classes and object-oriented programming in c# computers and programming (01204111)

Classes and Object-Classes and Object-Oriented Programming in Oriented Programming in C#C#

Computers and ProgrammingComputers and Programming(01204111)(01204111)

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OutlineOutline Array revisitedArray revisited Data encapsulation in C#Data encapsulation in C# Class and object creationClass and object creation Array of objectsArray of objects Member methodsMember methods ConstructorsConstructors

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Arrays RevisitedArrays Revisited Group multiple items of the same Group multiple items of the same

type into one "type into one "variablevariable" or "" or "objectobject"" Make programming easier to Make programming easier to

managemanage

What if we want to keep a few things What if we want to keep a few things that are of different types together?that are of different types together?

a1=3a1=3

a2=10a2=10

a0=7a0=7

a5=17a5=17

:: 77 101033 55 171788Array Array aa

00 2211 33 5544

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ExampleExample Imagine that you have to write a programImagine that you have to write a program

To store 100 students' namesTo store 100 students' names That's simple; just use an arrayThat's simple; just use an array

...and their scores...and their scores Also simple; create another arrayAlso simple; create another array

...and also their ...and also their IDID, , departmentdepartment, , facultyfaculty, , advisoradvisor, etc, etc

using System;class Scoring { public static void Main() { string [] name = new string[100]; double [] score = new double[100]; : }}

using System;class Scoring { public static void Main() { string [] name = new string[100]; double [] score = new double[100]; : }}

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More ExampleMore Example From the previous slide:From the previous slide:

We want to store students' ID, name, We want to store students' ID, name, score, department, faculty, advisor, etcscore, department, faculty, advisor, etc

We could write a program like this:We could write a program like this:

using System;class Scoring { public static void Main() { string [] name = new string[100]; int [] ID = new int[100]; double [] score = new double[100]; string [] dept = new string[100]; string [] faculty = new string[100]; string [] advisor = new string[100]; : }}

using System;class Scoring { public static void Main() { string [] name = new string[100]; int [] ID = new int[100]; double [] score = new double[100]; string [] dept = new string[100]; string [] faculty = new string[100]; string [] advisor = new string[100]; : }}

What a What a mess...mess...

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Data EncapsulationData Encapsulation A mechanism that bundles multiple A mechanism that bundles multiple

items of varying types into one item items of varying types into one item or "or "objectobject""

Dept="ME"Dept="ME"

Advisor="Arthur"Advisor="Arthur"

Name="Paula"Name="Paula"ID=48500000ID=48500000

4850000048500000PaulaPaulaMEMEArthurArthur

ID:ID:Name:Name:Dept:Dept:

Advisor:Advisor:

Object Object studentInfostudentInfo

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Encapsulation in C#Encapsulation in C# C# provides two kinds of data C# provides two kinds of data

encapsulation: encapsulation: structstruct and and classclass This course will focus on classes onlyThis course will focus on classes only

Objects created from a class can Objects created from a class can store a fixed number of itemsstore a fixed number of items may be of different typesmay be of different types

A A classclass is defined by programmer is defined by programmer

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Using ClassUsing Class

1.Define a class1.Define a class1.Define a class1.Define a class

2.Create an object from the class2.Create an object from the class2.Create an object from the class2.Create an object from the class

3.Access data in the object3.Access data in the object3.Access data in the object3.Access data in the object

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Defining ClassDefining Class

class StudentInfo { public int id; public string name; public string dept;}

class StudentInfo { public int id; public string name; public string dept;}

Must use "Must use "classclass" " keywordkeyword

Every class Every class needs a needs a namename

MembersMembers (or (or propertiesproperties) )

of objects to be createdof objects to be createdProtection level – Protection level – always use "always use "publicpublic" "

for nowfor now

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Defining Class (cont'd)Defining Class (cont'd) Where do we put the class Where do we put the class

definition?definition? Inside or outside a classInside or outside a class Outside a methodOutside a method

