Cursul 5 – 19 Martie 2019 [email protected]
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Din Cursurile trecute…
SOLID and Other Principles
GRASP
◦ Low coupling
◦ High cohesion
Unit Testing
De ce avem nevoie de modelare?
Cum putem modela un proiect?
SCRUM – roles, values, artifacts, events, rules
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SOLID Principles
◦ SRP – Single Responsibility Principle
◦ OCP – Open/Closed Principle
◦ LSP – Liskov Substitution Principle
◦ ISP – Interface Segregation Principle
◦ DIP – Dependency Inversion Principle
DRY – Don't Repeat Yourself
YAGNI – You Aren't Gonna Need It
KISS – Keep It Simple, Stupid
SOLID was introduced by Robert C. Martin in the an article called the “Principles of Object Oriented
Design” in the early 2000s
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Every object should have a single responsibility, and all its services should be narrowly aligned with that responsibility
“The Single Responsibility Principle states that every object should have a single responsibility, and that responsibility should be entirely encapsulated by the class.” – Wikipedia
“There should never be more than one reason for a class to change.” - Robert Martin
Low coupling & strong cohesion
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Classic violations
◦ Objects that can print/draw themselves
◦ Objects that can save/restore themselves
Classic solution
◦ Separate printer & Separate saver
Solution
◦ Multiple small interfaces (ISP)
◦ Many small classes
◦ Distinct responsibilities
Result
◦ Flexible design
◦ Lower coupling & Higher cohesion
Two responsabilities
Separated interfaces
interface Modem {
public void dial(String pno);
public void hangup();
public void send(char c);
public char recv();
}
interface DataChannel { public void send(char c);
public char recv();
}
interface Connection {
public void dial(String phn);
public char hangup();
}
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Open chest surgery is not needed when putting on a coat
Bertrand Meyer originated the OCP term in his 1988 book, Object Oriented Software Construction
“The Open / Closed Principle states that software
entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.” – Wikipedia
“All systems change during their life cycles. This must be borne in mind when developing systems expected to last longer than the first version.” - Ivar Jacobson
Open to Extension - New behavior can be added in the future
Closed to Modification - Changes to source or binary code are not required
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Change behavior without changing code?!
◦ Rely on abstractions, not implementations
◦ Do not limit the variety of implementations
In .NET – Interfaces, Abstract Classes
In procedural code - Use parameters
Approaches to achieve OCP
◦ Parameters - Pass delegates / callbacks
◦ Inheritance / Template Method pattern - Child types override behavior of a base class
◦ Composition / Strategy pattern - Client code depends on abstraction, "Plug in" model
Classic violations
◦ Each change requires re-testing (possible bugs)
◦ Cascading changes through modules
◦ Logic depends on conditional statements
Classic solution
◦ New classes (nothing depends on them yet)
◦ New classes (no legacy coupling)
When to apply OCP?
◦ Experience tell you
OCP add complexity to design (TANSTAAFL)
No design can be closed against all changes
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// Open-Close Principle - Bad example class GraphicEditor {
public void drawShape(Shape s) { if (s.m_type==1)
drawRectangle(s);
else if (s.m_type==2) drawCircle(s);
}
public void drawCircle(Circle r) {....}
public void drawRectangle(Rectangle r) {....}
}
class Shape { int m_type;
}
class Rectangle extends Shape { Rectangle() {super.m_type=1;}
}
// Open-Close Principle - Good example
class GraphicEditor {
public void drawShape(Shape s) { s.draw();
} }
class Shape {
abstract void draw();
}
class Rectangle extends Shape { public void draw() {
// draw the rectangle }
}
If it looks like a duck, quacks like a duck, but needs batteries – you probably have the wrong abstraction
Barbara Liskov described the principle in 1988
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"The Liskov Substitution Principle states that Subtypes must be substitutable for their base
types.“ - Agile Principles, Patterns, and Practices in C#
Substitutability – child classes must not
◦ Remove base class behavior
◦ Violate base class invariants
Normal OOP inheritance
◦ IS-A relationship
Liskov Substitution inheritance
◦ IS-SUBSTITUTABLE-FOR
The problem
◦ Polymorphism break Client code expectations
◦ "Fixing" by adding if-then – nightmare (OCP)
Classic violations
◦ Type checking for different methods
◦ Not implemented overridden methods
◦ Virtual methods in constructor
Solutions
◦ “Tell, Don’t Ask” - Don’t ask for types and Tell the object what to do
◦ Refactoring to base class- Common functionality and Introduce third class
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// Violation of Liskov's Substitution Principle class Rectangle{
int m_width;
int m_height;
public void setWidth(int width){
m_width = width;
}
public void setHeight(int h){
m_height = ht;
}
public int getWidth(){
return m_width;
}
public int getHeight(){
return m_height;
}
class Square extends Rectangle { public void setWidth(int width){
m_width = width;
m_height = width;
}
public void setHeight(int height){
m_width = height;
m_height = height;
} }
class LspTest
{private static Rectangle getNewRectangle()
{ // it can be an object returned by some factory ...
