A-level Computing 2009/AQA/Problem Solving, Programming, Operating Systems, Databases and Networking/Programming Concepts/Object-oriented programming (OOP)

Programming paradigms Pointers right|thumb|You can use the same class as a template to make (instantiate) many objects Object oriented programming is a type of programming paradigm based around programming classes and instances of classes called objects. These can be objects that appear on the screen (e.g., pictures, textboxes, etc.) or are part of the programming (e.g. actors, connections, particles, etc.). Classes Structures are very similar to Classes in that they collect data together. However, classes extend this idea and are made from two different things: Attributes - things that the object stores data in, generally variables. Methods - Functions and Procedures attached to an Object and allowing the object to perform actions Let's take a look at the following example: class car private maxSpeed as integer public fuel as integer public sub setSpeed(byVal s as integer) maxSpeed = s end sub public function getSpeed() as integer return maxSpeed end function public sub refuel(byVal x as integer) console.writeline("pumping gas!") fuel = fuel + x end sub public function getFuel() as integer return fuel end function public sub drive() fuel = fuel - 1 end sub end class You can see that the class is called car and it has: two attributes: maxSpeed, fuel five methods three procedures: setSpeed, refuel, drive two functions: getSpeed, getFuel center|300px Remember this is a class and therefore only a template, we need to 'create' it using an object Attributes These store information about the object. In the example above we store the fuel and maxSpeed. The attributes are attached to the classes, and if there are several instances (objects) of the classes then each will store its own version of these variables. Note that instead of the usual dim, there is the word private or public, we'll cover that in the encapsulation section. Methods Unlike structures, OOP allows you to attach functions and procedures to your code. This means that not only can you store details about you car (the attributes), you can also allow for sub routines such as drive() and refuel, which are attached to each class. Exercise: Object Orientation Intro What is the difference between a class and an object? A class is a template which cannot be executed An object is an instance of a class which can be executed one class can be used to make many objects What are the main components of a class? methods and attributes What is the difference between a structure and a class structures don't have methods OO - PIIE When talking about OOP you must remember the following: thumb|center|00 - PIIE! Where: 00 = Object Orientation and PIIE = Polymorphism / Inheritance / Instantiation / Encapsulation Instantiation As we have seen a class is a template for something, you can't actually execute a class, you must instantiate it, that is create an instance of an class in the form of an object. dim polo as new car 'instantiation 1 dim escort as new car 'instantiation 2 The code above creates an object called polo and escort, both of class type car (which we declared earlier). We can now use all the public attributes and methods: polo.refuel(100) 'assuming fuel starts at 0 polo.drive() escort.refuel(50) 'assuming fuel starts at 0 escort.drive() for x = 1 to 20 escort.drive() polo.drive() next polo.refuel(10) console.writeline("polo: " & polo.getFuel()) console.writeline("escort: " & escort.getFuel()) This would output the following: pumping gas! pumping gas! pumping gas! polo: 89 escort: 29 Exercise: Instantiating a car Write your own instantiation for a beetle car: dim beetle as new car 'instantiation What would the following output: dim ka as new car dim montego as new car ka.refuel(10) 'assuming fuel starts at 0 montego.refuel(50) 'assuming fuel starts at 0 montego.drive() montego.drive() for x = 0 to 10 montego.drive() next ka.refuel(10) console.writeline("ka: " & ka.getFuel()) console.writeline("montego: " & montego.getFuel()) pumping gas! pumping gas! pumping gas! ka: 20 montego: 37 Encapsulation 300px|thumb|You can only access private attributes and methods through an interface (public methods)You noticed that we didn't have to use the dim statement for the attributes and we used the word private instead. What this means is that these attributes are not directly accessible once you have instantiated the class. Let's take our polo class as an example: polo.fuel = 100 'this would be acceptable (but not in the exam!) In the example we access the fuel attribute of the polo class and give the car 100 units of fuel because fuel is declared as public, there are no restrictions in accessing it. However, when we try the following we run into trouble: polo.maxSpeed = 100 'this would be unacceptable The reason that this wouldn't work is because