E.g.,E.g.,using System;

class Test { class StdInfo { public int id; public string name; public string dept; }

public static void Main() { : }}

using System;

class Test { class StdInfo { public int id; public string name; public string dept; }

public static void Main() { : }}

using System;

class StdInfo { public int id; public string name; public string dept;}

class Test { public static void Main() { : }}

using System;

class StdInfo { public int id; public string name; public string dept;}

class Test { public static void Main() { : }}

oror

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Creating Object from ClassCreating Object from Class Syntax:Syntax:

oror

Example:Example:

class-name obj-name = new class-name();class-name obj-name = new class-name();

using System;class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { StdInfo student = new StdInfo(); : }}

using System;class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { StdInfo student = new StdInfo(); : }}

class-name obj-name;obj-name = new class-name();

class-name obj-name;obj-name = new class-name();

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Computer MemoryComputer Memory

Object Creation ProcessObject Creation Process

StdInfo student;

student = new StdInfo();

StdInfo student;

student = new StdInfo(); ????

just a reference, just a reference, not an actual objectnot an actual object

????????


Advisor:Advisor:

Object Object studentInfostudentInfo

studentstudent

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Accessing Object's Accessing Object's MembersMembers Syntax:Syntax:

Example:Example:

obj-name.member-nameobj-name.member-name

using System;class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { StdInfo student = new StdInfo(); student.id = 49041234; student.name = "Paula"; student.dept = "ME"; Console.WriteLine("ID: {0}", student.id); }}

using System;class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { StdInfo student = new StdInfo(); student.id = 49041234; student.name = "Paula"; student.dept = "ME"; Console.WriteLine("ID: {0}", student.id); }}

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Array of ObjectsArray of Objects Array of integersArray of integers

Object of type StdInfo (with reference Object of type StdInfo (with reference variable)variable)

Array of (references to) objects of type Array of (references to) objects of type StdInfoStdInfo

77 101033 55 171788

00 2211 33 5544

55 171788 77 101033

66 8877 99 11111010

4905123449051234PaulaPaulaMEME


4905234549052345LisaLisaEEEE


4905345649053456UmaUmaCPECPE


00 11 22

4905123449051234PaulaPaulaMEME


obj-varobj-var

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Creating Array of ObjectsCreating Array of Objects Syntax:Syntax:

Example:Example:

class-name[] array-name = new class-name[size];class-name[] array-name = new class-name[size];

using System;class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { StdInfo [] students = new StdInfo[50];

for (int i = 0; i < 50; i++) students[i] = new StdInfo(); : }}

using System;class Test { class StdInfo { public int id; public string name; public string dept; } public static void Main() { StdInfo [] students = new StdInfo[50];

for (int i = 0; i < 50; i++) students[i] = new StdInfo(); : }}

Create an array for Create an array for storing 50 references storing 50 references

to StdInfo objectsto StdInfo objects

Create an actual object Create an actual object StdInfo for each StdInfo for each

reference in the arrayreference in the array

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Accessing Objects in ArrayAccessing Objects in Array Syntax:Syntax:

Example:Example: Set Student#2's name to "Ariya"Set Student#2's name to "Ariya"

Display Student#3's departmentDisplay Student#3's department

array-name[index].memberarray-name[index].member

students[2].name = "Ariya";students[2].name = "Ariya";

Console.WriteLine("Department: {0}", students[3].dept);Console.WriteLine("Department: {0}", students[3].dept);

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Accessing DetailsAccessing Details

class StdInfo { public int ID; public string Name; public string Dept;}static void Main() { StdInfo[] students; students = new StdInfo[4]; : : students[2].Name = "Ariya";}

class StdInfo { public int ID; public string Name; public string Dept;}static void Main() { StdInfo[] students; students = new StdInfo[4]; : : students[2].Name = "Ariya";}

4905123449051234PaulaPaulaENVEENVE


4905234549052345LisaLisaMEME


4905345649053456UmaUmaCPECPE


4905456749054567MashaMashaEEEE


00

11

22

33

AriyaAriya

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Example: Example: Student RecordsStudent Records Get Get NN students' information with 3 fields students' information with 3 fields

ID, Name, ScoreID, Name, Score Then output a table of informationThen output a table of information

