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?

Answer 1

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)

Answer 2

╔═══════════════╦═══════════════════════════╦═════════════════════════════════╗
║               ║ 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)          ║
╚═══════════════╩═══════════════════════════╩═════════════════════════════════╝

Answer 3

source (You can learn the characteristics of module from there)

A Module is a collection of methods and constants. The methods in a module may be instance methods or module methods. Instance methods appear as methods in a class when the module is included, module methods do not. Conversely, module methods may be called without creating an encapsulating object, while instance methods may not.

Answer 4

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()).

Answer 5

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

 > f = Fruit.new
 => #<Fruit:0x007fe90c527c98> 
 > f.a
 => a
 > f.b
 => b

But there is no inheritance between modules:

module Apple
  module Green
    def green
      puts 'green'
    end
  end
end

class Fruit
  include Apple
end

> f = Fruit.new
 => #<Fruit:0x007fe90c462420> 
> f.green
NoMethodError: undefined method `green' for #<Fruit:0x007fe90c462420>

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
 => Fruit 
 > f.green
=> 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).

Answer 6

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.