CSCI 3328 Object Oriented Programming in C# Chapter 3: Introduction to Classes and Objects UTPA – Fall

Objectives In this chapter, you will – Become aware of reasons for using objects and classes – Become familiar with classes and objects 2

Introduction The book uses car analogy We humans are very good in recognizing and working with objects, such as a pen, a dog, or a human being We learned to categorize them in such a way that make sense to us. We may categorize them as animate object, inanimate objects, pets, friends, etc. 3

Introduction (cont'd) We some times classify objects based on their attributes, for example, green apples or red apples, fat or slim people, etc. If you think about it each object has many attributes. If I ask you list the attributes of an orange, you probably could list many things such as color, shape, weight, smell, etc. 4

Introduction (cont'd) In addition to attributes, all objects exhibit behaviors A dog eats, barks, wags its tail, plays, and begs – A dog exhibits many more other behaviors than this short list Another thing we need to remember about objects is that objects interact between each other 5

Objects Objects are packages that contain data and functions (methods) that can be performed on the data 6

Objects (cont'd) Data could be considered to be attributes and functions are considered to be behaviors of the object We can say that the attributes and behaviors are encapsulated into an object 7

Objects (cont'd) The objects interact between each other through their interfaces As an example a date object may have a set of data consisting of month, day and year, and methods consisting of assign date, display date, yesterday and tomorrow 8

Main Method Starting point of every application Console class has many methods including read and write – When using keyboard for input use ReadLine() You are going to use the Convert class all the time – Get to know this class well 9

Console.Write and WriteLine Console.WriteLine(“{0}\n{1}”, “Welcome to”, “C# Programming!”); Format items are enclosed in curly braces and contain a sequence of characters that tell the method which argument to use and how to format it. – Here, there are two arguments named {0} and {1} – The first argument will be followed by a new line character: \n 10

Addition Program using System; public class Addition { public static void Main( string[] args ) { int number1; // declare first number to add int number2; // declare second number to add int sum; // declare sum of number1 and number2 Console.Write( "Enter first integer: " ); // prompt user number1 = Convert.ToInt32( Console.ReadLine() ); Console.Write( "Enter second integer: " ); // prompt user number2 = Convert.ToInt32( Console.ReadLine() ); sum = number1 + number2; // add numbers Console.WriteLine( "Sum is {0}", sum ); // display sum } // end of function Main } // end of class Addition 11

Arithmetic Operators Unary: +, - Multiplicative: *, /, % Additive: +, - Relational operators – > = <= Equality operators – ==, != Precedence of operators 12 high low

If Statement using System; public class Comparison { public static void Main( string[] args ) { int number1; // declare first number to compare int number2; // declare second number to compare Console.Write( "Enter first integer: " ); number1 = Convert.ToInt32( Console.ReadLine() ); Console.Write( "Enter second integer: " ); number2 = Convert.ToInt32( Console.ReadLine() ); if ( number1 == number2 ) Console.WriteLine( "{0} == {1}", number1, number2 ); if ( number1 != number2 ) Console.WriteLine( "{0} != {1}", number1, number2 ); if ( number1 < number2 ) Console.WriteLine( "{0} < {1}", number1, number2 ); if ( number1 > number2 ) Console.WriteLine( "{0} > {1}", number1, number2 ); if ( number1 <= number2 ) Console.WriteLine( "{0} <= {1}", number1, number2 ); if ( number1 >= number2 ) Console.WriteLine( "{0} >= {1}", number1, number2 ); } // end Main 13

A Class is Like a Type You cannot use a class until an instance (an object) of it is made A class has properties (attributes) and methods – You can set and get the properties – Attributes are specified by the class’s instance variable – You can perform operations using methods 14

Example of a Class Declaration using System; public class GradeBook { private string courseName; // course name for this GradeBook public string CourseName // property { get { return courseName; } // end get set { courseName = value;} // end set } // end property CourseName public void DisplayMessage() { // use property CourseName to get the // name of the course that this GradeBook represents Console.WriteLine( "Welcome to the grade book for\n{0}!", CourseName ); // display property CourseName } // end method DisplayMessage } // end class GradeBook See next slide how to use this class 15

Using the Class GradeBook using System; public class GradeBookTest { public static void Main( string[] args ) { GradeBook myGradeBook = new GradeBook(); //object created Console.WriteLine( "Initial course name is: '{0}'\n", myGradeBook.CourseName ); Console.WriteLine( "Please enter the course name:" ); myGradeBook.CourseName = Console.ReadLine(); // set Console.WriteLine(); // output a blank line myGradeBook.DisplayMessage(); } // end Main } // end class GradeBookTest 16

