20. Object-Oriented Programming Fundamental Principles

Object-Oriented Programming Fundamental Concepts Svetlin Nakov Telerik Corporation www.telerik.com

Contents Fundamental Principles of OOP Inheritance Abstraction Encapsulation Polymorphism Cohesion and Coupling

Fundamental Principles of OOP Inheritance Inherit members from parent class Abstraction Define and execute abstract actions Encapsulation Hide the internals of a class Polymorphism Access a class through its parent interface

Classes and Interfaces Classes define attributes and behavior Fields, properties, methods, etc. Methods contain code for execution Interfaces define a set of operations Empty methods and properties, left to be implemented later public class Labyrinth { … } public interface IFigure { … }

Inheritance Inheritance allows child classes inherits the characteristics of existing parent class Attributes (fields and properties) Operations (methods) Child class can extend the parent class Add new fields and methods Redefine methods (modify existing behavior) A class can implement an interface by providing implementation for all its methods

Inheritance terminology derived class base class / parent class inherits derived interface base interface implements class interface implements

Inheritance – Benefits Inheritance has a lot of benefits Extensibility Reusability Provides abstraction Eliminates redundant code Use inheritance for buidling is-a relationships E.g. dog is-a animal (dogs are kind of animals) Don't use it to build has-a relationship E.g. dog has-a name (dog is not kind of name)

Inheritance – Example Person +Name: String +Address: String Employee +Company: String +Salary: double Student +School: String Base class Derived class Derived class

Class Hierarchies Inheritance leads to a hierarchy of classes and/or interfaces in an application: … Game MultiplePlayersGame BoardGame Chess Backgammon SinglePlayerGame Minesweeper Solitaire …

Inheritance in .NET A class can inherit only one base class E.g. IOException derives from SystemException and it derives from Exception A class can implement several interfaces This is .NET’s form of multiple inheritance E.g. List<T> implements IList<T> , ICollection<T> , IEnumerable<T> An interface can implement several interfaces E.g. IList<T> implements ICollection<T> and IEnumerable<T>

How to Define Inheritance ? We must specify the name of the base class after the name of the derived In the constructor of the derived class we use the keyword base to invoke the constructor of the base class public class Shape {...} public class Circle : Shape {...} public Circle (int x, int y) : base(x) {...}

Simple Inheritance Example public class Mammal { public int Age { get; set; } public Mammal(int age) { this.Age = age; } public void Sleep() { Console.WriteLine("Shhh! I'm sleeping!"); } }

Simple Inheritance Example (2) public class Dog : Mammal { public string Breed { get; set; } public Dog(int age, string breed) : base(age) { this.Breed = breed; } public void WagTail() { Console.WriteLine("Tail wagging..."); } }

S imple Inheritance Live Demo

Accessibility Levels Access modifiers in C# public – access is not restricted private – access is restricted to the containing type protected – access is limited to the containing type and types derived from it internal – access is limited to the current assembly protected internal – access is limited to the current assembly or types derived from the containing class

Inheritance and Accessibility class Creature { protected string Name { get; private set; } private void Talk() { Console.WriteLine("I am creature ..."); } protected void Walk() { Console.WriteLine("Walking ..."); } } class Mammal : Creature { // base.Talk() can be invoked here // this.Name can be read but cannot be modified here }

Inheritance and Accessibility (2) class Dog : Mammal { public string Breed { get; private set; } // base.Talk() cannot be invoked here (it is private) } class InheritanceAndAccessibility { static void Main() { Dog joe = new Dog(6, "Labrador"); Console.WriteLine(joe.Breed); // joe.Walk() is protected and can not be invoked // joe.Talk() is private and can not be invoked // joe.Name = "Rex"; // Name cannot be accessed here // joe.Breed = "Shih Tzu"; // Can't modify Breed } }

Inheritance and Accessibility Live Demo

Inheritance: I mportant A spect s Structures cannot be inherited In C# there is no multiple inheritance Only multiple interfaces can be implemented Instance and static constructors are not inherited Inheritance is transitive relation If C is derived from B, and B is derived from A, then C inherits A as well

Inheritance: Important Features A derived class extends its base class It can add new members but cannot remove derived ones Declaring new members with the same name or signature hides the inherited ones A class can declare virtual methods and properties Derived classes can override the implementation of these members E.g. Object.Equals() is virtual method

Abstraction Abstraction means ignoring irrelevant features, properties, or functions and emphasizing the relevant ones ... ... relevant to the given project (with an eye to future reuse in similar projects) Abstraction = managing complexity "Relevant" to what?

