Classes, Objects and iTunes
P.J. Drongowski
8 September 2004

Objectives

  * Show data structures in action in a real product
  * Show how classes and data objects are used

Motivation

  * Yep, we're going to talk about basic object-oriented design
  * User interface development helped inspire object-oriented design
  * Example: Files in the UNIX/Linux file system
    + We can discuss files collectively (a "class" of objects)
      - What a file is and what it stores
      - We can define rules of behavior
    + We can discuss files individually (instances of a class or an "object")
    + We can define operations on files and apply them to a specific file
      - Create file
      - Read and write file contents
      - Remove an existing file that we don't want anymore
  * Powerful ideas
    + Class
    + Object (instance of a class)
    + "Object factory"
  * From the user interface point of view, it's not a bad user model --
    a user brings an object into being, does things to an object, deletes it

Short iTunes demonstration

  * What is iTunes for people who haven't used it before?
    + Database of songs
      - Sorting
      - Searching
    + Rip songs from CDs
    + Buy and download songs from Apple music service
    + Burn your own CDs
    + Download songs to an iPod music player
  * Does anyone know any neat iTunes tricks?
  * The iTunes interface is patented!

Discussion points/questions

  * What can we observe about the iTunes interface?
    + What are the kinds of objects in iTunes?
    + What are the attributes of the objects?
    + What are the operations on the objects?
    + What are the relationships between objects?
    + Are relationships one-to-one? Many-to-one?

  * How can songs be described and represented in C++?
    + Kinds of object == class
    + A particular object == an instance of a class
    + Notion of class as an "object factory"
    + How are attributes represented as data memebers?
    + How are operations declared and defined?
    + How do we refer to objects?
      - Can declare and define an object like an object of primitive type
      - Can indirectly refer to an object using a "handle" or "pointer"

  * How are relationships represented?

Example

  // Class declaration
  class Song {
    private:
      char title[64] ;          // Song title
      int minutes, seconds ;    // Playing time
      int count ;               // Number of times played
      int month, day, year ;    // Last played
    public:
      Song() ;                  // Constructor
      ~Song() ;                 // Destructor
      void setTitle(char *t) ;  // Set song title
      char *getTitle() ;        // Get song title
      ...
      void play() ;             // Playback song thru speakers
  } ;

  // Instances of Song
  Song louieLouie, iFoughtTheLaw ;

Optional questions to investigate or ask

  * Where is song data (the actual AAC or MP3 file) stored?
      /users/XXX/Music/iTunes/iTunes Music/ARTIST/ALBUM/SONG
      /users/XXX/Music/iTunes/iTunes Music/Zero 7/Simple Things/Destiny.mp3
  * If we drew a picture of the iTunes database structure, what would it
    look like?
  * Consider the classes: Song, Artist and Album
    + Several songs are released on an album (many-to-one relationship)
    + Each song is recorded by a single artist (one-to-one relationship)
    + A song may be included on more than one album (many-to-one relationship)
  * In a 1-to-1 relationship, whould one of the objects be represented as
    an attribute of the other object?
    + A developer should consider if the redundancy is tolerable, i.e., if
      it's more space efficient to store the objects separately to avoid
      unnecessary copies of data
    + Also to be considered is if the subordinate object has relationships
      to other objects. If yes, it should be separate.

IS-A relation

  * Is a Compilation a special case of an Album? Artist? (Look into the
    iTunes library on disk for clues)
  * Example of IS-A relation: the program MASSIVE used to generate the
    armies in The Lord of The Rings
  * IS-A relationships are often implemented using inheritance