11. Objects and Classes

Using Objects and Classes
Defining Simple Classes
Objects and Classes
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Table of Contents
1. Objects
2. Classes
3. Built in Classes
4. Defining Simple Classes
 Properties
 Methods
 Constructors
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Objects and Classes
What is an Object? What is a Class?

Objects
 An object holds a set of named values
 E.g. birthday object holds day, month and year
 Creating a birthday object:
5
var birthday = new { Day = 22, Month = 6, Year = 1990 };
Birthday
Day = 22
Month = 6
Year = 1990 The new operator creates
a new object
var day = new DateTime(
2019, 2, 25);
Console.WriteLine(day);
Object
properties
Object
name
Create a new object
of type DateTime

Classes
 In programming, classes provide the structure for objects
 Act as template for objects of the same type
 Classes define:
 Data (properties), e.g. Day, Month, Year
 Actions (behaviour), e.g. AddDays(count),
Subtract(date)
 One class may have many instances (objects)
 Sample class: DateTime
 Sample objects: peterBirthday, mariaBirthday
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Objects - Instances of Classes
 Creating the object of a defined class is
called instantiation
 The instance is the object itself, which is
created runtime
 All instances have common behaviour
7
DateTime date1 = new DateTime(2018, 5, 5);

Objects and Classes – Example
8
DateTime peterBirthday = new DateTime(1996, 11, 27);
DateTime mariaBirthday = new DateTime(1995, 6, 14);
Console.WriteLine("Peter's birth date: {0:d-MMM-yyyy}",
peterBirthday); // 27-Nov-1996
Console.WriteLine("Maria's birth date: {0:d-MMM-yyyy}",
mariaBirthday); // 14-Jun-1995
var mariaAfter18Months = mariaBirthday.AddMonths(18);
Console.WriteLine("Maria after 18 months: {0:d-MMM-yyyy}",
mariaAfter18Months); // 14-Dec-1996
TimeSpan ageDiff = peterBirthday.Subtract(mariaBirthday);
Console.WriteLine("Maria older than Peter by: {0} days",
ageDiff.Days); // 532 days

Classes vs. Objects
 Classes provide structure for
creating objects
 An object is a single
instance of a class
9
class
DateTime
Day: int
Month: int
Year: int
AddDays(…)
Subtract(…)
Class actions
(methods)
Class name
Class data
(properties)
object
peterBirthday
Day = 27
Month = 11
Year = 1996
Object
name
Object
data

 You are given a date in format day-month-year
 Calculate and print the day of week in English
Problem: Day of Week
10
18-04-2016 Monday 27-11-1996
string dateAsText = Console.ReadLine();
DateTime date = DateTime.ParseExact(dateAsText, "d-M-yyyy",
CultureInfo.InvariantCulture);
Console.WriteLine(date.DayOfWeek);
ParseExact(…) needs a
format string + culture
(locale)
Check your solution here: https://judge.softuni.bg/Contests/1214/Objects-and-Classes-Lab
Wednesday

Using the Built-in API Classes
Math, Random, BigInteger, ...

 .NET Core provides thousands of ready-to-use classes
 Packaged into namespaces like System, System.Text,
System.Collections, System.Linq, System.Net, etc.
 Using static .NET class members:
 Using non-static .NET classes
Built-in API Classes in .NET Core
12
DateTime today = DateTime.Now;
double cosine = Math.Cos(Math.PI);
Random rnd = new Random();
int randomNumber = rnd.Next(1, 99);

 You are given a list of words
 Randomize their order and print each word at a separate line
Problem: Randomize Words
13
Note: The output is a sample. It
should always be different!
a b
b
a
PHP Java C#
Java
PHP
C#

Solution: Randomize Words
14
string[] words = Console.ReadLine().Split(' ');
for (int pos1 = 0; pos1 < words.Length; pos1++)
{
int pos2 = rnd.Next(words.Length);
// TODO: Swap words[pos1] with words[pos2]
}
Console.WriteLine(string.Join(Environment.NewLine, words));

 Calculate n! (n factorial) for very big n (e.g. 1000)
Problem: Big Factorial
15
50
3041409320171337804361260816606476884437764156
8960512000000000000
5 120 10 3628800 12 479001600
88
1854826422573984391147968456455462843802209689
4939934668442158098688956218402819931910014124
4804501828416633516851200000000000000000000

Solution: Big Factorial
16
using System.Numerics;
...
int n = int.Parse(Console.ReadLine());
BigInteger f = 1;
for (int i = 2; i <= n; i++)
f *= i;
Console.WriteLine(f);
Use the .NET API class
System.Numerics
.BigInteger
N!Check your solution here: https://judge.softuni.bg/Contests/1214/Objects-and-Classes-Lab

Defining Classes
Creating Custom Classes

 Specification of a given type of objects from
the real-world
 Classes provide structure for describing
and creating objects
Defining Simple Classes
class Dice
{
…
}
Class name
Class body
Keyword
18

Naming Classes
 Use PascalCase naming
 Use descriptive nouns
 Avoid abbreviations (except widely known, e.g. URL,
HTTP, etc.)
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class Dice { … }
class BankAccount { … }
class IntegerCalculator { … }
class TPMF { … }
class bankaccount { … }
class intcalc { … }

 Class is made up of state and behavior
 Properties store state
 Methods describe behaviour
Class Members
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class Dice
{
public int Sides { get; set; }
public string Type { get; set; }
public void Roll() { }
}
Properties
Method

 A class can have many instances (objects)
Creating an Object
21
class Program
{
public static void Main()
{
Dice diceD6 = new Dice();
Dice diceD8 = new Dice();
}
}
Use the new
keyword

 Describe the characteristics of a given class
Properties
22
class Student
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int Age { get; set; }
}
The getter provides
access to the field
The setter provides
field change

 Store executable code (algorithm)
Methods
23
class Dice
{
public int Roll()
{
return rnd.Next(1, Sides + 1);
}
}

 Special methods, executed during object creation
Constructors
24
class Dice
{
public Dice()
{
this.Sides = 6;
}
}
Overloading default
constructor
Constructor name is
the same as the name
of the class

 You can have multiple constructors in the same class
Constructors (2)
25
class Dice
{
public Dice() { }
public Dice(int sides)
{
this.Sides = sides;
}
p int Sides { get; set; }
}
class StartUp
{
public static void Main()
{
Dice dice1 = new Dice();
Dice dice2 = new Dice(7);
}
}

 Classes can define data (state) and operations (actions)
Class Operations
26
class Rectangle
{
public int Top { get; set; }
public int Left { get; set; }
public int Width { get; set; }
public int Height { get; set; }
int CalcArea()
{
return Width * Height;
}
Classes may hold
data (properties)
Classes may hold
operations
(methods)

Class Operations (2)
27
public int Bottom
{
get
{
return Top + Height;
}
}
public int Right
{
get
{
return Left + Width;
}
}
Calculated
property
Calculated
property
public bool IsInside(Rectangle r)
{
return (r.Left <= Left) && (r.Right >= Right) &&
(r.Top <= Top) && (r.Bottom >= Bottom);
}
Boolean
method

 …
 …
 …
Summary
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 Objects
 Holds a set of named values
 Instance of a class
 Classes define templates for object
 Methods
 Constructors
 Properties

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Trainings @ Software University (SoftUni)
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is licensed under the "Creative Commons Attribution-NonCom
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11. Objects and Classes

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