4.1.2 Programming Paradigms: Object-Orientated Programming

What is an object?

Containers of data and containers

What is insantiation?

The process of creating an object from a class

What is a class?

Specification of what properties (data) and methods (instructions) objects of their type will have

What is a class definitition?

Expression of a class on paper

→ Lists a class’ name, properties and methods

What does it mean if a property is private?

Can only be accessed from within an object

What does it mean if a method is public?

Allows an interface for accessing and modifying a class’ private properties

What is encapsulation ?

Combining methods and procedures to form an object

→ Object said to encapsulate its contents

What does encapsulation allow for?

Large programs to be split acorss a team of developers, each of whom can develop a class independently

What is inheritance?

Mechanism that allows a new class to acquire the properties and methods of an existing class, while also having its own properties and methods

What is good about inheritance?

Promotes code reuse

What kind of relationship does inheritance create?

‘is a’

What is polymorphism?

The ability of different classes to be treated as instances of the same class through a common interface, allowing methods to be used interchangeably

What is an overridden method?

Has the same name as a method in an inherited class but a different implementation

What kind of relationship does association create?

‘has a’

→ e.g. car and driverrelationship

→ Objects where one object contains or is composed of another

What are the types of association?

• Aggregation

• Composition

What is aggregation?

• Weaker kind of association

• Object will still exist if its containing object is destroyed

• e.g. a car and its passengers

What is composition?

• Stronger kind of association

• If the containing object is destroyed, the associated object is also destroyed

• e.g. car and its wheels

What are the strengths of using OOP? (5)

• Clear program structure

• Easier developing and testing

• Division of large projects

• Reuse of code

• Space efficiency of code

What are the design principles of OOP? (3)

• Encapsulate what varies

• Favour composition over inheritance

• Program to interfaces, not implementation

What is meant by ‘encapsulate what varies’?

Any requirements which are likely to change in the future should be encapsulated in a class

→ Any changes can easiy be made as required

What is meant by ‘favour composition over inheritance’?

Composition should be used over inheritance where possible

→ Composition is seen as more flexible relationship

→ Inheritance should be used when composition is not appropriate

What is meant by ‘program to interfaces, not implementation’?

• Allows unrelated classes to make use of similar methods

• An interface is designed as a collection of abstract procedures that can be implemented by unrelated classes

• When a new object is created, it can implement an interface which it provides with the correct properties and methods

What is a class diagram?

Visual representation of the relationships that exist between classesW

How are classes represented in a class diagram?

Boxes

How are relationships represented in a class diagram?

Connectors

How is inheritance shown in a class diagram?

Unfilled arrows which point from an inherited class towars the class which inherits it

→ Should always point upwards

How is associatation should in class diagrams?

• Aggregation: unfilled diamond-headed arrow

• Composition: filled diamond-headed arrow

How can class boxes be split?

• Name

• Properties

• Methods

How are public properties and methods indicated?

+

How are private properties and methods indicated?

-

How are protected properties and methods indicated?

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