First, we define a class as shown:First, we define a class as shown:

class StdInfo { public int id; public string name; public int score;}

class StdInfo { public int id; public string name; public int score;}

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ReadInfoReadInfo Method Method Reads all information for each Reads all information for each

studentstudent Returns an object of class StdInfoReturns an object of class StdInfo

static StdInfo ReadInfo() { StdInfo info = new StdInfo(); Console.Write("ID: "); info.id = int.Parse(Console.ReadLine()); Console.Write("Name: "); info.name = Console.ReadLine(); Console.Write("Score: "); info.score = int.Parse(Console.ReadLine()); return info;}

static StdInfo ReadInfo() { StdInfo info = new StdInfo(); Console.Write("ID: "); info.id = int.Parse(Console.ReadLine()); Console.Write("Name: "); info.name = Console.ReadLine(); Console.Write("Score: "); info.score = int.Parse(Console.ReadLine()); return info;}

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ShowInfoShowInfo Method Method Takes a StdInfo and displays the Takes a StdInfo and displays the

information on screeninformation on screen Returns nothingReturns nothing

static void ShowInfo(StdInfo info) { Console.WriteLine("{0,3} {1,-10} {2,2}", info.id, info.name, info.score);}

static void ShowInfo(StdInfo info) { Console.WriteLine("{0,3} {1,-10} {2,2}", info.id, info.name, info.score);}

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Put Them All TogetherPut Them All Togetherusing System;class StdRecords {

// Define class StdInfo here

// Define ReadInfo() here

// Define ShowInfo() here

static void Main() { Console.Write("How many students? "); int n = int.Parse(Console.ReadLine()); StdInfo[] students = new StdInfo[n]; for (int i = 0; i < n; i++) students[i] = ReadInfo(); for (int i = 0; i < n; i++) ShowInfo(students[i]); }}

using System;class StdRecords {

// Define class StdInfo here

// Define ReadInfo() here

// Define ShowInfo() here

static void Main() { Console.Write("How many students? "); int n = int.Parse(Console.ReadLine()); StdInfo[] students = new StdInfo[n]; for (int i = 0; i < n; i++) students[i] = ReadInfo(); for (int i = 0; i < n; i++) ShowInfo(students[i]); }}

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Object-Oriented Object-Oriented ProgrammingProgramming Classes are more than just a Classes are more than just a

mechanism to bundle data into mechanism to bundle data into objectsobjects

Objects may have its own behaviors Objects may have its own behaviors (defined by classes) to perform on its (defined by classes) to perform on its propertiesproperties E.g., they know how to display their E.g., they know how to display their

data on screen, or compute their data on screen, or compute their propertiesproperties

E.g., every circle object knows how to E.g., every circle object knows how to calculate its areacalculate its area

These are the concepts of Object-These are the concepts of Object-Oriented Programming (OOP)Oriented Programming (OOP)

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Object-Oriented View of Object-Oriented View of ClassesClasses A A classclass serves like a serves like a templatetemplate to to

create create objectsobjects of the same type of the same type A class defines a list of properties its A class defines a list of properties its

objects must have, but does not specify objects must have, but does not specify the values of these propertiesthe values of these properties

Class Circle Properties: radius, color

createcreate

create

create

createcreate

circle1 color = Yellow radius = 1

circle2 color = Red radius = 1.5

circle3 color = Blue radius = 2

objectsobjectsof classof classCircleCircle

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OOP and Graphical User OOP and Graphical User InterfaceInterface GUI components we have seen are GUI components we have seen are

objects of some classesobjects of some classes E.g., buttons are objects of class E.g., buttons are objects of class ButtonButton

inside inside System.Windows.FormsSystem.Windows.Forms namespacenamespacebutton1.Left = 60button1.Top = 31button1.Height = 56button1.Width = 115button1.Text = "OK"

button2.Left = 144button2.Top = 127button2.Height = 75button2.Width = 23button2.Text = "Cancel"button2.Color = Color.Red

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Member MethodsMember Methods Class may contain methodsClass may contain methods

