Difference between a class and a module

Question

I came from Java  and now I am working more with Ruby    One language feature I am not familiar with is the module  I am wondering what exactly is a module and when do you use one  and why use a module over a class

User · Answer

Basically  the module cannot be instantiated  When a class includes a module  a proxy superclass is generated that provides access to all the module methods as well as the class methods   A module can be included by multiple classes  Modules cannot be inherited  but this  mixin  model provides a useful type of  multiple inheritrance   OO purists will disagree with that statement  but don t let purity get in the way of getting the job done      This answer originally linked to http   www rubycentral com pickaxe classes html  but that link and its domain are no longer active

User · Answer

Class   When you define a class  you define a blueprint for a data type    class hold data  have method that interact with that data and are used to instantiate objects   Module   Modules are a way of grouping together methods  classes  and constants  Modules give you two major benefits      Modules provide a namespace and prevent name clashes  Namespace help avoid conflicts with functions and classes with the same name that have been written by someone else      Modules implement the mixin facility        including Module in Klazz gives instances of Klazz access to Module   methods          extend Klazz with Mod giving the class Klazz access to Mods methods

User · Answer

I m surprised anyone hasn t said this yet   Since the asker came from a Java background  and so did I   here s an analogy that helps   Classes are simply like Java classes   Modules are like Java static classes  Think about Math class in Java  You don t instantiate it  and you reuse the methods in the static class  eg  Math random

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-----------------------------------------------------------------------------                      class                        module                               --------------- --------------------------- ---------------------------------      instantiation    can be instantiated          can  not  be instantiated            --------------- --------------------------- ---------------------------------      usage            object creation              mixin facility  provide                                                               a namespace                        --------------- --------------------------- ---------------------------------      superclass       module                       object                               --------------- --------------------------- ---------------------------------      methods          class methods and            module methods and                                       instance methods             instance methods                   --------------- --------------------------- ---------------------------------      inheritance      inherits behaviour and can   No inheritance                                           be base for inheritance                                         --------------- --------------------------- ---------------------------------      inclusion        cannot be included           can be included in classes and                                                        modules by using the include                                                        command  includes all                                                               instance methods as instance                                                        methods in a class module          --------------- --------------------------- ---------------------------------      extension        can not extend with          module can extend instance by                            extend command               using extend command  extends                           only with inheritance       given instance with singleton                                                       methods from module               -----------------------------------------------------------------------------

User · Answer

The first answer is good and gives some structural answers  but another approach is to think about what you re doing  Modules are about providing methods that you can use across multiple classes - think about them as  libraries   as you would see in a Rails app   Classes are about objects  modules are about functions   For example  authentication and authorization systems are good examples of modules  Authentication systems work across multiple app-level classes  users are authenticated  sessions manage authentication  lots of other classes will act differently based on the auth state   so authentication systems act as shared APIs    You might also use a module when you have shared methods across multiple apps  again  the library model is good here

User · Answer

Bottom line  A module is a cross between a static utility class and a mixin   Mixins are reusable pieces of  partial  implementation  that can be combined  or composed  in a mix  amp  match fashion  to help write new classes   These classes can additionally have their own state and or code  of course

User · Answer

First  some similarities that have not been mentioned yet  Ruby supports open classes  but modules as open too  After all  Class inherits from Module in the Class inheritance chain and so Class and Module do have some similar behavior   But you need to ask yourself what is the purpose of having both a Class and a Module in a programming language  A class is intended to be a blueprint for creating instances  and each instance is a realized variation of the blueprint  An instance is just a realized variation of a blueprint  the Class   Naturally then  Classes function as object creation  Furthermore  since we sometimes want one blueprint to derive from another blueprint  Classes are designed to support inheritance     Modules cannot be instantiated  do not create objects  and do not support inheritance  So remember one module does NOT inherit from another   So then what is the point of having Modules in a language  One obvious usage of Modules is to create a namespace  and you will notice this with other languages too  Again  what s cool about Ruby is that Modules can be reopened  just as Classes   And this is a big usage when you want to reuse a namespace in different Ruby files   module Apple   def a     puts  a    end end  module Apple    def b     puts  b    end end  class Fruit   include Apple end    gt  f   Fruit new    gt    lt Fruit 0x007fe90c527c98 gt     gt  f a    gt  a   gt  f b    gt  b   But there is no inheritance between modules   module Apple   module Green     def green       puts  green      end   end end  class Fruit   include Apple end   gt  f   Fruit new    gt    lt Fruit 0x007fe90c462420 gt    gt  f green NoMethodError  undefined method  green  for   lt Fruit 0x007fe90c462420 gt    The Apple module did not inherit any methods from the Green module and when we included Apple in the Fruit class  the methods of the Apple module are added to the ancestor chain of Apple instances  but not methods of the Green module  even though the Green module was defined in the Apple module   So how do we gain access to the green method  You have to explicitly include it in your class   class Fruit   include Apple  Green end    gt  Fruit    gt  f green   gt  green   But Ruby has another important usage for Modules  This is the Mixin facility  which I describe in another answer on SO  But to summarize  mixins allow you to define methods into the inheritance chain of objects  Through mixins  you can add methods to the inheritance chain of object instances  include  or the singleton class of self  extend

User · Answer

namespace  modules are namespaces   which don t exist in java     I also switched from Java and python to Ruby  I remember had exactly this same question     So the simplest answer is that module is a namespace  which doesn t exist in Java  In java the closest mindset to namespace is a package    So a module in ruby is like what in java  class  No interface  No abstract class  No package  Yes  maybe     static methods inside classes in java  same as methods inside modules in ruby  In java the minimum unit is a class  you can t have a function outside of a class  However in ruby this is possible  like python     So what goes into a module  classes  methods  constants  Module protects them under that namespace    No instance  modules can t be used to create instances   Mixed ins  sometimes inheritance models are not good for classes  but in terms of functionality want to group a set of classes  methods  constants together  Rules about modules in ruby  - Module names are UpperCamelCase - constants within modules are ALL CAPS  this rule is the same for all ruby constants  not specific to modules  - access methods  use   operator - access constants  use    symbol     simple example of a module    module MySampleModule   CONST1    some constant     def self method one arg1      arg1   2   end end   how to use methods inside a module    puts MySampleModule method one 1    prints  3   how to use constants of a module    puts MySampleModule  CONST1   prints  some constant   Some other conventions about modules  Use one module in a file  like ruby classes  one class per ruby file

User · Answer

Module in Ruby  to a degree  corresponds to Java abstract class -- has instance methods  classes can inherit from it  via include  Ruby guys call it a  mixin    but has no instances  There are other minor differences  but this much information is enough to get you started

[ruby] Difference between a class and a module

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