Object Oriented Design Principles Arnon Rotem-Gal-Oz Product Line Architect

Preface Nothing really new here Just a summary of other peoples work: D.L. Parnas M. Fowler R.C. Martin, B. Meyer, B. Liskov W.J. Brown, R.C. Malveau, H.W. McCormick, T.J. Mowbray GoF N. Malik A. Holub And probably others I forgot

Agenda 7 Deadly Sins of Design The Rules Basic Stuff Evil Stuff More Stuff

7 Deadly Sins of Design Rigidity – make it hard to change Fragility – make it easy to break Immobility – make it hard to reuse Viscosity – make it hard to do the right thing Needless Complexity – over design Needless Repetition – error prone Not doing any

The Rules Apply Common Sense Dont get too dogmatic / religious Every decision is a tradeoff All other principles are just that Guidelines best practices Consider carefully if you should violate them - but, know you can.

Basic Stuff OCP open-closed principle SRP single responsibility principle ISP interface segregation principle LSP Liskov substitution principle DIP dependency inversion principle Dependency Injection

Open Closed Principle Software entities ( Classes, Modules, Methods, etc. ) should be open for extension, but closed for modification. Also Known As Protected Variation What Parnas meant when he coined Information hiding

Why OCP If not followed a single change to a program results in a cascade of changes to dependent modules. The program becomes fragile, rigid, unpredictable and un-reusable.

OCP Example TemplateMethod

OCP implications (examples) Use private modifier for class members Refrain from adding Getters automatically Use abstractions Extend by inheritance, delegation Inversion of Control (IoC)

Single Responsibility Principle A Class should have one reason to change A Responsibility is a reasons to change Can be tricky to get granularity right

Why SRP Single Responsibility = increased cohesion Not following results in needless dependencies More reasons to change. Rigidity, Immobility

SRP Example (1/2) The Rectangle has 2 responsibilities Algorithm Client Application Rectangle Draw() Area() DirectX Suddenly the Algorithm needs DirectX Dependency

SRP Example (2/2) Rectangle Area() Client Application Rectangle Renderer Draw() DirectX Algorithm Dependency

Interface Segregation Principle Many client specific interfaces are better than one general purpose interface Create an interface per client type not per client Avoid needless coupling to clients

Why ISP Otherwise – increased coupling between different clients Basically a variation on SRP

ISP example Client Application Rectangle Impl. DirectX Algorithm Dependency IRectangle Area() IRectangle Renderer Draw() Realization

Liskov Substitution Principle What is wanted here is something like the following substitution property: If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T. (Barbara Liskov, 1988)

Or in English Any subclass should always be usable instead of its parent class. Corollary - All derived classes must honour the contracts of their base classes IS A = same public behavior Pre-conditions can only get weaker Post-conditions can only get stronger

Why LSP Failing to follow LSP result in a mess OCP violations (if/then to identify types) Superclass unit test will fail Strange behavior

Trivial Example Why is that an LSP violation?

Real-Life Example.NET creates a transparent proxy and intercepts all calls to ContextBoundObjects Implementing IContextObjectSink On a Implementing IContextObjectSink On a class derived from ContrextAttribute lets you add a sink in the chain

Real-Life Example ServicedComponents violates LSP: violates LSP: You cannot add your own sinks (it only uses its own)

Dependency Inversion Principle Higher level modules should not depend on lower level modules Both should depend on abstractions Interfaces or Abstract classes Abstractions should not depend on details (Not to be confused with Dependency Injection and Inversion of Control) (Not to be confused with Dependency Injection and Inversion of Control)

Why DIP Increase loose coupling Abstract interfaces don't change Concrete classes implement interfaces Concrete classes easy to throw away and replace Increase mobility Increase isolation decrease rigidity decrease rigidity Increase testability Increase maintainablity Closely related to LSP

Reminder - Procedural Design

DIP

DIP implications Layers Interface based programming Separated Interface put interface in separate package than implementation put interface in separate package than implementation Dependency Injection

Old Way public class MyApp { public MyApp() { authenticator = new Authenticator(); database = new Database(); logger = new Logger(); errorHandler = new ErrorHandler(); } // More code here... }

Dependency Injection DIP says we should depend on an interface how do we get the concrete instance

New Way? public class MyApp { public MyApp() { authenticator = new IAuthenticator(); database = new IDatabase(); logger = new ILogger(); errorHandler = new IErrorHandler(); } // More code here... } OOPS

Dependency Injection Option 1 – Factory User depends on factory Factory depends on destination Option 2 – Locator/Registry/Directory The component still controls the wiring Instantiation Sequence Dependency on the Locator Option 3 – Dependency Injection An assembler controls the wiring

DI Options Setter Injection The component is passive Someone injects the dependency dependency

DI Options Setter Injection Constructor Injection Always initialized Better dependency visibility

Other DI Options Interface Injection Variation on setter injection Getter Injection Needs AOP (not clean)

Evil Stuff Switch statements If (type()) Singletons / Global variables Getters Helper Classes

More Stuff Package principles Not the core of this presentation Smells Anti-Patterns

Package Principles Reuse-Release Equivalency Principle Common Closure Principle Common Reuse Principle Acyclic Dependencies Principle Stable Dependencies Principle Stable Abstractions Principle

CodeSmells Something that's quick to spot Indication for a possible problem Not always the problem it self May not be a problem at all CodeSmell example – Long Method

DesignSmell (1) Many CodeSmells can also apply to design Long Parameter List Large Class (Swiss Army knife / Blob) Type Embedded in Name Uncommunicative Name Data Class Refused Bequest (LSP violations)

DesignSmell (2) Inappropriate Intimacy Lazy Class Feature Envy (Managers) Shotgun Surgery Parallel Inheritance Hierarchies Message Chains Component Without Interface Singletons

Design Related Anti-Patterns The Blob Functional Decomposition Poltergeist Golden Hammer Swiss Army Knife Kevorkian Component (Dead End) (Other Anti-Patterns deal with Architecture, Management, Code)

Thank you… Arnon Rotem-Gal-Oz