Post on 19-Jan-2016
Object-Oriented Design
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• Objects and concerns• Objects have a concern, meaning they have
a purpose• Not concerned as in worried
• All code should have a purpose
Why is that joke supposed to be funny?
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Classes
• Each class should have a clear purpose• One class usually corresponds to one kind of entity• Each class member corresponds to one attribute
• Only code related to that purpose goes in class• Functions for data that they modify• Code that needs to be modified at the same time
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Software Packages
• Every software package should have a purpose• Code in software package iff relates to purpose
• (iff -> if and only if)
• “Module” can refer to either a class or package• Every module should have a purpose• Code in module iff relates to purpose
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Example: Drug and Alcohol Counseling
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Survey DB
Survey
Surveyanswers
HealthInformation
All thispatient’s
answers (ever)
Counselee
Counselor
Create report
PostscriptPrinterPick up
Printout
Printout
Authenticat
e
User ID
Last name & PIN
• What are the key concerns?
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Key Concerns of Counseling Example
• Managing the users
• Performing the survey
• Generating the report
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Key Concerns of Counseling Example
• Managing the users• Authenticating counselees• Matching counselees to counselors
• Performing the survey• Generating the report
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Key Concerns of Counseling Example
• Managing the users
• Performing the survey• Representing the questions• Representing the answers• Storing answers• Allowing skips
• Generating the report
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Key Concerns of Counseling Example
• Managing the users
• Performing the survey
• Generating the report• Reading the data• Performing calculations of the report• Sending to printer
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UML Diagram (Review)
• One box per entity• Usually lists attributes• Interfaces and abstractions italicized
• Lines without arrowheads show references• Represents member variables in OO• Labeled with cardinality
• Lines with open arrowheads for specialization• Lines with regular arrowheads for dependencies
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Counselee Rec.
Counselor Rec.
Survey Instance
Questions
Answers
Authenticator
Report Maker
Survey Server
Question loader
Skip logic module
Answer storer
Data loader
Calculation module
Printer controller
Simple UML Diagram
note: UML details omitted for clarity
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Counselee Rec.
Counselor Rec.
Survey Instance
Questions
Answers
Authenticator
Report Maker
Survey Server
Question loader
Skip logic module
Answer storer
Data loader
Calculation module
Printer controller
Organize into Packages
Data Records
Authenticator
Survey
Report
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Coupling and Cohesion
• Coupling• When one module is involved in another module’s concern
• Cohesion• When a module is devoted to its own concern
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Levels of Coupling
• Content (worst)• A modifies B
• Common• Control• Stamp• Data• Uncoupled (best)
Coupling reduces maintainability!
A Bmodifies
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Levels of Coupling
• Content (worst)• Common
• A and B read/write same data
• Control• Stamp• Data• Uncoupled (best)
Coupling reduces maintainability!
A B
data
read/write
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Levels of Coupling
• Content (worst)• Common• Control
• A calls B
• Stamp• Data• Uncoupled (best)
Coupling reduces maintainability!
A
Bcalls
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Levels of Coupling
• Content (worst)• Common• Control• Stamp
• A provides structured data to B
• Data• Uncoupled (best)
Coupling reduces maintainability!
A
B
structured data
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Levels of Coupling
• Content (worst)• Common• Control• Stamp• Data
• A provides unstructured data to B
• Uncoupled (best)
Coupling reduces maintainability!
A
B
unstructured data
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Levels of Coupling
• Content (worst)• Common• Control• Stamp• Data• Uncoupled (best)
• None of the above
Coupling reduces maintainability!
AB
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Counselee Rec.
Counselor Rec.
Survey Instance
Questions
Answers
Authenticator
Report Maker
Survey Server
Question loader
Skip logic module
Answer storer
Data loader
Calculation module
Printer controller
Inter-package Couplings?