we have declared the maxSpeed attribute as private. If something is declared as private you can't access it externally, but how do you access it? The only way to access a private method or attribute is to use an interface, or public method. In the car code example we have: public sub setSpeed(byVal s as integer) 'declaring an interface maxSpeed = s end sub Because this method is public we can call it through: polo.setSpeed(100). And because setSpeed is declared inside the car object, it can have access to all the private attributes and methods. We also need to find out the speed of a car to display to the user, we can do this by creating a get routine: public function getSpeed() 'declaring an interface (a function as we are returning a value) return maxSpeed end function Because this method is public we can call it through: polo.getSpeed(). And because getSpeed is declared inside the car object, it can have access to all the private attributes and methods. In general attributes should always be declared as private and only accessible through interfaces, using a setX command to give the private variable X a new value, and using the getX command to return the value of X. You should never directly access X! Exercise: Encapsulation Declare a colour attribute for the car that can only be accessed through an interface private colour as string 'this must be private! Write an interface to set the colour public sub setColour(byVal c as string) 'declaring an interface, make sure it's public! colour = c end sub Write an interface to return the colour public function getColour() 'it must be a function to return a value return colour end function Create an actor class with the following: attributes: health, dexterity, x, y, strength methods: walkforward(), eat(), gethit(), displaystats(), setHealth(p), setLocation(px,py) Class actor Private health As Integer Private name As String Private x As Integer Private y As Integer Public Sub setName(ByVal p) name = p End Sub Public Sub walkforward() x = x + 1 End Sub Public Sub eat() health = health + 10 Console.WriteLine("Nom Nom Nom") End Sub Public Sub gethit() health = health - 10 Console.WriteLine("Argh!") End Sub Public Sub displaystats() Console.WriteLine("Character :" & name) Console.WriteLine("Health :" & health) Console.WriteLine("Location :" & x & ", " & y) End Sub Public Sub setHealth(ByVal p) health = p End Sub Public Sub setLocation(ByVal px, ByVal py) x = px y = py End Sub End Class For the actor class declared above instantiate: Wizard called Barry with 100 Health starting at 4,5 Orc called Herbert with 35 Health starting at 20,2, then report on his status dim wizard as new actor 'it doesn't have to be named wizard, it could have another name dim orc as new actor 'it doesn't have to be named orc, it could have another name wizard.setName("Barry") 'remember to use your get and set routines! wizard.setHealth(100) wizard.setLocation(4,5) orc.setName("Herbert") orc.setHealth(35) orc.setLocation(20,2) orc.displaystats() Programming Get and Set Routines Encapsulation is such a common thing that some languages have short cuts for making get and set routines. In VB.NET this involves using a property: Private _fuel As Integer Public Property Fuel() As Integer Get Return _fuel End Get Private Set(ByVal value As Integer) _fuel = value End Set End Property As we will never call _fuel directly it doesn't matter that it has an underscore at the beginning. Using this code we can perform the following methods without having the fuel attribute as public: lada.fuel = 23 Console.Writeline(lada.fuel) Inheritance right|thumb|inheritance diagram of vehicles, all sharing attributes and functions from the parent class 'Vehicle'. Note the direction of arrows Building on the car example above, what would happen if we wanted to declare an electric car? Well we'd probably want to store some information on the number of batteries that it has: class electricCar private maxSpeed as integer private fuel as integer 'fixed! private numBatteries as integer 'added public sub setnumBatteries(byVal n as integer) numBatteries = n end sub public function getnumBatteries() return numBatteries end sub public sub setSpeed(byVal s as integer) maxSpeed = s end sub public function getSpeed() as integer return maxSpeed end function public sub refuel(byVal x as integer) as integer '..... 