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G54PRG ProgrammingLecture

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Amadeo Ascó Adam Moore

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Object Oriented Theory II

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Previously

• Programming Methodologies– Unstructured Programming– Procedural Programming– Modular Programming

• Properties of Modular Programming

– Object Oriented Programming• Object Oriented Languages• Objects

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Overview

• Properties of OO Programming• Encapsulation• Classes• The Taxonomy of Insects• Hierarchical Taxonomy• Objects – Data Structures that Inherit• Multiple Inheritance• Polymorphism• Dynamic Method Binding

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Properties of OO Programming

• Encapsulation– Combining data with the code that acts upon that data to form a new

data-type - an object.

• Inheritance– Arranging objects into a hierarchy of descendant objects, with each

descendant inheriting access to all of its ancestors code and data.

• Polymorphism– A single action may be used in different ways in different contexts –

the implementation of that action being appropriate to the current usage.

• Dynamic method binding

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Encapsulation

• Objects model the real world - they are the ultimate form of data abstraction.

• Encapsulation means keeping all of the constituents of an object in the same place.

• Consider an orange:– Mathematical view - abstracted into separate components (area of

skin, weight, fluid volume, number of seeds etc).– Painters view - encapsulated on canvas an abstract whole.

• Encapsulation ensures that the relationships between the components of an object are preserved.

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Classes

• In most OO languages encapsulation is implemented by the class.

• Java, C++, Object Pascal, and many other programming languages implement OO in this way.

• Classes are user-defined data types that encapsulate code (methods) together with data (variables).

• Each object is a separate instance of a class, and therefore has its own state.

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The Taxonomy of Insects

F lie s

B e es W a sps A n ts

S o c ia l In se c ts B u tte rf lies B e e tles

W in ge d Inse c ts W in g le ss Inse c ts

In se c ts

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Hierarchical Taxonomy

• Consider:– How similar is an item to the others of its general class?– In what ways does it differ from them?

• Each category has a set of behaviours and characteristics that define it.

• The highest levels are the most general (i.e. the most simple)- lower levels become more specific.

• Once a characteristic is defined all categories below that in the hierarchy inherit that characteristic

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Objects – Data Structures that Inherit (1)

• Consider a program that handles graphics.• We might define a series of classes to draw

shapes on the screen.• The top level class is Location• This represents a position on screen

Location X co-ordinate (integer) Y co-ordinate (integer)

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Objects – Data Structures that Inherit (2)

• If we want to display a pixel we can use a Subclass, Point

Point (subclass of Location) ( inherited -X co-ordinate ) ( inherited -Y co-ordinate ) visible (boolean)

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Objects – Data Structures that Inherit (3)

• Instances of class point (objects)

Point 1

Point 3

Point 2

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Objects – Data Structures that Inherit (4)

• Classes contain data (X co-ordinate, Y co-ordinate and visible), encapsulated with code that operates on that data.

• A method called drawPoint

Point (subclass of Location) ( inherited -X co-ordinate ) ( inherited -Y co-ordinate ) visible (boolean) drawPoint (method)

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Objects – Data Structures that Inherit (5)

• Methods and variables may be public (i.e. invoked from anywhere), or private (i.e. only invoked from other methods in the class)

• A class may have a constructor (a method that is automatically invoked when an instance of the class is created).

• A class may have a destructor (a method that is automatically invoked when an object is destroyed).

• N.B. Java does not use destructors!

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Objects – Data Structures that Inherit (6)

Point (subclass of Location) public ( inherited -X co-ordinate ) public ( inherited -Y co-ordinate ) public visible (boolean) private drawPoint (method) private deletePoint (method) public Point (constructor) calls drawPoint public togglePoint calls drawPoint or deletePoint

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Objects – Data Structures that Inherit (7)

• Point may be subclassed as Circle or Square

Circle (subclass of Point) ( inherited -X co-ordinate ) ( inherited -Y co-ordinate ) ( inherited -visible ) radius -integer

Square (subclass of Point) ( inherited -X co-ordinate ) ( inherited -Y co-ordinate ) ( inherited -visible )

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Objects – Data Structures that Inherit (8)

Circle (subclass of Point) ( inherited -X co-ordinate ) ( inherited -Y co-ordinate ) ( inherited -visible ) radius integer Circle (constructor) togglePoint (inherited but overridden)

Square (subclass of Point) ( inherited -X co-ordinate ) ( inherited -Y co-ordinate ) ( inherited -visible ) length of side integer Square (constructor) togglePoint (inherited but overridden)

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Multiple Inheritance

• N.B. This is not implemented in Java!• It is implemented in C++• A class may have more than one parent

String

Drawable String

Point

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Polymorphism

• Although methods are inherited, their behaviour sometimes needs to be modified at different points in the hierarchy.

• The behaviour must be appropriate for the context of use.

• For example - the X,Y coordinates of location could be absolute pixel values or percentages of the screen. A polymorphic method would implement the appropriate functionality.

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Dynamic Method Binding

• Where several possible methods are available (e.g. polymorphic methods) the appropriate method does not need to be indicated to the compiler.

• The decision as to which method to use is made at runtime.

• In Java, this means that the VM selects the correct method to use at runtime.