Design principles

•The primary mechanism of OO design •No dependency should target a concrete class –Non-volatile classes (e.g. Java core library classes)...

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Design principles OOD: Lecture 4

Next lecture • UML: Thursday, Sep 20, at 8:15 am in 1211

Reminder: Readings • Wirfs-Brock, R., and B. Wilkerson (1989) “Object-oriented design: a responsibilitydriven approach” • Wikipedia's entry on Object-oriented Design

“Software rot” • Increased difficulty to adapt and maintain • Causes – Communication/Documentation breakdown • Maintainers not fully familiar with the original design principles -> change works, but…

– Design is not resilient in the face of change

Symptoms of rotting design • Rigidity – Every change causes a cascade of subsequent changes in dependent modules. “2 days -> 2 months”

• Fragility – Breaks in many places when a change is made

• Immobility – Reuse is more work than creating from scratch

• Viscosity: The law of least resistance when faced with a choice – Design viscosity: Hacks are easier/faster than preserving the design – Environment viscosity: Slow cycle time -> fastest choice

Dependency management • Rigidity, fragility, immobility, and viscosity are all four – arguably – caused by an improper dependency structure • Three groups of preventive principles / guidelines – Class design – Package cohesion – Package coupling

Principles of object-oriented class design

SOLID: • • • • •

SRP: The single responsibility principle OCP: The Open Closed principle LSP: The Liskov substitution principle ISP: The interface segregation principle DIP: The dependency inversion principle

SRP The single responsibility principle • A class should have one, and only one, reason to change. • A class should have a single responsibility • Example interface Modem public void public void public void public char }

{ // -- SRP Violation dial(String phoneNumber); hangup(); send(char c); receive();

SRP continued • Two responsibilities – Connection management: dial and hangup – Data communication: send and receive

• Better

– Nothing depends on the modem implementation class

OCP The Open Closed principle • A module should be open for extension but closed for modification. – Ability to change what the module does, without changing its source code – Techniques based on abstraction • Dynamic polymorphism • Static polymorphism

Dynamic polymorphism OCP violation example struct Modem { enum Type {hayes, courier, ernie) type; }; struct Hayes { Modem::Type type; // Hayes related stuff }; struct Courier { Modem::Type type; // Courier related stuff }; struct Ernie { Modem::Type type; // Ernie related stuff }; void LogOn(Modem& m, string& pno, string& user, string& pw) { if (m.type == Modem::hayes) { DialHayes((Hayes&)m, pno); } else if (m.type == Modem::courier) { DialCourier((Courier&)m, pno); } else if (m.type == Modem::ernie) { DialErnie((Ernie&)m, pno) // ... } }

OCP: Dynamic polymorphism continued class Modem public: virtual virtual virtual virtual };

{ void void char void

Dial(const string& pno) = 0; Send(char) = 0; Recv() = 0; Hangup() = 0;

void LogOn(Modem& m, string& pno, string& user, string& pw) { m.Dial(pno); // you get the idea. }

OCP: Static polymorphism • Templates/Generics template void LogOn(MODEM& m, string& pno, string& user, string& pw) { m.Dial(pno); // ... }

LSP The Liskov substitution principle • Derived classes must be substitutable for their base classes. • The contract of the base class must be honoured by the derived class • A derived class is substitutable for its base class if: – Its pre-conditions are no stronger than the base class method. – Its post-conditions are no weaker than the base class method.

• Or, in other words, derived methods should expect no more and provide no less.

LSP violation The Circle/Ellipse dilemma • A circle is-an ellipse

LSP violation 2 • A client code fragment: void f(Ellipse e) { Point a = new Point(0, 1); Point b = new Point(1, 0); e.setFoci(a, b); e.setMajorAxis(3); assert e.getFocus1() == a; assert e.getFocus2() == b; assert e.getMajorAxis() == 3; }

LSP violation 3 • Ugly client-side fix void f(Ellipse e) { if (e.getClass().equals( Ellipse.class)) { //… } else { throw new Exception( “Not a real ellipse”); }

ISP The interface segregation principle • Make fine grained interfaces that are client specific. Or Many client specific interfaces are better than one general purpose interface • [Kent] Do not change interfaces unless absolutely necessary, and especially do not change method signatures

DIP The dependency inversion principle • Depend on abstractions, not on concretions. • The primary mechanism of OO design • No dependency should target a concrete class – Non-volatile classes (e.g. Java core library classes) tend to cause less problems

Principles of package cohesion • REP: The release reuse equivalency principle • CCP: The common closure principle • CRP: The common reuse principle Note that these three exist in a balance. They can’t all three be completely satisfied at the same time

REP The release reuse equivalency principle • The granule of reuse is the granule of release. • Package together what would be reused together • Support and maintain older versions • Simplifies reuse

CCP The common closure principle • Classes that change together are packaged together. • Minimizes configuration management (CM) work – I.e. management, test, and release of packages

• Simplifies development and maintenance • Tends towards big packages

CRP The Common Reuse Principle • Classes that aren’t reused together should not be grouped together. • Complement of REP • Avoid forcing unnecessary client re-building • Simplifies reuse • Tends to small packages

Principles of package coupling • ADP: The acyclic dependencies principle • SDP: The stable dependencies principle • SAP: The stable abstractions principle

ADP The acyclic dependencies principle • The dependency graph of packages must have no cycles. • Cycles increase the work to re-build and eventually make every package depend on every other package • Breaking a cycle – New package: Break out of dependency target – Apply dependency inversion (DIP) + interface segregation (ISP)

ADP: Breaking a cycle Applying DIP & ISP

SDP The stable dependencies principle • Depend in the direction of stability. • A way of reducing the number of packages that are hard to change because changes would propagate to many other packages • 𝐼𝑛𝑠𝑡𝑎𝑏𝑖𝑙𝑖𝑡𝑦 =

𝐶𝑒 𝐶𝑎 +𝐶𝑒

• Depend upon packages whose Instability metric is lower than yours

SAP The stable abstractions principle • Abstractness increases with stability or Stable packages should be abstract packages. • Can be seen as a re-formulation of A dependency inversion (DIP) 1 • Abstract – stable – easy to extend (OCP) • Concrete – instable – easy to change I

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Next lecture • Thursday, Sep 20, at 8:15 am in 1211