'HOLD ON! end class This seems like a very long and tedious task rewriting all the same code again. You're right! It would be far better if we only had to declare all the new stuff we wanted to add. OOP allows for inheritance, where a new class can inherit the attributes and methods of a parent class: class electricCar inherits car 'declares what attributes and methods you are inheriting private numBatteries as integer 'added public sub setnumBatteries(byVal n as integer) 'interface numBatteries = n end sub public function getnumBatteries() 'interface return numBatteries end function end class This means that everything that car declared is now accessible from electricCar, as well as the new numBatteries attribute and methods. Let's instantiate this example and see what's possible dim gwiz as new electricCar gwiz.setnumBatteries(6) 'from electricCar gwiz.setSpeed(60) 'from car gwiz.drive() 'from car By using inheritance it makes creating new classes very quick and easy. It also allows for a modular approach to creating classes, where you might never use the base class at all, but only as a means of creating other child classes. Rather than having to rewrite the same functionality for similar objects, OOP allows you to reuse attributes and methods from parent classes. Exercise: Inheritance Declare a new class called limo that has attributes numSeats and colourSeats; and the ability to interface with them class limo inherits car 'declares what attributes and methods you are inheriting private numSeats as integer 'must be private private colourSeats as integer 'must be private public sub setnumSeats(byVal s as integer) 'interface set numSeats = s end sub public function getnumSeats() 'interface get return numSeats end function public sub setcolourSeats(byVal c as string) 'interface set colourSeats = c end sub public function getcolourSeats() 'interface get return colourSeats end function end class What are the benefits of using inheritance? Creating new classes from parent classes is very quick and easy. It allows for a modular approach to creating classes, where you might never use the base class at all. Inheritence diagrams CPT-OOP-inheritance.svg inheritance diagram of vehicles, all sharing attributes and functions from the parent class 'Vehicle'. The Truck inherits the Vehicle and adds its own attributes and methods The Car inherits the Vehicle and adds its own attributes and methods The Electric inherits the Car (and therefore the Vehicle) and adds its own attributes and methods The Petrol inherits the Car (and therefore the Vehicle) and adds its own attributes and methods Note the direction of arrows, you'll get marked down for putting them in the wrong direction. Exercise: Inheritance Diagrams 300px What is the parent class of car in the above diagram? vehicle Draw an inheritance diagram for the following classes: motor bikes, bikes, vehicles, pedal bikes, trikes. File:CPT-OOP-inheritance-bikes.svg Draw an inheritance diagram for the following classes: Monster, Character, Dragon, Hero, Orc File:CPT-OOP-inheritance-roleplay.svg Polymorphism When you were young you might have watched the Mighty Morphin' Power Rangers. These guys could morph from regular people into their power suits. Sadly polymorphism isn't that exciting, but it does allow you to change the function of attributes and methods when you are inheriting from a parent class. Consider our car example again. When we created the electricCar object we inherited from car and added the numBatteries attribute and methods. But what happens when we try and refuel, let's take a look at the code: public sub refuel(byVal x as integer) as integer console.writeline("pumping gas!") fuel = fuel + x end sub Well this just won't do. We are creating an electric car and we don't want to say that we are pumping gas; what would our sandal-socked yoghurt eating friends say! So for the electricCar object we want to inherit everything from car, but we want to morph(change) the refuel method. To do that we are going to use something called override: class electricCar inherits car private numBatteries as integer public sub setnumBatteries(byVal n as integer) numBatteries = n end sub public function getnumBatteries() as integer 'interface return numBatteries end sub '###### overrides morphs the inherited version, replacing it public overrides sub refuel(byVal x as integer) console.writeline("pure renewable energy!") fuel = fuel + x end sub '###### end class Exercise: Polymorphism In polymorphism what is the keyword we use to redefine a subroutine: overrides Write a definition of a limo class that uses polymorphism that makes sure that the maxSpeed never goes beyond 100: class limo inherits car public overrides sub setSpeed(byVal s as integer) if s > 100 then maxSpeed = 100 else maxSpeed = s end if end sub end class Write a definition of musclecar that inherits car, but uses 30 units of fuel per drive and displays "vroom vroom!" every time it drives. It should also store details on leatherSeating, allowing you to interface with this. class musclecar inherits car private leatherSeating as boolean public overrides sub drive() fuel = fuel - 30 console.writeline("vroom vroom!") end sub public sub setLeather(s as boolean) leatherSeating = s end sub public function getLeather() return leatherSeating end function end class Describe polymorphism Polymorphism allows you to inherit attributes from a parent class, but redefine some of the methods or attributes