Allow objects to perform computation Allow objects to perform computation on its own dataon its own data

Known as Known as member methodsmember methods Each member method can access Each member method can access

other members inside the same other members inside the same objectobject

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Example: Example: Member Member MethodsMethods Consider the following Person classConsider the following Person class

We can add a GetAge method to the We can add a GetAge method to the class to calculate a person’s ageclass to calculate a person’s age

class Person { public string name; public int birth_year;}

class Person { public string name; public int birth_year;}

class Person { public string name; public int birth_year;

public int GetAge() { return 2010 – birth_year; }}


public int GetAge() { return 2010 – birth_year; }}

In real program,In real program,2010 should not 2010 should not be hard-coded be hard-coded like this!like this!

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Thinking CornerThinking Corner Add two methods, Add two methods, CircumferenceCircumference

and and AreaArea, into the Circle class below, into the Circle class below So that each Circle object knows how to So that each Circle object knows how to

compute its own circumference length compute its own circumference length and areaand area

class Circle{ public double radius;

public double Circumference() { : }

public double Area() { : }}

class Circle{ public double radius;

public double Circumference() { : }

public double Area() { : }}

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ConstructorsConstructors A constructor is a special member method A constructor is a special member method

defined in a classdefined in a class It allows us to specify how each object of this It allows us to specify how each object of this

class gets constructedclass gets constructed It’s a method with the It’s a method with the same name as the classsame name as the class

and and without return typewithout return type (not even (not even voidvoid)) E.g.,E.g., class Person {

public string name; public int birth_year;

public Person() { birth_year = 1975; }}


public Person() { birth_year = 1975; }}

Person p = new Person();Console.WriteLine(p.birth_year);

Person p = new Person();Console.WriteLine(p.birth_year);

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Constructors with Constructors with ParametersParameters A constructor may also be defined to A constructor may also be defined to

accept parametersaccept parameters E.g.,E.g.,


public Person(string s) { name = s; birth_year = 1975; }}


public Person(string s) { name = s; birth_year = 1975; }}

Person p = new Person("John");Console.WriteLine(p.name);Console.WriteLine(p.birth_year);

Person p = new Person("John");Console.WriteLine(p.name);Console.WriteLine(p.birth_year);

The new operation The new operation passes the parameter passes the parameter to the newly created to the newly created

objectobject

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Referencing MembersReferencing Members In the previous example, the In the previous example, the

parameter name parameter name ss in the constructor in the constructor is not so meaningful, so we change it is not so meaningful, so we change it to to namename


public Person(string name) { name = name; birth_year = 1975; }}


public Person(string name) { name = name; birth_year = 1975; }}

Does nothing Does nothing because both because both

'name's refer to the 'name's refer to the parameter of the parameter of the

constructorconstructor

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Referencing Members: Referencing Members: thisthis VariableVariable To make a reference to the current To make a reference to the current

object, the special keyword object, the special keyword thisthis can can be usedbe used


public Person(string name) { this.name = name; this.birth_year = 1975; }}


public Person(string name) { this.name = name; this.birth_year = 1975; }}

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Thinking CornerThinking Corner Add a constructor to the Circle class Add a constructor to the Circle class

so that its objects can be created so that its objects can be created with provided with provided 'radius'radius' parameter' parameter

class Circle{ public double radius;}

class Circle{ public double radius;}

static void Main(){ Circle c1 = new Circle(30); Circle c2 = new Circle(2.5);}

static void Main(){ Circle c1 = new Circle(30); Circle c2 = new Circle(2.5);}

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Example: Balls in 2D Example: Balls in 2D Space (1)Space (1) Let us write a program (OOP style) to Let us write a program (OOP style) to

simulate ball movement in 2D spacesimulate ball movement in 2D space Each ball knows its own current Each ball knows its own current accelerationacceleration

and and velocityvelocity (on both x- and y-axes) (on both x- and y-axes) Each ball knows its current Each ball knows its current positionposition (x,y) (x,y) Each ball knows how to update its position Each ball knows how to update its position

and velocity after time and velocity after time tt (seconds) has passed (seconds) has passed Assuming constant accelerationAssuming constant acceleration

