ADVANCED PROGRAMMINGCHAPTER 4: INTRODUCTION TO INTRODUCTION TO CLASSES, OBJECTS, METHODS AND STRINGS

Dr Shahriar BijaniWinter 2016

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REFERENCES Visual C# 2012 How to Program, Paul Deitel

& Harvey Deitel, 5th Edition, Prentice Hall.

4.2 CLASSES, OBJECTS, METHODS, PROPERTIES AND INSTANCE VARIABLES

Review of classes: Car exampleMethods describe the mechanisms that

perform a tasks (e.g. acceleration)Hide complex tasks from the user; a

driver does not need to know how the accelerator works but can use it.

Classes must be defined before use; a car must be built before it can be driven

Many objects can be created from the same class; many cars can be built from same engineering diagram

4.2 CLASSES, OBJECTS, METHODS, PROPERTIES AND INSTANCE VARIABLES

Method calls send messages to an object to perform tasks; pressing the gas pedal sends a message to the car to accelerate

Objects have attributes; cars have color and speed gauge, each car knows how much fuel is in its own tank, but

not how much is in the tanks of other cars.

Properties, get Accessors and set Accessors The attributes are not necessarily accessible

directly. The car manufacturer does not want drivers to access the car’s engine to observe the amount of fuel in its tank.

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A CLASS EXAMPLE1 // Fig. 4.1: GradeBook.cs2 // Class declaration with one method.3 using System;45 public class GradeBook6 {7 // display a welcome message to the GradeBook user89 {10 Console.WriteLine( "Welcome to the Grade Book!" );11 } // end method DisplayMessage12 } // end class GradeBook

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CREATE AND CALL AN OBJECT1 // Fig. 4.2: GradeBookTest.cs2 // Create a GradeBook object and call its DisplayMessage method.3 public class GradeBookTest4 {5 // Main method begins program execution6 public static void Main( string[] args )7 {8 // create a GradeBook object and assign it to myGradeBook9 GradeBook myGradeBook = new GradeBook();1011 // call myGradeBook's DisplayMessage method12 myGradeBook.DisplayMessage();13 } // end Main14 } // end class GradeBookTest

Fig. 4.2: Create a GradeBook object and call its DisplayMessage method.

Welcome to the Grade Book!

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4.4 DECLARING A METHOD WITH A PARAMETER

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4.4 DECLARING A METHOD WITH A PARAMETER

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UML CLASS DIAGRAMS UML class diagram of class GradeBook with a public

DisplayMessage operation

Fig 4.6 GradeBook has a public DisplayMessage operation with a courseName parameter of type string.

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4.5 INSTANCE VARIABLES AND PROPERTIES

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Software Engineering Observation 4.2 Precede every field and method declaration with an

access modifier. Generally, instance variables should be declared private

and methods and properties should be declared public. No access modifier = private declare a private method, if it will be accessed only by

other methods of the class. Software Engineering Observation 4.3

private member variables and public methods and properties helps debugging

because problems with data manipulations are localized to the class’s methods and properties, since the private member variables are accessible only to these methods and properties.

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UML CLASS DIAGRAM WITH A PROPERTY

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4.8 AUTO-IMPLEMENTED PROPERTIESpublic string CourseName { get; set; }

the C# compiler can automatically create a private instance variable, and the get and set accessors for it.

Unlike a user-defined property, an auto-implemented property, must have both a get and a set accessor.

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4.9 VALUE TYPES VS. REFERENCE TYPES

Value Types A value type variable contains a value of that type! C#’s simple types (like int and double) are all value

types.

int count = 7;

All simple types, enum or a struct type are value types

Value typesbool

byte

char

decimal

double

enum

float

int

long

sbyte

short

struct

uint

ulong

ushort

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4.9 VALUE TYPES VS. REFERENCE TYPES Reference Types

A reference variable contains the address of a location in memory.

GradeBook myGradeBook = new GradeBook();

Reference-Type Instance Variables Initialized to null

string is a reference type. value null is not an empty string ( "" or string.Empty)

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4.10 INITIALIZING OBJECTS WITH CONSTRUCTORS

Constructors methods can initialize an object C# requires a constructor call for every

object that is created. The new operator calls the constructor.

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4.11 FLOATING-POINT NUMBERS AND TYPE DECIMAL type float represent precision with 7 significant digits. type double represent precision with15–16 significant

digits Type decimal provides 28–29 significant digits. To type a decimal literal, you must type the letter “M”

or “m”