Abstraction (2) Abstraction is something we do every day Looking at an object, we see those things about it that have meaning to us We abstract the properties of the object, and keep only what we need E.g. students get "name" but not "color of eyes" Allows us to represent a complex reality in terms of a simplified model Abstraction highlights the properties of an entity that we need and hides the others

In .NET abstraction is achieved in several ways: Abstract classes Interfaces Inheritance Abstraction in .NET +Color : long ButtonBase +click() Control Button RadioButton CheckBox

Abstraction in .NET – Example System.Object System.MarshalByRefObject System.ComponentModel.Component System.Windows.Forms.Control System.Windows.Forms.ButtonBase System.Windows.Forms.Button

Interfaces in C# An interface is a set of operations (methods) that given object can perform Also called "contract" for supplying a set of operations Defines abstract behavior Interfaces provide abstractions You shouldn't have to know anything about what is in the implementation in order to use it

Abstract Classes in C# Abstract classes are special classes defined with the keyword abstract Mix between class and interface Partially implemented or fully unimplemented Not implemented methods are declared abstract and are left empty Cannot be instantiated Child classes should implement abstract methods or declare them as abstract

Abstract Data Types Abstract Data Types (ADT) are data types defined by a set of operations (interface) Example: LinkedList<T> +Add(item : Object) +Remove(item : Object) +Clear() … «interface» IList<T> List<T>

Inheritance Hierarchies Using inheritance we can create inheritance hierarchies Easily represented by UML class diagrams UML class diagrams Classes are represented by rectangles containing their methods and data Relations between classes are shown as arrows Closed triangle arrow means inheritance Other arrows mean some kind of associations

UML Class Diagram – Example Shape #Position:Point struct Point +X:int +Y:int +Point interface ISurfaceCalculatable +CalculateSurface:float Rectangle -Width:float -Height:float +Rectangle +CalculateSurface:float Square -Size:float +Square +CalculateSurface:float FilledSquare -Color:Color +FilledSquare struct Color +RedValue:byte +GreenValue:byte +BlueValue:byte +Color FilledRectangle -Color:Color +FilledRectangle

Class Diagrams in Visual Studio Live Demo

Encapsulation Encapsulation hides the implementation details Class announces some operations (methods) available for its clients – its public interface All data members (fields) of a class should be hidden Accessed via properties (read-only and read-write) No interface members should be hidden

Encapsulation – Example Data fields are private Constructors and accessors are defined (getters and setters) Person -name : string -age : TimeSpan +Person(string name, int age) +Name : string { get; set; } +Age : TimeSpan { get; set; }

Encapsulation in .NET Fields are always declared private Accessed through properties in read-only or read-write mode Constructors are almost always declared public Interface methods are always public Not explicitly declared with public Non-interface methods are declared private / protected

Encapsulation – Benefits Ensures that structural changes remain local: Changing the class internals does not affect any code outside of the class Changing methods' implementation does not reflect the clients using them Encapsulation allows adding some logic when accessing client's data E.g. validation on modifying a property value Hiding implementation details reduces complexity  easier maintenance

Polymorphism Polymorphism = ability to take more than one form (objects have more than one type) A class can be used through its parent interface A child class may override some of the behaviors of the parent class Polymorphism allows abstract operations to be defined and used Abstract operations are defined in the base class' interface and implemented in the child classes Declared as abstract or virtual

Polymorphism (2) Why handle an object of given type as object of its base type? To invoke abstract operations To mix different related types in the same collection E.g. List<object> can hold anything To pass more specific object to a method that expects a parameter of a more generic type To declare a more generic field which will be initialized and "specialized" later

