Week 2 Design Examples and Designing 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 on Changability Change abounds… – During design – During coding – Upon delivery – After use Reuse for later projects

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, …

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

The Ideal Program … vs.

A Very Simple Example A sorted list class with: 1) getArray() : Array 2) getElementAt(int i) : Element

A Very Simple Example A List class with: 1) getArray() : Array 2) getElementAt(int i) : Element Its too slow, so we switch to a hash table 1) Everywhere you call getArray needs changing 2) Only List is changed Little things like this add up

UML Review: Class Diagrams Class Name Attribute : Type Operation (parameter) : Return Type Attribute : Type Operation (parameter) : Return Type

UML Review: Class Diagrams Association Aggregation Generalization Composition

A Class Diagram

Example 1: Theseus and the Minotaur

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?

Bonus! StompOn( stompee ) Or StompedOn( stomper )

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 klax_eng.php

A Good Klax Design?

Chiron 2 Architectural Style

C2 Klax

Spelling Klax?

Scrabble? How did you design for change?

Scrabble? How did you design for change? Could you redesign / refactor for – Changes to letter values?

Scrabble? How did you design for change? Could you redesign / refactor for – Changes to letter values? – Changes to bonus squares?

Scrabble? How did you design for change? Could you redesign / refactor for – Changes to letter values? – Changes to bonus squares? – AI opponents?

Scrabble? How did you design for change? Could you redesign / refactor for – 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 More broadly, you are asking: – What are the biggest change risks? – What kind of program is this, essentially? – Where is “the line”?

Assignment #1 Continued We’ll exchange designs now Review forms will be online tonight Send completed forms to Andre and I – – – Due 6:00 pm, Sunday