week 2 design examples and designing for change alex baker
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Week 2Design Examples andDesigning for Change
Alex Baker
Implementation Design
An implementation design is a road map– understandable, unambiguous, consistent, helpful, …
An implementation design describes a path from system design to the outcome
– correct, complete, concise, verifiable, effective, …
An implementation design describes what the implementers should do
– elegant, partitionable, recomposable, resilient, …
An implementation design is a guide towards future change
– evolvable, …
Emphasis: Changability
Subtle
Requires foresight and careful balancing
An implementation design describes what the implementers should do
– elegant, partitionable, recomposable, resilient, …
An implementation design is a guide towards future change
– evolvable, …
Emphasis: Changability
Reusable Extensible Maintainable Portable Well-Documented Adaptable
Simplicity Readability Minimalistic Elegant Well-Organized Concise
Efficiency Powerful
Changability: Basic Principles
Low Coupling: Reducing interdependency– Changes don’t propagate– Reuse is facilitated
High Cohesion: Grouping functionality– Easier to find things– Metaphor guides decisions
Information Hiding
Information Hiding Made Very Simple
A List class with:1) getArray() : Array
2) getElementAt(int i) : Element
Information Hiding Made Very Simple
A List class with:1) getArray() : Array
2) getElementAt(int i) : Element
Its too slow, so we switch to a hash table1) Everywhere you call getArray needs changing
2) Only List is changed
Little things like this add up
The Ideal Program
…
vs.
Why the Emphasis on Changability?
Change abounds…– During coding– After use– Reuse for later projects
Why is there so Much Change?
Designs
Outcomes
Why is there so Much Change?
Designs
Outcomes
Engineering
PhysicsPrinciplesExisting Examples
Why is there so Much Change?
Designs
Outcomes
Software Engineering
PhysicsPrinciplesExisting Examples
Why is there so Much Change?
Designs
Outcomes
Software Engineering
PhysicsPrinciplesExisting Examples
Why is there so Much Change?
Designs
Outcomes
Software Engineering
PhysicsPrinciplesExisting Examples
Why the Emphasis on Changability?
Change abounds…– During coding– After use– Reuse for later projects
You usually just don’t know for sure…
Changes During Coding
Refinement of the high level design
Dependencies reveal themselves– Oh, I need to know [X].– I can’t access that data?
‘It turns out this class is HUGE’
Need to redesign as you go Can changes be made within the design?
Changes during Use
Potential breakdowns at several levels
– Customers have needs
– Customers make requests
– Developers provide software
Reuse
A changing of context
…
Change Happens
Let’s design for it
Our Approach
Lots of examples (3 problems, many solutions, today)
Some overarching lessons
Build up an intuition
UML Review: Class Diagrams
Class Name
Attribute : Type
Operation (parameter) : Return Type
Attribute : Type
Operation (parameter) : Return Type
Operation (parameter) : Return Type
UML Review: Class Diagrams
Association
Aggregation
Generalization
Composition
A Class Diagram
Example #1: Theseus and the Minotaur
http://www.logicmazes.com/theseus.html
Original Theseus Design
Original Theseus Design
What if we want to add “Water” ?
Original Theseus Design
What if we want to add “Water” ?
?
Original Theseus Design
?
What if we want to add “Water” ?
How do we fix this?
TntM: Changes
Changing the board size
Adding terrain types
Adding more monsters
Adding a second player
“Intelligent elements”
TntM 2
TntM 2: Object Interfaces & Inheritance
TntM: Changes
Changing the board size
Adding terrain types
Adding more monsters
Adding a second player
“Intelligent elements”
How far is too far?
Changing board size?
Pushable blocks?
Intelligent elements?
Real time gameplay?
Physics challenges?
TntM 2
Bonus!
StompOn( stompee )
Or
StompedOn( stomper )
Aside: Three Degrees of Support
Is it a good solution?
Can they build that solution?
Can that solution be used to make other good solutions?
Example #2 Klax
Blocks fall from above, use a paddle to catch Deposit blocks in one of 5 columns Try to make lines of 3 in a row or more
http://www.123games.dk/game/puzzle/klax3d/klax_eng.php
A Good Klax Design?
Chiron 2 Architectural Style
C2 Klax
Spelling Klax?
Spelling Klax?
Example #3: Scrabble
Function Overload
Changes to rack
Class Overload
(besides missing details)
Is this a good design?
Overengineered?
What change are you designing for?
Multiple point values for a given tile type?
More types of board objects?
Overengineered?
Meanwhile:
Board doesn’t actually work
Populating the tiles is likely a hassle
Tilepool vs. Player.currentTiles
Inelegance lead to problems
A Nice Approach?
A Nice Approach?
Scrabble
Can a design accommodate:
– Changes to letter values?
– Changes to bonus squares?
– AI opponents?
– Different dictionaries?
Summary
Designing for change is a matter of:– Controlling dependency– Proper encapsulation– Generalizing where you can
Which changes?– What are the biggest change risks?– What kind of program is this, essentially?– Where is “the line”?
Practically:
What is the core we know will be true?– What variations on that will we see?
If there’s something that needs to change:– Can it be changed in one place?– Does anything else need to be changed when it changes?
Can each class ignore its context?
Discipline
Assignment #1 Continued
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