Virtual M ethod s Virtual method is method that can be used in the same way on instances of base and derived classes but its implementation is different A method is said to be a virtual when it is declared as virtual Methods that are declared as virtual in a base class can be overridden using the keyword override in the derived class public virtual void CalculateSurface()

The override Modifier Using override we can modify a method or property An override method provides a new implementation of a member inherited from a base class You cannot override a non-virtual or static method The overridden base method must be virtual, abstract, or override

Polymorphism – How it Works? Polymorphism ensures that the appropriate method of the subclass is called through its base class' interface Polymorphism is implemented using a technique called late method binding Exact method to be called is determined at runtime , just before performing the call Applied for all abstract / virtual methods Note: Late binding is slower than normal (early) binding

Polymorphism – Example override CalcSurface() { return size * size; } override CalcSurface() { return PI * radius * raduis; } Abstract class Abstract action Concrete class Overriden action Overriden action Figure +CalcSurface() : double Square -x : int -y : int -size : int Circle -x : int -y : int -radius: int

Polymorphism – Example (2) abstract class Figure { public abstract double CalcSurface(); } abstract class Square { public override double CalcSurface() { return … } } Figure f1 = new Square(...); Figure f2 = new Circle(...); // This will call Square.CalcSurface() int surface = f1.CalcSurface(); // This will call Square.CalcSurface() int surface = f2.CalcSurface();

Class Hierarchies: Real World Example

Real World Example: Calculator Creating an application like the Windows Calculator Typical scenario for applying the object-oriented approach

Real World Example: Calculator (2) The calculator consists of controls: Buttons, panels, text boxes, menus, check boxes, radio buttons, etc. Class Control – the root of our OO hierarchy All controls can be painted on the screen Should implement an interface IPaintable with a method Paint() Common properties: location, size, text, face color, font, background color, etc.

Real World Example: Calculator (3) Some controls could contain other (nested) controls inside (e. g. panels and toolbars) We should have class Container that extends Control holding a collection of child controls The Calculator itself is a Form Form is a special kind of Container Contains also border, title ( text derived from Control ), icon and system buttons How the Calculator paints itself? Invokes Paint() for all child controls inside it

Real World Example: Calculator (4) How a Container paints itself? Invokes Paint() for all controls inside it Each control knows how to visualize itself What is the common between buttons, check boxes and radio buttons? Can be pressed Can be selected We can define class AbstractButton and all buttons can derive from it

Calculator Classes TextBox Paint() «interface» IPaintable -location -size -text -bgColor -faceColor -font Control Container Form Calculator AbstractButton Button CheckBox RadioButton MainMenu MenuItem Panel

Cohesion Cohesion describes how closely all the routines in a class or all the code in a routine support a central purpose Cohesion must be strong Well-defined abstractions keep cohesion strong Classes must contain strongly related functionality and aim for single purpose Cohesion is a useful tool for managing complexity

Good and Bad Cohesion Good: hard disk, cdrom, floppy BAD: spaghetti code

Strong Cohesion Strong cohesion example Class Math that has methods: Sin() , Cos() , Asin() Sqrt() , Pow() , Exp() Math.PI , Math.E double sideA = 40, sideB = 69; double angleAB = Math.PI / 3; double sideC = Math.Pow(sideA, 2) + Math.Pow(sideB, 2) - 2 * sideA * sideB * Math.Cos(angleAB); double sidesSqrtSum = Math.Sqrt(sideA) + Math.Sqrt(sideB) + Math.Sqrt(sideC);

Bad Cohesion Bad cohesion example Class Magic that has these methods: Another example: public void PrintDocument(Document d); public void SendEmail( string recipient, string subject, string text); public void CalculateDistanceBetweenPoints( int x1, int y1, int x2, int y2) MagicClass.MakePizza("Fat Pepperoni"); MagicClass.WithdrawMoney("999e6"); MagicClass.OpenDBConnection();

Coupling Coupling describes how tightly a class or routine is related to other classes or routines Coupling must be kept loose Modules must depend little on each other All classes and routines must have small, direct, visible, and flexible relations to other classes and routines One module must be easily used by other modules

Loose and Tight Coupling Loose Coupling: Easily replace old HDD Easily place this HDD to another motherboard Tight Coupling: Where is the video adapter? Can you change the video controller?

