BIM313 – Advanced Programming Techniques Object-Oriented Programming 1

Contents Object-Oriented Programming – Objects – Constructors, Destructors – OOP Techniques (Interfaces, Inheritance, Polymorphism, Operator Overloading, Events) 2

Object-Oriented Programming (OOP) 3

Contents What is OOP? OOP Techniques Using OOP in Console Application 4

What is OOP? The type of programming we have covered so far is known as functional (or procedural) programming OOP is an approach to creating computer applications which uses objects Code reuse is easier in OOP We have already used some objects in our programs (e.g. Console, Exception, etc.) 5

Objects An object has both member variables and methods – structs in C contain only member variables Objects are instantiated from classes Objects are also called instances The terms class and object are often confused, and it is important to understand the distinction 6

Class vs. Object Class Class is only a definition Classes are coded in source files You write a class once Classes are abstract Object Object is the implementation Objects are created at runtime You can create many objects from a class Objects are concrete 7

Class vs. Object 8

Everything is an object Everything in C# and.NET Framework is an object! even the int variables are objects too! 9

Object Members Properties – Data contained in an object – Determine the state of the object – They may be read-only, write-only, or both readable and writable – Example: Columbian filter coffee with milk and two sugars Methods – Functions of objects are called methods – Example: AddSugar(), Mix(), etc. 10

Visibility public – Public variables or methods are accessible by other objects private – Private variables or methods are accessible only by the methods of the object itself 11

The Life Cycle of an Object Construction – Initialization of the object – Implemented by constructors – There may be several constructors – The code snippet ctor can be used to create a constructor in Visual Studio Destruction – Resources used by the object are freed – Implemented by a destructor function – Each class may have only one destructor function 12

Construction CupOfCoffee myCup = new CupOfCoffee(); CupOfCoffee myCup = new CupOfCoffee(“Columbian”); CupOfCoffee myCup = new CupOfCoffee(“Columbian”, true, true, 2); 13 Source Filtered?Milk? Sugar?

Constructor Syntax class MyClass { public MyClass() { // Default constructor code } public MyClass(int myInt) { // Non-default constructor code } 14

Destruction You can make some extra operations when an object is about to be destroyed – e.g. Saving the object data into a file Generally the default destructor does all the work for you and you don’t need to write a destructor 15

Destructor Syntax class MyClass { ~MyClass() { // Destructor body } Use the Finalize() method to call the destructor 16

Static and Instance Class Members Static Members – Shared between all instances of a class – You don’t need to instantiate an object to use a static member – You can access static member with the class name – Examples: Math.Sin(), Main(), Console.WriteLine(), int.Parse(), Convert.ToDouble(), etc. – static methods can access only the static members of a class Instance Members – All objects have separate instance members – Instance members require an instance to be used – You can access instance members with the name of the objects – Examples: Length() (of strings and arrays), ToString(), etc. 17

OOP Techniques Interfaces Inheritance Polymorphism Operator Overloading Events 18

Interfaces An interface is a collection of methods and properties that are grouped together to encapsulate specific functionality Interfaces are only some definitions, and they should be implemented in classes – i.e. the class supports all functionality defined in the interface You can’t instantiate an interface Interfaces cannot contain any code Interface names generally start with ‘I’ A class may implement more than one interfaces 19

Interface Syntax public interface IMyInterface { … } public class MyClass : IMyInterface { … } 20

Exercise: Sorting Cars Create an enumeration of ‘CarBrands’ Create a class ‘Car’ Add two members, ‘Brand’ and ‘Price’ to ‘Car’ class definition Create a constructor (use ‘ctor’ code snippet) Create a ‘Display()’ method in ‘Car’ class Create an array of cars in Main Change ‘Car’ definition so that it implements IComparable interface – IComparable objects can be sorted Implement the CompareTo() method Sort the cars in Main 21

Implementing an Interface in VS 22

Solution (Page 1) enum CarBrand { Mercedes, BMW, Honda, Toyota, Volkswagen, Mazda } class Car : IComparable { // Properties: CarBrand Brand; decimal Price; // Constructor: public Car(CarBrand brand, decimal price) { this.Brand = brand; this.Price = price; } // Methods public void Display() { Console.WriteLine(this.Brand + " - " + this.Price + " TL."); } public int CompareTo(Car other) { return (int)this.Price - (int)other.Price; } 23

Solution (Page 2) class Program { static void Main(string[] args) { Car[] cars = new Car[] { new Car(CarBrand.Volkswagen, 70000), new Car(CarBrand.Mercedes, ), new Car(CarBrand.Honda, 65000) }; Array.Sort(cars); Console.WriteLine("Cars sorted in price:\n"); foreach (Car car in cars) { car.Display(); } 24

Inheritance Any class may inherit from another Inherited class will have all members of base class Classes in C# may derive only from a single base class directly (No multiple-inheritance) Interfaces may inherit from other interfaces (maybe multiple) Syntax: class InheritedClass : BaseClass { … } 25

Inheritance Syntax public class MyClass : BaseClass { … } class MyClass : BaseClass, Interface1, Interface2 { … } 26

Visibility in Inheritance Public members of the base class are accessible from the derived class Private members of the base class are not accessible from the derived class Protected members of the base class are accessible from the derived class but not accessible from other classes 27

Virtual Members Virtual members of the base class can be overridden by the derived class 28

Abstract Classes Abstract classes can’t be instantiated directly You have to derive another class from the abstract class and then the derived class can be instantiated Abstract classes may have abstract members, which have no implementation in the base class, so an implementation must be supplied in the derived class 29

Sealed Classes A sealed class may not be used as a base class, so no derived classes are possible 30

Polymorphism Using the same method with different implementations in base and derived classes is called polymorphism Example: ToString() method of the Object class 31

Operator Overloading if (carA.Price > carB.Price) { … } You can use the following code if you overload the operators: if (carA > carB) { … } 32

Events When you click a button or move the mouse in a Windows program, events are raised When an event is raised, an event handler method is executed You can add custom events into your own classes 33

Reference Types vs. Simple Types Simple Types – int, float, double, etc. – string – object – struct Reference Types: Contents are stored in a separate memory location (heap) – Classes (Objects created with the new keyword) – Arrays 34

Shallow Copying vs. Deep Copying Shallow Copying – When you make a copy of an object, value types are copied correctly but reference types point to the previous copies – Accomplished by MemberwiseClone method of the object class MemberwiseClone() is a protected member of the object class Deep Copying – Reference types are copied too – Implement the ICloneable interface 35

Properties You can’t make range-check on public members – Age of a person can only be positive – If negative values are assigned, program may crash By using properties, you can check values before they are assigned You can create read-only or write-only properties 36

Properties Syntax public int MyIntProp { get { // Property get code. } set { // Property set code. } 37

Properties Example private int m_Age; public int Age { get { return m_Age; } set { if (value < 0) m_Age = 0; else m_Age = value; } 38

Read-Only Property Example private string FirstName; private string LastName; public string FullName { get { return FirstName + “ ” + LastName; } 39

.:. Application.:. Create a Curve class which stores a list of points Create a Point class to be used in the Curve class Add PrintPoints() and AddPoint() methods into the Curve class Make the Curve class cloneable – try shallow and deep copies Create a property named CurveLength Overload the ‘–’ operator so that Point1 – Point2 gives the distance between them Overload the ‘ ’ operators for the Curve class which compares the curve lengths 40