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Object-orientated Programming (OOP)

Unit 4 Computing

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Programming Languages

All computer languages can be put in one of the following categories:

• Low Level Languages

• High Level Languages

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Low Level Languages

• are machine orientated – an assembly language program written on one machine will not work on any other type of machine i.e. not portable.

• one assembly language instruction = one machine code instruction

• use mnemonics

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High Level Languages

• are not machine orientated i.e. they are portable.

• have statements that are close to English.

• one statement = many machine code instructions

• are problem orientated – different hlls have structures and facilities appropriate for a particular type of problem.

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Match the descriptions to the correct type of high level language• a sequence of instructions which are

executed in a programmer-defined order

• a set of facts & rules which are used to answer questions

• includes real-time facilities, interaction with hardware interfaces and supports concurrent programming

• execution of code depends upon an event e.g. user clicking a mouse button

• Event-driven• Imperative• Declarative• Languages

for real-time embedded systems

• Procedural

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Event-driven

• Programs written in an imperative language start at the beginning and run until they finish in a programmer-defined order.

• Programs written for a Windows-like environment need to be event-driven – nothing happens until the user does something.

• Object-Orientated Programming (OOP) languages were developed for writing event-driven programs as traditional hlls were not suitable for this.

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Classes

• Objects and classes are the two key ideas• A class is a collection of data structures and the

methods that can be used with these data structures

• Methods are sometimes also called activities/procedures/code/functions/behaviours

• Sometimes instead of data structures the terms properties/characteristics/record/attributes might be used.

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Classes Example 1

• For example the mammal class might have attributes height, weight, and age; and have methods eat, drink, breathe and sleep (a slightly simplified example).

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Classes Example 2

• The camera class might have attributes shutter speed, aperture, flash setting; and have methods take picture, turn flash on, turn flash off, adjust aperture, adjust shutter speed.

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Objects

• An object is an instance of a class

• An object is a collection of data structures and methods

• Rover the Dog is an object of class mammal

• The Canon S50 is an object of class Camera

• Mr T is an object of class mammal

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Objects continued…

• A dialogue box class would have attributes including position and size.

• Methods would include OnOK, OnCancel, OnClose – describing what to do when the user clicks on the OK or Cancel buttons, or on the X.

This is an instance of the dialogue box class i.e. an

object

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Objects v Classes

• A class can be thought of as a blueprint for an object.

• This blueprint can then be used to create one object or thousands of objects.

• A class does not represent an object; it represents all the information that a typical object should have as well as the methods that it should have.

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Object-orientated programming

• To write a program using an OOP you first define the different classes.

• Next you create objects (called instantiation – similar to declaring variables).

• That’s it.• When the program is run events will cause

objects to interact with each other via messages and cause things to happen.

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Encapsulation

• is defined as the joining together of methods and data into objects.

• all objects have private and public areas.• the data structures are put in the private

area – this means that they can only be used within this object (like a local variable) i.e. by the object’s methods.

• the methods are put in the public area so that they can be called by other objects.

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Defining a class• We are now going to

define the class for the camera class.

• The first thing you have to do is to say that you are defining a class and to give it a name.

• You then need to state what methods

• and attributes there are.

camera = classprivate:

shutterspeed : real;aperture : real;flashsetting : integer;

public:takepicture;turnflashon;turnflashoff;adjustaperture;adjustshutterspeed;

end;

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Instantiation

Now that you have defined a class you can instantiate objects. This is very similar to declaring variables in a high level programming language.

• canonS50 : camera;

• mycamera : camera;

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Exercise 11) Define the mammal class,

as described in the picture on the right.

2) Declare the following objects: Lassie, Cletus, Skippy the Bush Kangaroo.

3) Define a class TV – TVs can be turned on/off, change channel and adjust volume; they have attributes on?, volumelevel and channel.

