The Role of Design in the First-Year Computer Science Curriculum

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The Role of Design in the First-Year Computer
Science Curriculum

• CCSC Third Annual Northeastern Conference
• Friday 24-April-98
• Rick Mercer - Penn State
• rhm1_at_psu.edu
• These slides are at
• www.bk.psu.edu/faculty/mercer

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First some quotes

• Design is the set of activities including (1)
  defining an architecture for the software that
  satisfies the requirements and (2) specifying an
  algorithm for each software component
• Alan Davis

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• Good design comes from experience, experience
  comes from bad design
• Fred Brooks

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• Freshman can't design
• anonymous

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A CCSCNC Aside

• Imagine this is week 13 of your first course and
  you can
• construct and manipulate objects send messages
• implement classes
• you know a few OO design heuristics guidelines
• you know control structures and arrays
• this is the first of several lectures on OO
  design
• You have adopted Computing Fundamentals with
  Standard C, 2nd Edition, Rick Mercer, Franklin,
  Beedle, and Associates

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Snapshot of the next two weeks

• Week 13 Outline
• Introduce object-oriented design in the context
  of a case study JukeBox
• Role play some scenarios one team up front at
  first
• Develop Component/Responsibility/Helper (CRH)
  cards
• Week 14 Outline
• Design classes methods, instance variables
• Select 4 to 5 person teams pick new project
• Analyze the problem, design a solution

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Object-Oriented Design

• Object-oriented design activities
• (1) identify the key abstractions (classes) and
• (2) assign the major responsibility to each class

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A Problem Specification

• The student affairs office has decided to put
  some new found activity fee funds toward a music
  juke box in the student center. The JukeBox
  should allow a student to play individual songs.
  No money is required. Instead, a student swipes a
  magnetic Student ID card in a card reader to
  select songs. Each student is allowed to play up
  to 1500 minutes worth of "free" JukeBox music in
  their academic careers at the school, but never
  more than two selections on any given date. A
  student may select a CD from the available
  collection of CDs and then select an individual
  song from the CD. Students may not play entire
  CDs.

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Goals for the First Phase

• 1. Find classes that model (shape) the system as
  a natural and sensible set of abstractions.
• 2. Determine the purpose, or main responsibility
  of each class.
• what an instance of the class must be able to do
• what each object must know about itself
• 3. Look for helpers
• To help complete its responsibility, one object
  may delegate responsibility to other objects.

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One Possible Architecture The major components
and the system boundary
CardReader Gets Student ID
StudentCollection Stores valid Students
Student
Track- Selector Gets track to play
JukeBox Coordinates activities
CDCollection Stores all CDs that can be played
CDPlayer Plays a Track
CD
Track
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• Scenario What happens when a Student, with a
  valid account, wants to select a Track?
• ...
• JukeBox Okay, now I should check to make sure
  the Student is able to select this Track before
  telling the CDPlayer to play it. I seem to
  remember I just can not simply play a Track
  without checking on a few things. I know the
  current Student and the selected Track. What do I
  do now?
• Who is role playing Track?
• Who is role playing Student?

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Object-Oriented Design Heuristics guidelines to
good design

• Minimize the number of message sends between a
  class and its helper.
• Avoid all-powerful (omnipotent) classes.
• Distribute system intelligence among the classes
  as evenly as possible. The top-level classes in a
  design should share the work uniformly.

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CRH cards to Class definitions

• CRH Cards capture analysis and some design
  decisions

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CRH cards help design classes

• // First draft, no constructors yet
• class cdCollection
• public
• void addCD(const CD aCD)
• // Add a CD to the collection if possible
• bool removeCD(const CD aCD)
• // Remove CD if possible or return false
• private
• vector ltCDgt my_data
• int my_size

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A design pattern Iterator

• class CD
• public
• CD()
• CD(string initArtist, string initTitle, int
  initNum)
• //--modifiers
• void addTrack(const track nextTrack)
• // accessors
• string artist() const
• string title() const
• int cdNumber() const
• track getTrack(int trackNum) const
• int size() const
• //--iterator functions
• void first()
• void next()
• bool isDone() const
• track currentItem() const

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Summary

• Team projects and putting things in context helps
  case studies
• Students design when
• select classes and responsibilities the
  architecture
• consider design heuristics and patterns
• Object-oriented design is a natural, doable way
  to introduce design
• Given the chance, Freshman can design