by: md rezaul huda reza [email protected]. objects and classes. c# oo programming

UNIVERSITY OFSOUTH ASIA

Lecture 5:

By: Md Rezaul Huda [email protected]

CSE- 629VISUAL PROGRAMMING

WITH INTERNET)

VISUAL PROGRAMMING

Objects and Classes.C# OO Programming

UNIVERSITY of South Asia

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THIS WEEK… Objects and classes The C# class – has both methods and

fields (variables) as members Code generated by Visual Studio Static (class) variables and methods Properties

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OBJECTS. REAL WORLD EXAMPLES

Examples of real-world objects:

your pet, your desk, your bicycle (car?)

What do these real-world objects share? Answer: two characteristics

StateBehaviour

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OBJECTS. STATES AND BEHAVIOURS

State example:ColourBreedHungry

BehaviourRunningWagging tail Eating

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SOFTWARE OBJECT: MYCAR State:

• make: Mercedes• currentSpeed: 50mph• colour: red• currentGear: 4

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Behaviour: • brake• change gears• accelerate

This particular car is one instance of

a general car


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SOFTWARE OBJECT: MYCAR -CONT

State = fields/variables • Make: Mercedes• currentSpeed: 50mph• colour: red• currentGear: 4

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Behaviour = methods

• brake• change gears• accelerate

Object variables are formally known as fields (instance variables )

Object methods are formally known as instance methods


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CLASSES Definition: A class is a blueprint, or

prototype, that defines the variables and the methods common to all objects of a certain kind.

Example : Class Car

Variables: make, currentSpeed, colour, currentGear

Methods: brake, change gears,accelerate

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THE DIFFERENCE BETWEEN OBJECTS AND CLASSESthe term "object" is sometimes used to refer to both classes and instances

Question: You, as an object, would be an instance of what class?

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Use to make

ObjectsClass (the blueprint)


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EXAMPLE OF CLASS

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public class Car {

string make; int currentSpeed;string colour; int currentGear;

public void Brake() { ….

}public int ChangeGears (int cg){ ….

}public void Accelerate() { ….

} }

Fields

Methods


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EXAMPLE OF OBJECTS

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public class Car {

......}

public partial class Form1 : Form { .... private Car myPorsche;private Car myTractorpublic Form1(){

myPorsche = new Car();myTractor = new Car();

} }


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CREATING A CLASS IN VS In Solution Explorer right click on the name of

the project than Add / Class

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Class Name with cs

extension


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ENCAPSULATION A class encapsulates:

Fields (Instance variables) represent the data

Instance methods represent the behavior

The program does not worry how the class works internally

What is special about a class: both data and behavior are presented in one “unit”

The methods and variables that constitute a class are called members of the class

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CREATING CLASSES IN C# Class declaration

declares the name of the class along with other attributes

Class Body within curly braces { and } declarations for all fields (instance variables) and

static fields (class variables) and implementations for all instance methods

and static methods (class methods)

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CONSTRUCTORS These are also part of a class definition A constructor is a sort of class method used to

create new instances - objects of the defined by the class

They are defined and used (called) Example of using/calling a constructor:

Form f; f = new Form();

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class (type)Variable/field

constructor


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UNIVERSITY of South AsiaMORE ON CONSTRUCTORS

Same identifier (name) as the class name

Do not return a value Void is not included Overloading possibleDefault constructor (the “no-args” constructor)No arguments Example: Form() It is defined by default if no other constructors

are defined

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CONSTRUCTORS 1 has the same name as the class

public Car () {make = “ Mercedes”; currentSpeed = 0;colour = “Red”; currentGear = 0;

}

Note: classes don't have to have constructors. A default constructor is automatically provided by the runtime system, but it doesn't do anything. To perform some initialization, you will have to write some constructors for your class.

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No arguments in this case No return type

inside: instantiating fields


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UNIVERSITY of South AsiaCREATING AN OBJECT USING CONSTRUCTOR

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Declaring a constructor

Calling a constructor when an object is

created

public class Car {

string make, colour;int currentSpeed, currentGear;

public Car () {make = “ Mercedes”; currentSpeed = 0;colour = “Red”; currentGear = 0;

}

public void Brake() {… …}public int ChangeGears (int cg){ …. }public void Accelerate() { …. }

}………………

public Car myCar = new Car();


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THE MEMBERS OF A TYPICAL CLASS At least one constructor (the default

one) Methods Fields (the data, sometimes called

member variables) Properties (accessed like fields, but

actually methods)

