Object Inheritence in Ruby (versus JavaScript)

Prerequisites

• ga-wdi-boston/ruby-vs-js-objects

Objectives

By the end of this, students should be able to:

• Diagram the Ruby method lookup chain, and compare it to the JavaScript prototype lookup chain.
• Compare Ruby inheritance with Ruby composition.
• Give two reasons why inheritance and composition are desirable.
• Write a class which inherits from another class.
• Describe why composition is preferred to inheritance.

Preparation

1. Fork and clone this repository.
2. Install dependencies with bundle install.

JavaScript Inheritance

Remember how JavaScript method lookup works? We use prototypes. Here's an example:

User.prototype.totalDistance = function() {
  let result = 0;

  for (let i = 0; i < this.runs.length; i++) {
    result += this.runs[i].distance;
  }

  return result;
}

We defined methods by attaching them to the prototype of the User object. New instances of User will "inherit" their methods from the User prototype. In this example, all instances of User share the common property totalDistance.

Ruby Inheriterance

In Ruby, we have classes instead of prototypes. We attach methods to the class since Ruby doesn't use prototypes. Ruby will be our first example of linking different objects together in a hierarchy. This process is called "inheritance" and the result is what we mean when we say an object "inherits" behavior from another object.

Some objects can be classified in multiple ways. These multiple classifications often make sense as a hierarchy. For example, a Dog is a kind of Pet. It's also a kind of Animal. In Ruby, we can share code (data or behavior) between two classes using inheritance. Let's look at an example of inheritance. Note that a Ruby class can only inherit from one other class, so whether you name that class Pet or Animal will depend on your application.

class Animal
  def eat
    puts "Nom nom nom"
  end
end

class Dog < Animal
end

dog = Dog.new
dog.eat #=> "Nom nom nom"

class Dog < Animal
  def speak
    puts "WOOF"
  end

  def eat
    puts "Slop slop slop"
  end
end

dog.speak #=> "WOOF"
dog.eat #=> "Slop slop slop"

animal = Animal.new
animal.eat #=> "Nom nom nom"
animal.speak #=> NoMethodError

Demo: Drawing the Method Lookup Chain in Ruby and JavaScript

In Ruby, method lookup occurs through classes. In JavaScript, method lookup occurs through inspecting the .prototype property on constructor functions.

Let's draw the method lookup chain, first through prototypes in JavaScript, and then through classes in Ruby.

Lab: Drawing the Method Lookup Chain in Ruby

Please diagram the method lookup chain using the following requirements:

• The class DenverBroncos has an instance method called lose.
• DenverBroncos inherits from the class FootballTeam.
• The class FootballTeam has an instance method called play_game.
• Diagram creating a new instance of the DenverBroncos: broncos_2015 = DenverBroncos.new.
• Diagram how Ruby finds and executes the methods called on broncos_2015: broncos_2015.lose and broncos_2015.play_game.

There is a Better Way (Composition)

Classical inheriterance has its limitations and we don't want to limit ourselves, that's where composition comes in. Let's discuss the code below:

class Person
  def initialize
    @car = Car.new
  end
end

class Car
  def accelerate
    # code here
  end
end

Whenever I create a new instance of a person, on initialization an instance of a car is created as well. This instance of car has all of the methods defined in the Car class. It inherits all of methods defined in the Car class.

An Even Better Way (Mixins)

We've dealt with mixin before in our bootstrap-sass training ga-wdi-boston/html-css-layout-bootstrap-sass and the concept is similar. We want to make chunks of code that are resuable across of multiple classes. These "chunks" are called modules. Take a look at the code below:

module Sleeper
  def go_to_sleep
    # code here
  end
end

class Person
  include Sleeper
end

class Computer
  include Sleeper
end

In the code above we defined a module called Sleeper. We also define a Person class and a Computer class. By using the keyword include followed by the name of the module (in this case Sleeper) we have access to the methods we defined in our module. This is great because it allows us to keep our code D-R-Y, not to mention it allows us to be lazy developers (the good kind of lazy).

Lab: Model Shapes Using Classes

In a previous lab, you were asked to create and use a Shape class.

A Rectangle is a Shape, and a Square is a Rectangle.

Create a Rectangle that inherits from Shape. You will need to override the constructor method inside Rectangle to take two sides of different lengths. Since all rectangles have four sides, you can set a default value for @sides inside Rectangle's constructor.

Requirements for Rectangles:

• Rectangles should be instantiated with Rectangle.new(3, 4) to create a rectangle with a length of 3 and a width of 4.
• Instances of Rectangle should respond to the #area method and give the correct result.
• Do not override anything that doesn't need to be overriden.

Next, create a Square class that inherits from Rectangle.

Requirements for Squares:

• Squares should be instantiated with Square.new(4) to create a square with all sides equal to 4.
• Instances of Square should respond to the #area method and give the correct result.
• Do not override anything that doesn't need to be overriden.

Additional Resources

• Useful links to documentation
• Stack Overflow: Inheriterance

License

Source code distributed under the MIT license. Text and other assets copyright General Assembly, Inc., all rights reserved.