Inheritance and Polymorphism

Namespaces, Cohesion and Coupling

Veselin GeorgievNational Academy for

Software Developmentacademy.devbg.org

Svetlin NakovTelerik

Corporationwww.telerik.com

Contents

1. Cohesion and Coupling

2. Inheritance

3. Polymorphism

4. Namespaces

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Cohesion and Coupling

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 Classes must contain strongly

related functionality and aim for single purpose

Cohesion is a useful tool for managing complexity

Well-defined abstractions keep cohesion strong

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Good and Bad Cohesion Good cohesion: hard disk, CD-ROM, floppy

BAD: spaghetti code

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Strong Cohesion Strong cohesion example

Class Math that has methods:

Sin(), Cos(), Asin(), Sqrt(), Pow(), Exp()

Math.PI, Math.Edouble 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);

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Bad Cohesion Example of bad cohesion Class Magic that has all these methods:

Another example:MagicClass.MakePizza("Fat Pepperoni");

MagicClass.WithdrawMoney("999e6");

MagicClass.OpenDBConnection();

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)

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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

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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?

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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); }}

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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 Example{ static void Main() { MathParams.operand = 64; MathUtil.Sqrt(); Console.WriteLine(MathParams.result); }}

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Spaghetti Code

Combination of bad cohesion and tight couplingclass 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() {…}}

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Inheritance

Inheritance Inheritance is the ability of a class to implicitly gain all members from another class

Inheritance is fundamental concept in OOP The class whose methods are inherited is called base (parent) class The class that gains new functionality is called derived (child) class

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Inheritance (2) All class members are inherited

Fields, methods, properties, … In C# classes could be inherited

The structures in C# could not be inherited Inheritance allows creating deep inheritance hierarchies In .NET there is no multiple inheritance, except when implementing interfaces

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InheritanceLive Demo

Polymorphism

Polymorphism Polymorphism is fundamental concept in OOP The ability to handle the objects of a specific class as instances of its parent class and to call abstract functionality

Polymorphism allows creating hierarchies with more valuable logical structure Allows invoking abstract functionality without caring how and where it is implemented

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Polymorphism (2) Polymorphism is usually implemented through:

Virtual methods (virtual)

Abstract methods (abstract)

Methods from an interface (interface)

In C# to override virtual method the keyword override is used C# allows hiding virtual methods in derived classes by the keyword new

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Polymorphism – Example

class Person { public virtual void PrintName() { Console.WriteLine("I am a person."); }}

class Trainer : Person{ public override void PrintName() { Console.WriteLine("I am a trainer."); }}

class Student : Person{ public override void PrintName() { Console.WriteLine("I am a student."); }}

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Polymorphism – Example (2)

static void Main(){ Person person = new Person(); person.PrintName(); // I am a person.

Person trainer = new Trainer(); trainer.PrintName(); // I am a trainer.

Person student = new Student(); student.PrintName(); // I am a student.}

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PolymorphismLive Demo

Namespaces

Namespaces Similar to namespaces in C++ and

packagesin Java

Ensure logical grouping of type definition aggregations

May contain classes, structures, interfaces, enumerators and other namespaces

Can not contain methods and data Allows definition of types with equal

names (they must be in different namespaces)

Can be allocated in one or several files

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Namespaces (2) To include a namespace – using directive is used

using allows direct use of all types in the namespace

Including is applied to the current file

The directive is written at the begging of the file

When includes a namespace with using its subset of namespaces is not included

using System.Windows.Forms;

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Namespaces (3)

Types, placed in namespaces, can be used and without using directive, by their full name:

using can create allies for namespaces :

using IO = System.IO;using WinForms = System.Windows.Forms;

IO.StreamReader reader = IO.File.OpenText("file.txt");

WinForms.Form form = new WinForms.Form();

System.IO.StreamReader reader = System.IO.File.OpenText("file.txt");

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Namespaces – Practices

Divide the types in your applications in namespaces always when they are too much (above 15-20)

Classify the types logically in namespaces according to their purpose

If the types are too much use subsets of namespaces

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Namespaces – Practices (2)

Distribute all public types in files identical with their names

Arrange the files in directories, corresponding to their namespaces

The directory structure from your project course-code have to reflect the structure of the defined namespaces

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Namespaces – ExampleNamespaces – Examplenamespace SofiaUniversity.Data{ public struct Faculty { // ... } public class Student { // ... } public class Professor { // ... } public enum Specialty { // ... }}

Namespaces – Example (2)

Namespaces – Example (2)

namespace SofiaUniversity.UI{ public class StudentAdminForm :

System.Windows.Forms.Form { // ... } public class ProfessorAdminForm :

System.Windows.Forms.Form { // ... }}namespace SofiaUniversity { public class AdministrationSystem { public static void Main() { // ... } }}

Namespaces – Example (3)

Recommended directory structure and classes organization in them

Recommended directory structure and classes organization in them

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Questions??

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Inheritance and Polymorphism

Exercises

1. A bank holds different types of accounts for its customers: deposit accounts, loan accounts and mortgage accounts. Customers could be physical persons or companies.

All accounts have customer, balance and interest rate (monthly based). Deposit accounts are allowed to deposit and withdraw money. Loan and mortgage accounts can only deposit money.

All customers have name. Persons have social security number and monthly income. Companies have Bulstat.

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Exercises (2)

All accounts can calculate their interest amount for a given period (in months). In the common case it is calculated as follows:

numberOfMonths * interestRate.

Loan accounts have no interest for the first 3 months if are held by persons 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.

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Exercises (3)

Mortgage accounts have ½ interest for the first 12 months for companies and no interest for the first 6 months for persons.

Your task is to write a program to model the bank system. You should identify the classes, base classes and abstract actions and implement the calculation of the interest for the different accounts.

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