Subclasses

Part of the Object Oriented Programming section of Coddy's Ruby journey. Lesson 22 of 57.

Inheritance builds a new class on an existing one. class Dog < Animal makes Dog a subclass of Animal, its superclass. A dog has every method of an animal, including initialize, plus the methods Dog adds:

class Animal
  attr_reader :name

  def initialize(name)
    @name = name
  end

  def eat
    "#{name} is eating"
  end
end

class Dog < Animal
  def fetch
    "#{name} fetches the ball"
  end
end

rex = Dog.new("Rex")
puts rex.eat
puts rex.fetch

Output:

Rex is eating
Rex fetches the ball

The added methods belong to the subclass only. A plain Animal does not have them, and a sibling class such as Cat does not see the methods of Dog:

class Animal
  def breathe
    "breathing"
  end
end

class Dog < Animal
  def fetch
    "fetching"
  end
end

class Cat < Animal
end

puts Cat.new.breathe
begin
  Cat.new.fetch
rescue NoMethodError => e
  puts e.message
end

Output:

breathing
undefined method `fetch' for an instance of Cat

superclass names a class's parent. A Ruby class has exactly one superclass, and a class declared without < inherits from Object, which gives every object methods such as to_s and inspect:

class Animal
end

class Dog < Animal
end

puts Dog.superclass
puts Animal.superclass
puts Object.superclass

Output:

Animal
Object
BasicObject

Inheritance fits an is-a relation: a dog is an animal, a savings account is an account. Shared behaviour goes into the superclass once, and each subclass adds what makes it different. In this section each class lives in its own file, and the superclass's file is loaded first:

# main.rb
require_relative 'animal'   # the superclass first
require_relative 'dog'      # class Dog < Animal needs Animal
require_relative 'cat'

Challenge

Easy

Complete the classes. Vehicle stores a name and a number of wheels and has describe, which returns Mini has 4 wheels. Car and Bike are subclasses of Vehicle that do not define initialize. Car adds honk, returning Mini says beep, and Bike adds ring, returning BMX rings.

The supplied code reads lines car Mini or bike BMX, creates a Car with 4 wheels or a Bike with 2, and prints describe, the sound (honk or ring) and the superclass of the object's class.

Your code goes in vehicle.rb, car.rb and bike.rb. main.rb holds the supplied input/output code and cannot be edited.

Try it yourself

require_relative 'vehicle'
require_relative 'car'
require_relative 'bike'

# Supplied input/output code: keep it as it is
input = []
while (line = gets)
  input << line.chomp
end
input.each do |line|
  kind, name = line.split(" ", 2)
  if kind == "car"
    v = Car.new(name, 4)
    sound = v.honk
  else
    v = Bike.new(name, 2)
    sound = v.ring
  end
  puts v.describe
  puts sound
  puts v.class.superclass
end