(x,y)(x,y)

v = (vx,vy)

a = (ax,ay)

(x,y)(x,y)

v = (vx,vy)

3 seconds 3 seconds passedpassed

(0,0)(0,0)

++

++

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Example: Balls in 2D Example: Balls in 2D Space (2)Space (2) Let us define the class Let us define the class BallBall so that so that

each Ball object has the following each Ball object has the following properties:properties: double sx,sydouble sx,sy – current position on the – current position on the

x- and y- axes (in meters)x- and y- axes (in meters) double vx,vydouble vx,vy – current velocity on the – current velocity on the

x- and y- axes (in m/s)x- and y- axes (in m/s) double ax,aydouble ax,ay – current acceleration on – current acceleration on

the x- and y- axes (in m/sthe x- and y- axes (in m/s22))class Ball { public double sx, sy; public double vx, vy; public double ax, ay;}

class Ball { public double sx, sy; public double vx, vy; public double ax, ay;}

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Example: Balls in 2D Example: Balls in 2D Space (3)Space (3) Add a method Update to update the Add a method Update to update the

position and velocity of the ballposition and velocity of the ball High school physics applies hereHigh school physics applies here

class Ball { : public void Update(double t) // t = time elapsed { sx = sx + 0.5*ax*t*t + vx*t; sy = sy + 0.5*ay*t*t + vy*t; vx = vx + ax*t; vy = vy + ay*t; }}

class Ball { : public void Update(double t) // t = time elapsed { sx = sx + 0.5*ax*t*t + vx*t; sy = sy + 0.5*ay*t*t + vy*t; vx = vx + ax*t; vy = vy + ay*t; }}

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Example: Balls in 2D Example: Balls in 2D Space (4)Space (4) Finally, add a constructor to allow Finally, add a constructor to allow

convenient creation of Ball objectsconvenient creation of Ball objects

class Ball { : public Ball(double sx, double sy, double vx, double vy, double ax, double ay) { this.sx = sx; this.sy = sy; this.vx = vx; this.vy = vy; this.ax = ax; this.ay = ay; }}

class Ball { : public Ball(double sx, double sy, double vx, double vy, double ax, double ay) { this.sx = sx; this.sy = sy; this.vx = vx; this.vy = vy; this.ax = ax; this.ay = ay; }}

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Example: Balls in 2D Example: Balls in 2D Space (5)Space (5) Test the programTest the program Simulate two Ball objectsSimulate two Ball objects

Ball b1 moves at constant velocity Ball b1 moves at constant velocity (ax = ay = 0)(ax = ay = 0)

Ball b2 moves under Earth's gravity Ball b2 moves under Earth's gravity (ax = 0, ay = 9.8)(ax = 0, ay = 9.8)

static void Main(){ Ball b1 = new Ball(0,0,10,20,0,0); Ball b2 = new Ball(0,100,0,0,0,-9.8); b1.Update(10); b2.Update(10); Console.WriteLine("After 10 seconds…"); Console.Write("b1 is at position ({0},{1})", b1.sx, b1.sy); Console.WriteLine(" with velocity [{0} {1}]", b1.vx, b1.vy); Console.Write("b1 is at position ({0},{1})", b2.sx, b2.sy); Console.WriteLine(" with velocity [{0} {1}]", b2.vx, b2.vy);}

static void Main(){ Ball b1 = new Ball(0,0,10,20,0,0); Ball b2 = new Ball(0,100,0,0,0,-9.8); b1.Update(10); b2.Update(10); Console.WriteLine("After 10 seconds…"); Console.Write("b1 is at position ({0},{1})", b1.sx, b1.sy); Console.WriteLine(" with velocity [{0} {1}]", b1.vx, b1.vy); Console.Write("b1 is at position ({0},{1})", b2.sx, b2.sy); Console.WriteLine(" with velocity [{0} {1}]", b2.vx, b2.vy);}

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Example: Projectile Motion Example: Projectile Motion (1)(1) Simulate projectile motion on earthSimulate projectile motion on earth