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Exercise 1 Question 1 answer

mammal = class

private:

age : integer;

height : integer;

weight : integer;

public:

sleep;

eat;

breathe;

drink;

end

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Exercise 1 Question 2 answer

Lassie : mammal;

Cletus : mammal;

Skippy the Bush Kangaroo : mammal;

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Exercise 1 Question 3 answer

TV = classprivate:

on? : boolean;volume : integer;channel : integer;

public:turn on/off;changeChannel;adjustVolume;

end

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Inheritance 1

• Object-orientated programming allows for classes to be defined in terms of other classes.

• For example bears, cats, mice and humans are all subclasses of the mammal class.

• The mammal class is called the Base Class or SuperClass.

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Inheritance 2

• Each subclass inherits state and methods from the Base Class.

• Humans, mice, cats and bears all inherit some attributes from the mammal class: age, gender, height in cm, weight in kg.

• They also inherit some behaviours from the mammal class: eat, drink, sleep and breathe.

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Inheritance 3

• Subclasses can also have their own attributes and methods that they have not inherited.

• For instance bears can also do stuff in woods and break into cars. All bears have a name.

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Exercise 2

1) Write a class definition of the bear class, that inherits from the mammal class (defined in Exercise 1). Hint: You do not need to restate everything that is inherited from the mammal class.

2) Instantiate the following objects of class bear: Rupert, Winnie, Paddington.

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Exercise 2 answers

bear = class derived from mammalprivate:

name : string;public:

do_stuff_in_woods;break_into_cars;

end

winnie : bear;rupert : bear;paddington: bear;

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Inheritance Diagrams

Inheritance can also be represented diagrammatically.

Bear Cat

Mammal Notice that the arrows point upwards

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Exercise 3

1) Draw an inheritance diagram for bikes, tandems, mountain bikes and racing bikes all inherit from the bike super class.

2) Draw an inheritance diagram for reptiles: lizards and snakes inherit from reptile; pythons and cobras inherit from snake; geckos and chameleons from lizard.

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Exercise 3 Question 1 answer

Bike

Tandem Mountain Bike

Racing Bike

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Exercise 3 Question 2 answer

Reptile

Snake

Cobra Python

Lizard

Gecko Chameleon

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Polymorphism

• Two classes are said to be polymorphic if they have some of the same data structures or behaviours.

• There are two types of polymorphism – independent and inherited.

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Inherited polymorphism

• Here are the bear and mammal classes from earlier.

• If we also have a human class, that is derived from the mammal class, then the human and bear classes will have inherited polymorphism on the eat, breathe, sleep and drink methods; the age, weight, height and gender attributes.

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Independent polymorphism

• Bear and human classes (as shown on the right) also have independent polymorphism on the name attribute.

• This is when two classes have some of the same behaviours or data structures but these have not been inherited.

human = class derived from mammal

private:

name : string;

public:

talk;

end;

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Independent polymorphism 2

Tigers and Fish might both have a swim method. This could not be inherited from an animal base class as they both have to swim in very different ways. Therefore, this is another example of independent polymorphism.

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Containment

• Containment is the idea that an object can contain other objects.

• A house object could have two door objects and six window objects.

• A dialogue box object could have an OK button object and a cancel button object.

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Messages

• In an object-orientated program objects are instantiated and then events cause them to communicate with each other by sending messages.

• To look at how messages work we will be using two classes: vet and cat.

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Vet class

The vet class inherits from the human class. Vets also have the methods

• drive car• send dog to ‘sleep’

(the best thing for all dogs)

• castrate cat

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Cat class

• The cat class is derived from the mammal class.

• Cats also have the methods: purr, hack up fur ball and receive surgery.

• Cats have the attributes: name, fur colour and number of testicles.

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Example continued

• We are going to send a message between Tiddles the Cat and John the Vet.

• Tiddles has been a naughty boy and his owner would like to change the value of his number of testicles attribute to 0.

• You cannot just say: tiddles.NumberOfTesticles = 0 as the data is kept in the private area of an object.

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Example continued 2

• The owner has to take Tiddles to the vet.

• The castrate cat method of John the Vet will then send a message to the receive surgery method of Tiddles the Cat.

• The receive surgery method will then change the number of testicles attribute of Tiddles the Cat from two to zero.