Data Records
Authenticator
Survey
Report
content: A modifies Bcommon: A / B read/write same datacontrol: A calls Bstamp: A -> B structured datadata: A->B unstructured datauncoupled
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Levels of Cohesion
• Functional / informational (best) • A and B work together for one purpose
• Communicational• Procedural• Temporal• Logical• Coincidental (worst)
Cohesion increases maintainability
A B
one purpose
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Levels of Cohesion
• Functional / informational (best) • Communicational
• A and B use the same data
• Procedural• Temporal• Logical• Coincidental (worst)
Cohesion increases maintainability
A B
data
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Levels of Cohesion
• Functional / informational (best) • Communicational• Procedural
• A executes, then B executes…• A & B have vaguely related purpose
• Temporal• Logical• Coincidental (worst)
Cohesion increases maintainability
A B
purpose
time
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Levels of Cohesion
• Functional / informational (best) • Communicational• Procedural• Temporal
• A executes, then B executes…• A & B have NO related purpose
• Logical• Coincidental (worst)
Cohesion increases maintainability
A B
time
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Levels of Cohesion
• Functional / informational (best) • Communicational• Procedural• Temporal• Logical
• Either A or B might be executed
• Coincidental (worst)
Cohesion increases maintainability
A B
A ∨ B
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Levels of Cohesion
• Functional / informational (best) • Communicational• Procedural• Temporal• Logical• Coincidental (worst)
• None of the above
Cohesion increases maintainability
A
B
Counselee Rec.
Counselor Rec.
Survey Instance
Questions
Answers
Authenticator
Report Maker
Survey Server
Question loader
Skip logic module
Answer storer
Data loader
Calculation module
Printer controller
Intra-package Cohesion?
Functional: A&B have one purposeCommunicational: A&B share same dataProcedural: A runs, then B, relatedTemporal: A runs, then B, unrelated Logical: either A or B might be executedNo cohesion
Counselee Rec.
Counselor Rec.
Survey Instance
Questions
Answers
Authenticator
Report Maker
Survey Server
Question loader
Skip logic module
Answer storer
Data loader
Calculation module
Printer controller
Tip #1: “Don’t talk to strangers”
This would be bad:
Counselee Rec.
Counselor Rec.
Survey Instance
Questions
Answers
Authenticator
Report Maker
Survey Server
Question loader
Skip logic module
Answer storer
Data loader
Calculation module
Printer controller
Tip #2: Move Code to Where It’s Used
Reduce inter-package coupling
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Tip #3: Split Modules to Reduce Cycles
• Our example had no cycles• But here’s a way to correct them:
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Tip #4: In Reuse, Prefer Composition over Inheritance
• Use composition to add features or to reuse code• Use inheritance to add a new version of an entity
Survey Instance
Questions
Answers
Survey Server
Question loader
Skip logic module
Answer storerTiming data
Answer check
Logging module
Tip #4: In Reuse, Prefer Composition over Inheritance
• Use composition to add features or to reuse code• Use inheritance to add a new version of an entity
Survey Instance
Questions Answers
numeric question
multiple choice question
free-text question
numeric answer
multiple choice answer
free-text answer
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Interface and Polymorphism
• An interface is a promise:• I can provide this output if you provide a valid input• If you can meet these preconditions, then I can meet these
postconditions
• Polymorphism: If A, B, C, and D provide same interface, then they all make the same promise• May keep the promise in different ways
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Incremental vs. Iterative
• Use incremental development when:• Much of the system’s value resides in one subsection• One part of the system must be completed (logically) before another
• Use iterative development when:• System’s value is spread out over much of the system• The whole system needs to work before you can build it up
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Incremental Examples
• Adding new kinds of print outs• Printout carries much of system’s value
• Adding a new data export module• Logically, main system needs to work before worrying about
exporting data
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Iterative Examples
• Tweaking reports and user interface (surveyor) to improve usability• Improvements to existing system pieces
• Adding new kinds of questions (and answers), changing reports• Changes spread across system