Cannon ball exits the cannon at Cannon ball exits the cannon at position (0,0)position (0,0)

Ask user for initial velocityAsk user for initial velocity Report the position of the cannon ball Report the position of the cannon ball

every secondevery second

v = (50,50)

a = (0,-9.8)

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Example: Projectile Motion Example: Projectile Motion (2)(2)static void Main(){ Console.Write("Enter initial vx: "); double vx = double.Parse(Console.ReadLine()); Console.Write("Enter initial vy: "); double vy = double.Parse(Console.ReadLine()); Ball b = new Ball(0, 0, vx, vy, 0, -9.8); Console.WriteLine("Time sx sy vx vy"); Console.WriteLine("----------------------------------"); for (int i = 0; i <= 10; i++) // simulate for 10 seconds { Console.WriteLine("{0,2}{1,8:f2}{2,8:f2}{3,8:f2}{4,8:f2}", i, b.sx, b.sy, b.vx, b.vy); b.Update(1); } Console.ReadLine();}

static void Main(){ Console.Write("Enter initial vx: "); double vx = double.Parse(Console.ReadLine()); Console.Write("Enter initial vy: "); double vy = double.Parse(Console.ReadLine()); Ball b = new Ball(0, 0, vx, vy, 0, -9.8); Console.WriteLine("Time sx sy vx vy"); Console.WriteLine("----------------------------------"); for (int i = 0; i <= 10; i++) // simulate for 10 seconds { Console.WriteLine("{0,2}{1,8:f2}{2,8:f2}{3,8:f2}{4,8:f2}", i, b.sx, b.sy, b.vx, b.vy); b.Update(1); } Console.ReadLine();}

Enter initial vx: 50Enter initial vx: 50Enter initial vy: 50Enter initial vy: 50Time sx sy vx vyTime sx sy vx vy-------------------------------------------------------------------- 0 0.00 0.00 50.00 50.000 0.00 0.00 50.00 50.00 1 50.00 45.10 50.00 40.201 50.00 45.10 50.00 40.20 2 100.00 80.40 50.00 30.402 100.00 80.40 50.00 30.40 3 150.00 105.90 50.00 20.603 150.00 105.90 50.00 20.60 4 200.00 121.60 50.00 10.804 200.00 121.60 50.00 10.80 5 250.00 127.50 50.00 1.005 250.00 127.50 50.00 1.00 6 300.00 123.60 50.00 -8.806 300.00 123.60 50.00 -8.80 7 350.00 109.90 50.00 -18.607 350.00 109.90 50.00 -18.60 8 400.00 86.40 50.00 -28.408 400.00 86.40 50.00 -28.40 9 450.00 53.10 50.00 -38.209 450.00 53.10 50.00 -38.2010 500.00 10.00 50.00 -48.0010 500.00 10.00 50.00 -48.00

Format the value to have 2 decimal Format the value to have 2 decimal places and width of 8 charactersplaces and width of 8 characters

40

Thinking CornerThinking Corner Modify the program in the previous Modify the program in the previous

example to ask user for starting example to ask user for starting speed and angle of the cannon ball, speed and angle of the cannon ball, instead of vx,vyinstead of vx,vy

ss

41

Challenging CornerChallenging Corner Write a GUI application that creates Write a GUI application that creates

several Ball objects, then simulates their several Ball objects, then simulates their movements and draw them on a windowmovements and draw them on a window Use a Timer to update the time and draw the Use a Timer to update the time and draw the

balls at new locationsballs at new locations Make balls bounce when they hit wallsMake balls bounce when they hit walls

42

ConclusionConclusion Multiple related data items can be Multiple related data items can be

bundled into an object by defining a bundled into an object by defining a class for itclass for it

Object-Oriented Programming (OOP) Object-Oriented Programming (OOP) allows programmers to view data as allows programmers to view data as objects that have their own objects that have their own behaviorsbehaviors

classes and object-oriented programming in c# computers and programming (01204111)

Documents

public int id public

public string dept

class keywordevery class

public static void main

new double100 string

new string100 string

new string100 string

class definition