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class name

fields

methods


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class Dog { private string barkSound; private string breed; private string dogSpeech;

public Dog(){barkSound = "Woof!";breed = "cocker spaniel";

}

public string GetSpeech(){dogSpeech = "Hello. I am a " + breed +

". " + barkSound; return dogSpeech; }

public void SetSound(String bS){this.barkSound = bS;

} }

Class declaration

Fields

Constructor

Methods

Class body

Another C# CLASS Example

Class body

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OVERLOADING CONSTRUCTORS Same as overloading methods

The compiler differentiates constructors based on the number of parameters (arguments) in the list and/or yhe parameters types

A class can have any number of constructors with the same name

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Arguments list

No arguments

public Dog(){

barkSound = "Woof!";breed = "cocker spaniel";

}

public Dog( string bs, string b ){

barkSound = bs;breed = b;

}


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CREATING OBJECTSDeclaring objects of class Dog:

private Dog lady ;

private Dog tramp ;

At this point lady and tramp are declared but not yet instantiated.

We say that such an object has a null value. Objects are instantiated by using the class’

constructor. Both lady and tramp are of type Dog.

Reference type

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INSTANTIATING THE OBJECTS Instantiate using the keyword new: lady = new Dog();

tramp = new Dog(“woof woof”, “moggy”);

Note: Every time you instantiate a new object from a class, you get a new copy of each of the class's instance variables.

As local variables, inside methods, you can use “var”:var lady = new Dog(); private Dog lady = new Dog();

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THE DIFFERENCE BETWEEN OBJECTS AND PRIMITIVE VARIABLES

public double a = 31.3public int b = 20

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Primitive data types, not composed of any other types

Private Dog lady = new Dog(); Object, composed of other types

31.3

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reference

a:

b:

lady: Woof ! cocker spaniel

barkSound breed


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REFERENCE TYPES AND VALUE TYPE

• Value types: int, short, double, bool….• Reference types : objects that store references to

the actual data, not the value– built-in reference types: • object• string

• To instantiate a reference object in C#, you have to use new keyword.

• In C# we create a reference and then point the reference at an object allocated the heap with the new keyword, like:

Object myobj;myobj = new Object();

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OBJECT SHARING Objects get instantiated by a constructor method from

their class. - private Dog lady = new Dog(); - private Dog tramp = new Dog(“woof woof”, “moggy”);

Another way is simply by assigning the value of

another object. - private Dog myDog = lady;

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lady:

trump:

myDog:

Woof ! cocker spaniel

barkSound breed

woof woof moggy

barkSound breed


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IT’S A DOG’S… CLASS!

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class Dog { string barkSound; string breed;

public Dog(){barkSound = "Woof!";breed = "cocker spaniel";

}

public string GetSpeech(){var dogSpeech = "Hello. I am a " + breed + ". " + barkSound;

return dogSpeech; }

public void SetSound(string barkSound){this.barkSound = barkSound;

} }

I’m a field (instance variable). Each generated Dog Object has a new copy of me.

I am instantiated to “Woof” in the constructor. That means all created Dogs will bark “Woof”

if nobody changes my content.

I’m a “setter” method !I can make it possible for a field to vary over time.

By the way, did you notice THIS ?

I am an instance method.

I am a local variable. I could be declared as a string


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SO, WHAT IS THIS ? two different variables with the same name, barkSound !!

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class Dog {

String ; ….. ….. public void setSound(string ) { = ; } }

barkSound

barkSoundbarkSound barkSoundthis.

defined as a parameterfor a new barking sound

Field for each dog that is instantiated

this is used to refer to the current instance of Dog.

the current value of the instance variable (this.barkSound) is replaced with the new value passed as an argument (barkSound) to setBark.


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UNIVERSITY of South AsiaRELATIONSHIPS BETWEEN CLASSESclass DogChorus {

Dog lady;

Dog buddy;

public DogChorus() {

lady = new Dog();

buddy = new Dog();

buddy.SetSound("Ruff!");

}

public string GetOutput(){

return lady.GetSpeech()+ ”\n” + buddy.GetSpeech();

}

}

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class Dog { string barkSound, breed;

public Dog(){ barkSound = "Woof!"; breed = "cocker spaniel"; } public string GetSpeech(){ string dogSpeech = "Hi. I’m a" + breed + ". " + barkSound; return dogSpeech; } public void SetSound(string bs){ barkSound = bS; } }

has a

Question: How is encapsulation applied in the Dog class?


