2. Classes and Objects

Learning Objectives

• be familiar with how to create and use classes
• understand the difference between classes and objects
• see the relationship between classes and OOP

Classes

Classes can be seen as blueprints for creating objects which are different instances of the same class. Classes allow us to define the common characteristics and behaviours of a group of related objects.

Take for instance, the wrapper classes like String in Java or std::string in C++, we have the stored data (the text string) and a bunch of methods which act on that data (e.g. length() returns the length of that string data). This is an abstraction and encapsulation like we've seen before, except that we can have different instances instead of being confined to a Singleton.

For instance:

-- CarClass.lua
local Car = {}
Car.__index = Car

function Car.new(color, speed)
    local self = setmetatable({}, Car)
    self.color = color
    self.speed = speed
    return self
end

function Car:Drive()
    print("The " .. self.color .. " car is driving at " .. self.speed .. " mph.")
end

return Car

Here, Car.new is the constructor that initializes a new object of class Car and returns it, and Car:Drive is a method available to all instances of the Car class.

Objects

Objects are the concrete realization of a class. While a class defines the general structure and behaviour, an object represents a specific instance created from that class, with its own unique data.

For instance, we can create Car objects like this:

local Car = require(game.ServerScriptService.CarClass)

local redCar = Car.new("red", 60)
local blueCar = Car.new("blue", 80)

redCar:Drive() -- Outputs: "The red car is driving at 60 mph."
blueCar:Drive() -- Outputs: "The blue car is driving at 80 mph."

Object-Oriented Programming

Object-oriented programming (OOP) is the programming paradigm based upon the concepts of objects in order to model our programming problems.

One way to look at OOP is as a mirror of our natural world, to break everything down into objects which have certain properties, exhibit certain behaviours, and allow certain actions.