Week 7 Discussion/Lab Notes

Administrivia

• Everyone should have scores for HW1-4, PROJ1-2, and MT1 online.
• Quiz 2 will be during discussion next week. It will cover object-oriented programming.
• Get in groups of 4 for Projects 3-4.

Discussion Outline

• Midterm 1
• 1. What would Scheme do?
• use your reader; look it up in the Scheme specification (use the index!)
• 3. box-and-pointer diagrams
• only cons (and list and quote, sort of) creates new pairs
• 4. new count-change
• how does the original cc work?
• cc tree
• 5. sets
• you do not know how sets are represented
• you can not assume they are lists
• they could be procedures (we know this from Project 2, message-passing, ...)
• they could be numbers
• ... etc. (who knows?)
• the point of data abstraction is that you don't need to know the implementation!
• 8. unbalanced trees
• why are there 2^n - 1 ways to produce an unbalanced tree?
• In general, if the problem supplies suggestions/hints, follow them!
• remember to load obj.scm/obj.ss
• remember to reinstantiate objects after modifying the class
• use begin instead of sequence
• return values, side-effects, set!
• class-based OOP
• instance-vars and instantiation arguments have implicit methods
• isA, hasA, usesA relationships

Object-Oriented Programming (OOP)

Object-oriented programming is a powerful programming metaphor particularly for modeling items in the real world. (OOP can be applied to less tangible, more abstract entities as well, but it's questionable whether OOP is the best programming style to use for these cases.)

In this class we'll be using class-based OOP. This is the model that most OOP languages use, where we define general classes and create particular instances of them. (Another model is prototype-based OOP, which, as far as I know, isn't widely used in practice.)

Read the material in the course reader about object-oriented programming.

Objects have different relationships with other objects.

• isA: This is used when we use inheritance; that is, when one object is a child of another. For example, suppose we have a Dog parent class and a Beagle child class. We say a beagle isA dog. Note that this is not true the other way around; not every dog is a beagle!
• hasA: This is used when we have container classes. A container is an object that has internal variables bound to other objects. These containers are like physical containers in the real world. For example, suppose we have a Car class and an Engine class. We say that a car hasA engine.
• usesA: This is used when one object manipulates another object or asks it to do something. For example, suppose we have a Person class and a Door class. A person usesA door by opening it, closing it, locking it, etc.

Object-Oriented Design (OOD)

How should we structure and organize our classes? What kind of relationships should classes have with each other? These are the topics of object-oriented design. Unfortunately there aren't any steadfast rules about this; different situations require different design decisions, and you learn what's good and what's bad mostly from experience and common sense. Some general design principles:

• avoid multiple inheritance when possible
• use metaphors from the real world
• keep it simple

You'll deal with OOD much more in CS61B (Data Structures) and CS169 (Software Engineering).