Object Oriented Programming

Question 1

Object-Oriented Programming (OOP)

Answer 1

Paradigm where the program is composed of interacting objects, each responsible for its own data and operations

Question 2

Class

Answer 2

Template for an object that defines its attributes and methods

Question 3

Object

Answer 3

A specific instance of a class

Question 4

Attribute

Answer 4

Data associated with the class

Question 5

Method

Answer 5

A functionality of a class

Question 6

Object state

Answer 6

The actual data values of a particular object’s attributes

Question 7

Instantiation

Answer 7

Creation of a new object

Question 8

How to instantiate object

Answer 8

By calling class’s constructor method → Object returned can be assigned to identifier variable

Question 9

Types of methods

Answer 9

• Getter methods
• Setter methods

Question 10

Getter methods

Answer 10

Written as functions and return the object’s state

Question 11

Setter methods

Answer 11

Written as procedures and used to change object’s state

Question 12

Encapsulation

Answer 12

Wrapping all of an object’s related data and methods under one entity → Objects can’t affect the way other objects function

Question 13

Benefits of encapsulation

Answer 13

• Promotes modularity → Self-contained units of functionality that can be re-used
• Reduces complexity (easier to understand how object fits into system if it’s self-contained)
• Can change implementation of entity without impacting other parts of program
• Improve security (information hiding)
• Easier to maintain and test code (easier to interface with individual components)

Question 14

Inheritance

Answer 14

• Principle that allows us to create a new class that is a modified version of another class
• Subclass gains all the attributes and methods of the superclass
• Subclasses can be overidden and properties can be polymorphised
• New data and behaviour can also be added to subclasses

Question 15

When to used inheritance

Answer 15

• Is [subclass] a [superclass]?
• E.g. is a Fox an Animal?

Question 16

Benefits of inheritance

Answer 16

• Reuse of code and avoids duplication → Saves time and effort when writing & testing
• Hierarchy of classes → Better organisation of code
• Specialised classes designed for specific tasks that inherit from a general class (e.g. Fox class inherits from Animal)
• Polymorphism and overriding → ↑ Flexible and powerful code

Question 17

Overriding

Answer 17

When a subclass replaces and inherited method

Question 18

Polymorphism

Answer 18

When a subclass has a different implementation of an inherited method to the superclass

Question 19

Inheritance diagram

Answer 19

Child classes point to parent class with unfilled arrow

Question 20

Object association

Answer 20

When an object either contains or has reference to another related object (‘has a’ relationship

Question 21

Types of association

Answer 21

• One-to-one
• One-to-many
• Many-to-many
• Aggregation
• Composition

Question 22

Aggregation

Answer 22

• Weaker form of association
• When an object (whole) is composed of or contains multiple other objects (parts)
• Lifecycle of part not dependent on whole
• E.g. Team and Player have an aggregative association

Question 23

Composition

Answer 23

• Strong form of association (and aggregation)
• Lifcycle of part depends on whole
• E.g. Hotel and Room have a compositive association (rooms can’t exist if hotel is destroyed)

Question 24

Reasons to favour composition over inheritance

Answer 24

• ↑ Flexibility (stuck with inherited attributes & methods vs Picking and choosing required properties)
• ↑ Explicit & easier to understand (properties of composite class clearer)
• ↑ Robust (changing superclass may change unintentionally change subclass)
• ↑ Efficient (if subclass doesn’t use all inherited properties from superclass → Wasteful)

Question 25

Information hiding

Answer 25

• Practice of keeping internal details of class hidden from rest of code → Prevents exposing inner workings of class
• Implemented using access modifiers and encapsulation
• Object’s state manipulated using getter and setter methods

Question 26

Access modifiers

Answer 26

• Public (accessible by any object from any class in program)
• Protected (accessible by other objects of same class or subclasses)
• Private (only accessible to that object)

Question 27

What access modifiers to use?

Answer 27

• Attributes should be private (prevents non-interface access)
• Most methods should be public (allows access to interface)

Question 28

Programming to interface

Answer 28

• Designing code to depend on interfaces rather than concrete implementations → Multiple classes can have different implementations of same method
• Part of encapsulation

Question 29

Interface

Answer 29

A collection of abstract methods that a group of unrelated classes must implement (e.g. multiple appliances must be able to switch on and off, so this can be implemented as an interface)

Question 30

Coding interfaces

Answer 30

Public interface Switches
	Procedure SwitchOn
	Procedure SwitchOff
	Procedure SetTimer(time)
	Function GetTimer
	etc
End

Class Microwave implements Switches

Question 31

Interfaces in Python

Answer 31

Implemented as abstract classes

from abc import ABC, abstractmethod

class TwoDimensionalShape(ABC):
	@abstractmethod
	def get_area(self):
		pass

Question 32

Encapsulate what varies

Answer 32

• Encapsulating aspects of program that are likely to change into separate classes or modules → ↑ Flexible and maintainable code
• Separates stable parts of code from more volatile ones → Easier to understand and modify code without having to modify code that doesn’t need to change
• Can be implemented using interface

Question 33

Advantages of OOP

Answer 33

• • Methodology forces designers to go through an extensive planning phase → better designs with fewer weaknesses
• Encapsulation - source code for an object can be written, tested and maintained independently
• Once created, knowledge of an object’s specific implementation isn’t necessary for a programmer to use it
• New objects can be easily created with small differences from existing ones
• Reusability - objects that are already defined coded and tested may be used in many different programs → OOP provides good framework for code libraries
• Software maintenance - an object-oriented program is easier to maintain due to rigidly enforced modular structure

Question 34

UML (Unified Modelling Language) Diagrams

Answer 34

Models the classes in a system and the relationships between them

Question 35

Class Boxes

Answer 35

• Name of class
• Attributes : Type
• Methods (: Return type)

Question 36

Relationship lines

Answer 36

• Inheritance = Pointy arrow
• Aggregation = Clear diamond arrow
• Composition = Filled diamond arrow

Question 37

Visibility markers

Answer 37

• Public = +
• Private = -
• Protected = #

Question 38

Formal class definition

Answer 38

MediaFile = Class:
Public:
Procedure PlayFile
Function GetTitle : String
Function GetDuration : Integer
Private:
Title : String
Duration : Integer

MusicFile = Subclass(MediaFile):
Public:
Function GetArtist : String
Function GetSampleRate : Integer
Funcion GetBitDepth : Integer
Private:
Artist : String
SampleRate : Integer
BitDepth : Integer