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Object Oriented Design in the

6 package-level principles +

DRY YAGNI+&

SOLID

classestypesJava

Haskell Ruby

ECMAScript

?

Single Responsibility

Principle

SOLID

Don’tRepeatYourself

Sell

Track

Buy

class Good implements InvItem, Purchase, Sellable{ InventoryImpl inventory; PurchaseImpl purchase; SellableImpl sellable; ... }

Java

Ruby

class Good include SellableItem include InventoryTracking include Purchaseend

Ruby

joy discipline

Haskell

joydiscipline

data Good = Good SaleInfo InvInfo PurchInfo

instance ReceiptItem Good where format Good s _ _ = desc s + price s

Haskell

dictionaries

functions

ECMAScript

ECMAScript

var good = _.extend({}, invInfo, saleInfo, purInfo)

{ name: “Canteen”, sku: 37671287, price: 1399, format: function() {...} cost: 715}

InterfaceSegregation

Principle

SOLID

String receiptLine(Sellable item)

Java

ECMAScript

function receiptLine(item) { console.log(“Cha-ching! profit ” + item.price - item.cost) return format(item.desc, item.price) }

Haskell

print :: ReceiptItem -> String

LiskovSubstitution

Principle

SOLID

Java

List sneaky = new MyList()...

if (sneaky.length == -1) { System.out.println(“WAT!”)}

LSP

PLS

Principle of

Least

Surprise

Ruby

def form_receipt_line(item_sold) ... line_with_desc_and_priceend

violate expectations

access global state

modify input

change the world

DependencyInversionPrinciple

SOLID

App

Hibernate

App

HibernatePersistence

API

HibernatePersistence APIAppfunction

another function

data

ECMAScript

find items

format receipt

lineitem data

var items = _.map(codes, findItem)var lines = _.map(items, formatReceiptLine)

> cat data.csv | grep 42 | cut -f2 | sort

ksh

Open/ClosedPrinciple

SOLID

Extension

Closed

App

Module

LibraryClosed for

modification

> cat data.csv | grep 42 | cut -f2 | sort

package principles

3 x cohesion3 x coupling

couplingcohesion

Common Reuse

Principle

Common ChangesPrinciple

Reuse-ReleaseEquivalency

Principle

App

LibraryReuse This

right problem

best solutionevidence of actual use

name and location

code reviews documentation

testsexisting functionality

maintenanceburden

machinery

bugslegacy code

Planning for reuse is premature

optimization

Premature optimizationis the root of all evil

Donald Knuth

AcyclicDependencies

Principle

App

Module

Library

App

Module

Library

Module Module

Library LibraryLibraryLibrary

App

Stable Abstractions

Principle

Stable Dependency

Principle

Increasing stability

Concrete

Abstract

Reusable

You Are Here

Abstract

DRY

getTaxRate capitalize

customerRewards formatDate

YouAin’tGonnaNeed It

ConcreteYou Are Here Simple

Time Reading

UseChange

$ $$$

6 package-level principles +

DRY YAGNI+&

SOLID

SmallClear intent

older = better

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