Principles of Effective Code Design and Best Practices" (66 characters)

Object/Class Design 1

Principles of straight-line code Make dependences obvious: (e.g., through passing arguments, return values) firstResult = doThing1(); secondResult = doThingY(firstResult); Vs. doThing1(); doThingY(); 2

Grouping related items (example) Ordering implicit, but emphasizes grouping MarketingData marketingData = new MarketingData(); marketingData.ComputeQuarterly(); marketingData.ComputeAnnual(); marketingData.Print(); SalesData salesData = new SalesData(); salesData.ComputeQuarterly(); salesData.ComputeAnnual(); salesData.Print(); 3

Which is better? This code takes an array of strings, it processes all of the strings before a dash one way and all of the remaining strings another way. Assume there is only one dash in the array of strings. boolean dashFound = false; for (String arg : args) { if (arg.equals("-")) { dashFound = true; } else if (!dashFound) { process1(arg); } else { process2(arg); } } A int i = 0; while(i < args.length && !args[i].equals("-")) { process1(args[i]); i++; } B i++; // skip the dash for( ; i < args.length ; i++) { process2(args[i]); } C D Control flow is fine for both Control flow is problematic for both 4

Which is better? public int[] copyIntArray(int[] input) { int [] copy = new int[input.length]; int i = 0; for (int value: input) { copy[i++] = value; } return copy; } A public int[] copyIntArray(int[] input) { int [] copy = new int[input.length]; for (int i = 0; i < input.length; i++) { copy[i] = input[i]; } return copy; } B C D Control flow is fine for both Control flow is problematic for both 5

Which is best? A if (getAmountOfGasInTank() >= gasNeeded(destination)) { // avoid unnecessary stops; reduce wear on engine } else { fillGasTank(); } if (getAmountOfGasInTank() < gasNeeded(destination)) { fillGasTank(); } else { // avoid unnecessary stops; reduce wear on engine } if (gasNeeded(destination) < getAmountOfGasInTank()) { // avoid unnecessary stops; reduce wear on engine } else { fillGasTank(); } if (gasNeeded(destination) >= getAmountOfGasInTank()) { fillGasTank(); } else { // avoid unnecessary stops; reduce wear on engine } B C D 6

Design is hard Design is an art, not a science Large/infinite design space, not enumerable Requirements, constraints, trade-offs, and priorities You get better with practice / experience / seeing good design / hearing critiques of others designs 7

Virtues of a good design (software) Manages complexity Loose coupling Reusability Ease of maintenance Standard techniques Extensibility High Fan-in Low-to-medium Fan-out 8

Good Design Manages Complexity Seven plus or minus two (Miller s Law) The goal of all software-design techniques Break complicated problems into simple problems Separation of concerns Focus on one at a time 9

Keep Coupling Loose small interfaces (few methods, few arguments/method) obvious (interactions through parameter passing) flexible 10

Abstract Data Types Define a class based around conceptual structures Encapsulation / information-hiding Make interfaces more informative (self-documenting) Easier to reason about correctness Treat even simple items as ADTs Good for extensibility 11

Inheritance can provides 2 things Shared interface: Public methods Ensure methods have consistent meaning in derived class Liskov Substitution Principle Shared implementation Code in shared super class, not replicated in each derived Could be private data/methods 12

hasA vs. isA relationship Which is a candidate for inheritance? A) hasA relationship B) isA relationship C) both hasA and isA relationships D) neither hasA and isA relationships 13

Inheritance vs. Interfaces Inheritance should be a isA relationship Interfaces are for capabilities ( mixin s) 14

Designing Good Interfaces Sufficiently Expressive General Minimal 15