Object Oriented Design

• From chapter 4.
  • Read it, through page 149.
• He begins by discussing design and the importance of design.
• Design becomes more important as
  • You work on larger projects.
  • You work on projects with multiple programmers.
  • As you work on projects where the team changes as the project progresses.
• If PennWest is not a warning, I don't know what is.
• He begins with a number of examples you should read.
• He moves on to discuss abstraction and reuse
  • He claims that these two are fundamental design rules to follow.
• Abstraction - Preserving the information that is relevant in a given context and forgetting the information that is irrelevant in that context. John Guttag.
  • His entry in the book is good, but please read the wikipedia article, or at least the portion on data abstraction.
• He gives an example of a TV.
  • You know how to use a TV.
  • But you don't know how a TV is implemented.
• The same is true of an object or abstract data type (ADT)
• An abstract data type is a data type whose properties (domain, and operations) are specified independently of any implementation.
• Let's consider a bug in our game.
  • Hopefully we have implemented a functional decomposition by now.
  • In my opinion, a major problem is that you have to know how a bug works at all levels of the implementation.
    • In main, int player1[BODY_PART_COUNT]{0}
    • In Win, we step through the array.
    • In PrintBug we step through the array...
  • And none of the information is collected
    • The essence of a bug is spread throughout the code.
  • Neither of these is that bad in this code because
    • It is a tiny program (200 lines)
    • It is a simple program.
    • And only one person is working on it.
• The real problem is an abstraction problem.
  • I need to think of the bug, game rules, players, ... at all times during the program.
  • By introducing abstraction I can avoid this.
• Think about the Die class.
  • The programmer level (user) I only need to know the interface.
  • At the implementation level I need to know the details of implementation.
  • Thus the Die class is an abstraction.
  • The Die class has been separated into an interface and an implementation
• Reuse
  • "You don't build your own oven in which to bake your cookies. Nor do you churn you own butter, mill your own flower or form y our own chocolate chips."
  • This is tough, you need to
    • Design with the proper functionality.
    • Design with the proper interface.
    • Implement only the "correct" features.
    • Don't provide too much extra
  • Header and implementation files are useful.
• Again, my die class.
  • Note it doesn't do an I/O.
    • That will be program specific.
  • It does provide the required interface, perhaps
    • It might be nice to have a CurrentValue() function.
    • Would adding that alter the current interface?
    • Would that break programs that use the current Die class?