return new Square();
}
public static void main (String args[])
{ Rectangle r = LspTest.getNewRectangle();
r.setWidth(5);
r.setHeight(10);
// user knows that r it's a rectangle. It assumes that he's able to set the width and height as for the base class
System.out.println(r.getArea());
// now he's surprised to see that the area is 100 instead of 50.
}}
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You want me to plug this in. Where?
“The Interface Segregation Principle states that
Clients should not be forced to depend on methods they do not use.” - Agile Principles, Patterns, and Practices in C#
Prefer small, cohesive interfaces - Interface is the interface type + All public members of a class
Divide "fat" interfaces into smaller ones
◦ “fat” interfaces means classes with useless methods,
increased coupling, reduced flexibility and maintainability
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Classic violations
◦ Unimplemented methods (also in LSP)
◦ Use of only small portion of a class
When to fix?
◦ Once there is pain! Do not fix, if is not broken!
◦ If the "fat" interface is yours, separate it to smaller ones
◦ If the "fat" interface is not yours, use "Adapter" pattern
Solutions
◦ Small interfaces
◦ Cohesive interfaces
◦ Focused interfaces
◦ Let the client define interfaces
◦ Package interfaces with their implementation
//Bad example (polluted interface)
interface Worker { void work();
void eat();
}
ManWorker implements Worker { void work() {…};
void eat() {30 min break;};
}
RobotWorker implements Worker { void work() {…};
void eat() {//Not Appliciable for a RobotWorker};
}
//Solution: split into two interfaces
interface Workable {
public void work();
}
interface Feedable{
public void eat();
}
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Would you solder a lamp directly to the electrical wiring in a wall?
“High-level modules should not depend on low-level modules. Both should depend on abstractions.”
“Abstractions should not depend on details.
Details should depend on abstractions.” -
Agile Principles, Patterns, and Practices in C#
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Framework
Third Party Libraries
Database
File System
Web Services
System Resources (Clock)
Configuration
The new Keyword
Static methods
Thread.Sleep
Random
How it should be
◦ Classes should declare what they need
◦ Constructors should require dependencies
◦ Dependencies should be abstractions and be shown
How to do it
◦ Dependency Injection
◦ The Hollywood principle "Don't call us, we'll call you!"
Classic violations
◦ Using of the new keyword, static methods/properties
How to fix?
◦ Default constructor, main method/starting point
◦ Inversion of Control container
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//DIP - bad example
public class EmployeeService {
private EmployeeFinder emFinder //concrete class, not abstract. Can access a SQL DB for instance public Employee findEmployee(…) {
emFinder.findEmployee(…)
} }
//DIP - fixed
public class EmployeeService {
private IEmployeeFinder emFinder //depends on an abstraction, no an implementation public Employee findEmployee(…) {
emFinder.findEmployee(…) }
}
Don't Repeat Yourself (DRY)
You Ain't Gonna Need It (YAGNI)
Keep It Simple, Stupid (KISS)
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Repetition is the root of all software evil
"Every piece of knowledge must have a single, unambiguous representation in the system.“
- The Pragmatic Programmer
"Repetition in logic calls for abstraction.