Setting and Getting Values for Instance Variables Controlled ways of allowing users to access variables Usually get and set are made public so others can access them Private courseName instance and public CourseName property Note the uppercase/lowercase difference Properties contain accessors that handle the details of returning and modifying data After defining the property you can just use = to set it or get it by referring to it such as in read. C# auto implements get/set for private instance variables. Example: public string CourseName {get;set;} – You can assign values like: myGradeBook.CourseName = Console.ReadLine(); 17

Value Types vs. Reference Types Reference type contains the address of the location in memory where the data resides (refer to an object) 18

Initializing Objects with Constructors By default, when an object is created, it is initialized to null by default If we rather have a different value initially, we can use constructors – The “new” operator automatically calls the constructor – See the next two slides for example 19

GradeBook Class With a Constructor using System; public class GradeBook { public string CourseName { get; set; } public GradeBook( string name ) //constructor { CourseName = name; // set CourseName to name } // end constructor public void DisplayMessage() { Console.WriteLine( "Welcome to the grade book for\n{0}!", CourseName); } // end method DisplayMessage } // end class GradeBook 20

Test the Class using System; public class GradeBookTest { public static void Main( string[] args ) { GradeBook gradeBook1 = new GradeBook("CS101 Introduction to C# Programming" ); // invokes constructor GradeBook gradeBook2 = new GradeBook("CS102 Data Structures in C#"); // invokes constructor Console.WriteLine( "gradeBook1 course name is: {0}", gradeBook1.CourseName ); Console.WriteLine( "gradeBook2 course name is: {0}", gradeBook2.CourseName ); } // end Main } // end class GradeBookTest 21

About Some Data Types in C# Value Types Size (in bits) Range sbyte to 127 byte 8 0 to 255 short to ushort 16 0 to int to uint 32 0 to long to ulong 64 0 to char 16 0 to bool 8 true, false enum types and struct types Reference types include class types, interface types, delegate types, and array types Pointer types 22

Floating-Point Numbers and Decimal Real numbers can be stored as float, double and decimal In C#, float and double are treated as double by default Float and double stores approximation of real numbers in available spaces Decimal only stores limited range of decimal points precisely Single precision floating point numbers have seven significant digits, whereas the double gives 15 to 16 significant digits If you indicate a real number with M such as 18.33M, it will be treated as a decimal number M stands for money 23

Example of Decimal Number class Program { static void Main(string[] args) { decimal x = 3M; Console.WriteLine("{0:C}", x); Console.ReadLine(); } 24 string format specifier

String Format Specifiers C or c – Currency D or d – decimal X or x – hexadecimal E or e – scientific notion 25

Bank Account Example public class Account { private decimal balance; // instance variable that stores the balance public Account( decimal initialBalance ) // constructor { Balance = initialBalance; // set balance using property } // end Account constructor public void Credit( decimal amount ) // credit (add) an amount to the account { Balance = Balance + amount; // add amount to balance } // end method Credit public decimal Balance // a property to get and set the account balance { get {return balance;} // end get set {if ( value >= 0 ) balance = value; } // end set // validate that value is greater than or equal to 0; } } // end class Account 26

Create and Manipulate an Account using System; public class AccountTest { public static void Main( string[] args ) { Account account1 = new Account( 50.00M );// create Account object Account account2 = new Account( -7.53M ); // create Account object Console.WriteLine( "account1 balance: {0:C}", account1.Balance ); // display initial balance of each object using a property Console.WriteLine( "account2 balance: {0:C}\n", account2.Balance ); // display Balance property decimal depositAmount; // deposit amount read from user Console.Write( "Enter deposit amount for account1: " ); // prompt and obtain user input depositAmount = Convert.ToDecimal( Console.ReadLine() ); Console.WriteLine( "adding {0:C} to account1 balance\n", depositAmount ); account1.Credit( depositAmount ); // add to account1 balance Console.WriteLine( "account1 balance: {0:C}", account1.Balance ); // display balances Console.WriteLine( "account2 balance: {0:C}\n", account2.Balance ); Console.Write( "Enter deposit amount for account2: " ); depositAmount = Convert.ToDecimal( Console.ReadLine() ); Console.WriteLine( "adding {0:C} to account2 balance\n", depositAmount ); account2.Credit( depositAmount ); // add to account2 balance Console.WriteLine( "account1 balance: {0:C}", account1.Balance ); Console.WriteLine( "account2 balance: {0:C}", account2.Balance ); } // end Main } // end class AccountTest 27

Your Second Assignment Write a program to calculate an estimate on a paint job. User will enter Price of paint per gallon, and length and height of each wall. Amount of paint needed will be calculated from sqft as well as labor cost. Create and use classes. 28

Summary Class – Constructor – Destructor – Fields – Properties – Methods (events) Resources: us/library/0b0thckt(v=vs.80).aspxhttp://msdn.microsoft.com/en- us/library/0b0thckt(v=vs.80).aspx 29