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class DogChorus { Dog lady; Dog buddy; public DogChorus() { lady = new Dog(); buddy = new Dog(); buddy.SetSound("Ruff!"); } public string GetOutput(){ return lady.GetSpeech()+ ”\n” + buddy.GetSpeech(); }}

public void SetSound(String barkSound){ this.barkSound = barkSound;}

A DOGS … CHORUS!

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buddy changes its barking characteristic from the default Woof to Ruff. How?

public Dog(){ barkSound = "Woof!"; breed = "cocker spaniel";}

Class Dog - lady instance

“Woof”

Class Dog -buddy instance

“Ruff”

creating an instance of Dog referred to as lady

“Ruff”


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THIS

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Within a class, this refers to the current instance of the class. The this keyword can be used to access members from within constructors and instance methods this is often used in constructors:

public Dog ( string barkSound, string breed)

{

this.barkSound = barkSound;

this.breed = breed;

}


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MODIFIERS

Appear in method/ constructor headings

variable declarations

the declaration heading for classes and other class members

Indicates how it can be accessed

Types of modifiers

Access (to do with visibility)

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ACCESS MODIFIERS (PROTECTIONLEVELS FOR CLASS MEMBERS) Class members and classes can have one of the

following protection levelspublic – accessible to everyone internal – accessible within the same

assembly/namespaceprivate – accessible only inside classprotected – accessible for descendants –later lecture

Default protection levelsClass members (fields, instance methods) – privateClasses– internal

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VISIBILITY. PUBLIC OR PRIVATE? Public makes the methods available to other

pieces of program Private: nobody can access the private variables.

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private string barkSoundprivate string breedprivate string dogSpeechFields (variables)public Dog()constructorpublic string GetSpeech()public void SetSound(

string bs)methods

Dog() constructor

GetSpeech() SetSound(string bs)methods

Class seen by the programmer Object seen by other pieces of program


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THE EFFECTS OF PUBLIC AND PRIVATE ACCESSIBILITY

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violateEncapsulationUnless properties

enforceencapsulation

provide services to clients – for example, otherclasses

support othermethods in theclass

public private

variables

methods


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UNIVERSITY of South AsiaUSING ACCESS MODIFIERS TO IMPLEMENT ENCAPSULATION: DATA FIELDS

As a general rule, no fields should be declared publicAny changes to the object's state (its

fields/variables) should be accomplished by that object's methods

We should make it difficult, if not impossible, for one object to "reach in" and directly alter another object's state

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GETTERS AND SETTERS instance methods to access and change

values of fields Getter/Accessor

Returns just the current value Standard naming convention → add ”Get” in front

of instance variable name Accessor for noOfSquareYardsis GetNoOfSquareYards( )

Setters/MutatorsNormally includes one parameter Method body → single assignment statement Standard naming convention add ”Set” to the front

of the instance variable identifier Mutator for noOfSquareYardsis SetNoOfSquareYards( )

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private string breed;

public string GetBreed{

return breed;}

public void SetBreed (string breed){

this.breed = breed; }

EXAMPLES

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GetterNo parameter.

Return type

Setter.Parameter.

No return type.


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IMPLEMENTING DATA ENCAPSULATION USING PROPERTIES

Use C# properties to provide access to data safelyFields(instance variables) should be declared

private, with public properties that allow safe access to them

Public properties allow clients to:Get (obtain the values of) private data

getter (accessor): controls formatting and retrieval of dataSet (assign values to) private data

setter (mutator): ensure that the new value is appropriate for the data member

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USING EXISTING PROPERTIES IN VS

Properties in C# give access to fields

Accessing a property is like using a getter method

Changing a property is like using a setter method

Accessing or changing some properties of an object can be done without writing code - directly in Visual Studio by changing the object’s Properties list

Naming in C# for properties

Use the same name as the instance variable ( field), but start with uppercase character


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UNIVERSITY of South AsiaCREATING C# PROPERTIES

A special construct using set and get keywords Automatically created in VS.

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private string breed;public string Breed{

get { return breed; }set { breed = value;}

}

Matching data type

The value keyword represents the

implied parameter

How could we make this a read-only property?