Repetition in process calls for automation.“ - 97 Things Every Programmer Should Know
Variations include:
◦ Once and Only Once
◦ Duplication Is Evil (DIE)
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Magic Strings/Values
Duplicate logic in multiple locations
Repeated if-then logic
Conditionals instead of polymorphism
Repeated Execution Patterns
Lots of duplicate, probably copy-pasted, code
Only manual tests
Static methods everywhere
Don’t waste resources on what you might need
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"A programmer should not add functionality until deemed necessary.“ – Wikipedia
"Always implement things when you actually need them, never when you just foresee that you need them.“ - Ron Jeffries, XP co-founder
Time for adding, testing, improving
Debugging, documented, supported
Difficult for requirements
Larger and complicate software
May lead to adding even more features
May be not know to clients
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You don’t need to know the entire universe when living on the Earth
"Most systems work best if they are kept simple.“ - U.S. Navy
"Simplicity should be a key goal in design and unnecessary complexity should be avoided.“
- Wikipedia
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GRASP = General Responsibility Assignement Software Patterns (Principles)
Descrise de Craig Larman în cartea
Applying UML and Patterns. An Introduction to Object Oriented Analysis and Design
Ne ajută să alocăm responsabilități claselor și obiectelor în cel mai elegant mod posibil
Exemple de principii folosite în GRASP:
Information Expert
(sau Expert),Creator
,High Cohesion
,Low Couplig
,Controller
Polymorphism, Pure Fabrication, Indirection,
Să facă:
◦ Să facă ceva el însuși, precum crearea unui obiect sau să facă un calcul
◦ Inițializarea unei acțiuni în alte obiecte
◦ Controlarea și coordonarea activităților altor obiecte
Să cunoască:
◦ Atributele private
◦ Obiectele proprii
◦ Lucrurile pe care le poate face sau le poate apela
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Traducere: șablon, model
Este o soluție generală la o problemă comună
Fiecare pattern are un nume sugestiv și ușor de reținut (ex. composite, observer, iterator,
singleton, etc.)
Problemă: dat un anumit comportament
(operație), cărei clase trebuie să-i fie atribuit?
O alocare bună a operațiilor conduce la sisteme care sunt:
◦ Ușor de înțeles
◦ Mai ușor de extins
◦ Refolosibile
◦ Mai robuste
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Soluție:
asignez o responsabilitate clasei care are
informațiile necesare pentru îndeplinirea acelei responsabilități
Recomandare:
începeți asignarea responsabilităților evidențiind clar care sunt responsabilitățile
Carei clase trebuie sa-i fie asignată metoda getTotal()? Mai trebuie alte metode?
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Clasă Responsabilități
Sale să cunoască valoarea totală a cumpărăturilor SalesLineItem să cunoască subtotalul pentru un produs ProductSpecification să cunoască prețul produsului
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Problemă: cine trebie să fie responsabil cu crearea unei instanțe a unei clase?
Soluție: Asignați clasei B responsabilitatea de a crea instanțe ale clasei A doar dacă cel puțin una dintre următoarele afirmații este adevărată:
◦ B agregă obiecte de tip A
◦ B conține obiecte de tip A
◦ B folosește obiecte de tip A
◦ B are datele de inițializare care trebuie transmise la
instanțierea unui obiect de tip A (B este deci un Expert în ceea ce privește crearea obiectelor de tip A)
Factory pattern
este o variantă mai complexă47
Cine este responsabil cu crearea unei instanțe a clasei SalesLineItem?
Deoarece Sale conține (agregă) instanțe de tip
SalesLineItem, Sale este un bun candidat pentru a i se atribui responsabilitatea creării acestor instanțe
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Cuplajul este o măsură a gradului de dependență a unei clase de alte clase
Tipuri de Dependență:
◦ este conectată cu
◦ are cunoștințe despre
◦ se bazează pe
O clasă care are cuplaj mic (redus) nu depinde de “multe” alte clase; unde “multe” este
dependent de contex
O clasă care are cuplaj mare depinde de multe alte clase
Probleme cauzate de cuplaj:
◦ schimbări în clasele relaționate forțează schimbări locale
◦ clase greu de înțeles în izolare (scoase din context)
◦ clase greu de refolosit deoarece folosirea lor presupune și prezența claselor de care
depind
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Forme comune de cuplaj de la clasa A la clasa B sunt:
◦ A are un atribut de tip B
◦ O instanță a clasei A apelează un serviciu oferit de un obiect de tip B
◦ A are o metodă care referențiază B
(parametru, obiect local, obiect returnat)
◦ A este subclasă (direct sau indirect) a lui B
◦ B este o interfață, iar A implementează această interfață
Don’t talk to strangers
Orice metodă a unui obiect trebuie să apeleze doar metode aparținând
◦ lui însuși
◦ oricărui parametru al metodei
◦ oricărui obiect pe care l-a creat
◦ oricăror obiecte pe care le conține
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Diagrama de clase
Diagrama de colaborare
Exista legături între toate clasele
Elimină cuplajul dintre Register și Payment
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Coeziunea este o măsură a cât de puternic sunt focalizate responsabilitățile unei clase
O clasă ale cărei responsabilități sunt foarte
strâns legate și care nu face foarte multe lucruri are o coeziune mare
O clasă care face multe lucruri care nu sunt relaționate sau face prea multe lucruri are o coeziune mică (slabă)
Probleme cauzate de o slabă coeziune:
◦ greu de înțeles
◦ greu de refolosit
◦ greu de menținut
◦ delicate; astfel de clase sunt mereu supuse la schimbări
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Sunt principii vechi în design-ul software
Promovează un design modular
Modularitatea este proprietatea unui sistem care a fost descompus într-o mulțime de module
coezive și slab cuplate
Problemă: Cine este responsabil cu tratarea unui eveniment generat de un actor?
