius
;; are automatically generated
  ;; template
;; (define getArea (s)
;;    (cond [(Rectangle? s)
;;            ... (Rectangle-width s) ...
;;            ... (Rectangle-height s) ...]
;;          [(Circle? s)
;;            ... (Circle-radius s) ...]))
public class Rectangle implements IShape {
  // ...
  public double getArea() {
    return _width * _height;
  }
}		 ;; actual code
(define getArea (s)
   (cond [(Rectangle? s)
             (* (Rectangle-width s)
                (Rectangle-height s))]
         [(Circle? s)
             (* PI (Circle-radius s)
                   (Circle-radius s))]))
public class Circle implements IShape {
  // ...
  public double getArea() {
    return Math.PI * _radius * _radius;
  }
}
/**
 * A pizza has a price and a shape.
 */
public class Pizza {
  private double _price;
  private IShape _shape;
  public Pizza(double p, IShape s) {
    _price = p;
   _shape = s;
  }
  public double getPrice() {
    return _price;
  }
  public IShape getShape() {
    return _shape;
  }
}		 ;; A Pizza is 
;;   (make-Pizza p s) where
;; p is a number representing the price,
;; and s is a Shape
(define-struct Pizza(price shape))
;; make-Pizza, Pizza-price, Pizza-shape
;; are automatically generated.

 

Unified Modeling Language (UML) Diagram

UML is a de-facto standard for diagramming OO designs.  It is language neutral.

The above Pizza design is represented in UML as follows.

Last revised 01/11/2006 by Dũng X. Nguyễn