Note – no parenthesis

()


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GENERATE PROPERTIES IN VS USING REFACTOR Right click on the field:

Click Refactor, than Encapsulate field

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GENERATE PROPERTIES IN VS Leave the name as it comes by default

(good practice)

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USING PROPERTIES Access through the object (instance of a class)

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class Dog{private string breed;public string Breed{ get {return breed;} set {breed = value;}}

...}

class DogChorus { Dog tramp; public DogChorus() { tramp = new Dog();

string myBreed = tramp.Breed; tramp.Breed= “mutt”; .... }


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THE STATIC MODIFIER

“Static” Indicates that a variable or a method belongs to the class itself rather than to an instance

Methods with the static modifier = class methods (static methods)The Main( ) method must include static in the first line

Example: Suppose that all dogs have 4 legs. You can define a static :

static int dogLegs = 4;All instances (dogs) share this variable and will

have 4 legs. If you define dogLegs as an instance variable each instance

(e.g. lady, tramp) would have its own copy of the variable, and you assign a different value for each dog (using property, constructor,etc)

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UNIVERSITY of South AsiaCLASS METHODS (STATIC METHODS)

Class (static) methods can only operate on class (static) variables and cannot access the instance variables defined in the class*.

To specify that a method is a class method, use the static keyword in the method declaration.

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class Dog { private static int dogLegs; public static int getLegs() { return dogLegs; } public static void setLegs(int dl) { dogLegs= dl; } }

* Note: class methods cannot access instance variables unless the method created an instance of the class first and accesses the variable through it

Static (class) variable

Static (class) method


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CLASS (STATIC) METHODS 2 Methods declared with static modifier Can be defined in any class Not associated with an object The class method M in the class C is

invoked using the notation C.M( … )E. g. Dog.GetLegs();

cannot refer to instance variables It can refer to and manipulate class

variables

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CALLING STATIC METHODS EXAMPLE

double aValue = 78.926;double result1, result2;result1 = Math.Floor(aValue); // result1 = 78result2 = Math.Sqrt(aValue); // result2 = 8.88403061678651MessageBox.Show (“aValue rounded to 2 decimal places”

+ Math.Round(aValue, 2));

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Call it through the class name, not an instance

Each call returns a value

Round is called through a class. Must be static

A Value rounded to 2 decimal places is 78.93

MessageBox class as a static method Show()


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THE MEMBERS OF A TYPICAL CLASS

At least one constructor Methods (functions in other languages) Fields (the data, sometimes called member

variables) Properties (accessed like fields, but actually

methods)

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class name

fields

methods

properties


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RELATIONSHIP BETWEEN C# ELEMENTS

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WHAT IS A NAMESPACE? Namespaces provide scope for the

classes defined within a group -an “umbrella”

System: most important and frequently used namespace

Each namespace enclosed in curly braces—{ }

Can define your own namespace

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EXPLORING A SIMPLE APPLICATION

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using allows us to call classes from other namespaces

What is the namespace for?

What is a class? DO you notice any new word?

Method called. Where is it declared?

Method declared.

Body of the method


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.NET 2010 PARTIAL CLASSES

namespace WindowsApplication1

{ public partial class Form1 :

Form { public Form1() { InitializeComponent(); }….. }}

Allows class implementation across more than one file.

This permits breaking down very large classes

Useful if some parts of a class are automatically generated.

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The partial class Form1

Program class • Always in a

windows application

• Automatic generated code.

• Created to keep class Form1 simple

The partial class Form1 (Designer)


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partial class Form1 (Designer)

Method that creates buttons labels, etc


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CLASS PROGRAM

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Who is Form1() ?


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CLASS DIAGRAM FOR THE DOGS’ WORLD

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Note: the code is available on teachmat


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VIEWING THE CLASS DIAGRAM IN VS

In Solution ExplorerRight click on the name

of the namespaceChoose “View class

diagram”

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GARBAGE COLLECTION 1 The idea of a “dog” does not disappear, but what about any particular

dog? In real life trump would be born, have a life cycle and at some point, die …

In C#, objects are: Created

the new operator – converts a chunk of raw memory to an object

living the object’s methods/properties are called using the dot operator [.] Other references' to the object

Destroyed Only when all the references to the object have disappeared

How do you know when an object is dead? Answer: If nobody can call it - It can not be reached.

From time to time the garbage collector chases all the references in all the objects to find all the live objects.

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GARBAGE COLLECTION 2 When you initialize a variable using the new

operator, you are in fact reserving some space in the heap memory. The memory is "allocated" for the variable.

When that variable is no longer needed, such as when your program closes, it (the variable) must be removed from memory and the space it was using can be made available to other variables or other programs.

The .NET Framework solves the problem of garbage collection by "cleaning" the memory after you. This is done automatically when necessary so that the

programmer doesn't need to worry about this issue.

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ANY QUESTION ? ? ?


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THANK YOU

by: md rezaul huda reza [email protected]. objects and classes. c# oo programming

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