Loose Coupling – Example class Report { public bool LoadFromFile(string fileName) {…} public bool SaveToFile(string fileName) {…} } class Printer { public static int Print(Report report) {…} } class Program { static void Main() { Report myReport = new Report(); myReport.LoadFromFile("C:\\DailyReport.rep"); Printer.Print(myReport); } }

Tight Coupling – Example class MathParams { public static double operand; public static double result; } class MathUtil { public static void Sqrt() { MathParams.result = CalcSqrt(MathParams.operand); } } class MainClass { static void Main() { MathParams.operand = 64; MathUtil.Sqrt(); Console.WriteLine(MathParams.result); } }

Spaghetti Code Combination of bad cohesion and tight coupling: class Report { public void Print() {…} public void InitPrinter() {…} public void LoadPrinterDriver(string fileName) {…} public bool SaveReport(string fileName) {…} public void SetPrinter(string printer) {…} } class Printer { public void SetFileName() {…} public static bool LoadReport() {…} public static bool CheckReport() {…} }

Summary OOP fundamental principals are: inheritance, encapsulation, abstraction, polymorphism Inheritance allows inheriting members form another class Abstraction and encapsulation hide internal data and allow working through abstract interface Polymorphism allows working with objects through their parent interface and invoke abstract actions Strong cohesion and loose coupling avoid spaghetti code

Questions? Object-Oriented Programming Fundamental Concepts http://academy.telerik.com

Exercises We are given a school. In the school there are classes of students. Each class has a set of teachers. Each teacher teaches a set of disciplines. Students have name and unique class number. Classes have unique text identifier. Teachers have name. Disciplines have name, number of lectures and number of exercises. Both teachers and students are people. Your task is to identify the classes (in terms of OOP) and their attributes and operations, define the class hierarchy and create a class diagram with Visual Studio.

Exercises (2) Define class Human with first name and last name. Define new class Student which is derived from Human and has new field – grade . Define class Worker derived from Human with new field weekSalary and work-hours per day and method MoneyPerHour () that returns money earned by hour by the worker. Define the proper constructors and properties for this hierarchy. Initialize an array of 10 students and sort them by grade in ascending order. Initialize an array of 10 workers and sort them by money per hour in descending order.

Exercises (3) Define abstract class Shape with only one virtual method CalculateSurface() and fields width and height . Define two new classes Triangle and Rectangle that implement the virtual method and return the surface of the figure (height*width for rectangle and height*width/2 for triangle). Define class Circle and suitable constructor so that on initialization height must be kept equal to width and implement the CalculateSurface() method. Write a program that tests the behavior of the CalculateSurface( ) method for different shapes ( Circle , Rectangle , Triangle ) stored in an array.

Exercises (4) Create a hierarchy Dog , Frog , Cat , Kitten , Tomcat and define suitable constructors and methods according to the following rules: all of this are Animals. Kittens and tomcats are cats. All animals are described by age, name and sex. Kittens can be only female and tomcats can be only male. Each animal produce a sound. Create arrays of different kinds of animals and calculate the average age of each kind of animal using static methods. Create static method in the animal class that identifies the animal by its sound.

Exercises (5) A bank holds different types of accounts for its customers: deposit accounts, loan accounts and mortgage accounts. Customers could be individuals or companies. All accounts have customer, balance and interest rate (monthly based). Deposit accounts are allowed to deposit and with draw money. Loan and mortgage accounts can only deposit money.

Exercises (6) All accounts can calculate their interest amount for a given period (in months). In the common case its is calculated as follows: number_of_months * interest_rate. Loan accounts have no interest for the first 3 months if are held by individuals and for the first 2 months if are held by a company. Deposit accounts have no interest if their balance is positive and less than 1000. Mortgage accounts have ½ interest for the first 12 months for companies and no interest for the first 6 months for individuals.

Exercises (7) Your task is to write a program to model the bank system by classes and interfaces. You should identify the classes, interfaces, base classes and abstract actions and implement the calculation of the interest functionality.

20. Object-Oriented Programming Fundamental Principles

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