Aceste evenimente sunt asociate cu operații ale sistemului
Un Controller este un obiect care nu ține de interfața grafică și care este responsabil cu
recepționarea sau gestionarea unui eveniment
Un Controller definește o metodă
corespunzătoare operației sistemului
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Soluție: asignează responsabilitatea pentru
recepționarea sau gestionarea unui eveniment unei clase care reprezintă una dintre
următoarele alegeri:
◦ Reprezintă întregul sistem sau subsistem (fațadă controller)
◦ Reprezintă un scenariu de utilizare în care apare evenimentul;
În mod normal, un controller ar trebui să delege altor obiecte munca care trebuie făcută;
Controller-ul coordonează sau controlează activitatea, însă nu face prea multe lucruri el însuși
O greșeală comună în design-ul unui controller este să i se atribuie prea multe responsabilități (fațade controller)
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Testarea unei funcţii, a unui program, a unui ecran, a unei funcţionalităţi
Se face de către programatori
Predefinită
Rezultatele trebuie documentate
Se folosesc simulatoare pentru Input şi Output
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NetBeans - TikiOne JaCoCoverage:
http://plugins.netbeans.org/plugin/48570/tikio ne-jacocoverage
Java Code Coverage for Eclipse:
http://www.eclemma.org/
IntelliJ – Running with coverage:
https://www.jetbrains.com/help/idea/2016.3/r unning-with-coverage.html
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SOLID
Design Patterns
◦ Definitions, Elements, Example, Classification
JUnit Testing
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Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides:
Design Patterns: Elements of
Reusable Object-Oriented Software
(GangOfFour) Ovidiu Gheorghieş, Curs 7 IP
Adrian Iftene, Curs 9 TAIP:
http://thor.info.uaic.ro/~adiftene/Scoala/2011/
TAIP/Courses/TAIP09.pdf
Craig Larman.
Applying UML and Patterns. An Introduction to Object Oriented Analysis and Design
Ovidiu Gheorghieș, Curs 6 IP
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WebProjectManager: http://profs.info.uaic.ro/~adrianaa/uml/
Diagrame de Stare și de Activitate:
http://software.ucv.ro/~soimu_anca/itpm/Diagrame%20de%20 Stare%20si%20Activitate.doc
Deployment Diagram:
http://en.wikipedia.org/wiki/Deployment_diagram
http://www.agilemodeling.com/artifacts/deploymentDiagram.
htm
GRASP:
http://en.wikipedia.org/wiki/GRASP_(Object_Oriented_Design)
http://web.cs.wpi.edu/~gpollice/cs4233-
a05/CourseNotes/maps/class4/GRASPpatterns.html Introduction to GRASP Patterns:
Gang-Of-Four: http://c2.com/cgi/wiki?GangOfFour,
http://www.uml.org.cn/c%2B%2B/pdf/DesignPatterns.pdf
Design Patterns Book: http://c2.com/cgi/wiki?DesignPatternsBook
About Design Patterns: http://www.javacamp.org/designPattern/
Design Patterns – Java companion:
http://www.patterndepot.com/put/8/JavaPatterns.htm
Java Design patterns:
http://www.allapplabs.com/java_design_patterns/java_design_patter ns.htm
Overview of Design Patterns:
http://www.mindspring.com/~mgrand/pattern_synopses.htm
Gang of Four: http://en.wikipedia.org/wiki/Gang_of_four
JUnit in Eclipse: http://www.vogella.de/articles/JUnit/article.html
JUnit in NetBeans: http://netbeans.org/kb/docs/java/junit-intro.html
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https://scotch.io/bar-talk/s-o-l-i-d-the- first-five-principles-of-object-oriented- design
https://www.slideshare.net/enbohm/solid- design-principles-9016117
https://siderite.blogspot.com/2017/02/solid -principles-plus-dry-yagni-kiss-final.html
https://thefullstack.xyz/dry-yagni-kiss-tdd- soc-bdfu
