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Ruby Inheritance

Inheritance lets one class take the methods of another class. The class that takes them can add its own methods and replace one it took.

Methods taken as they are

Dog adds no methods of its own. The new call and the greet call both use methods taken from Animal.

class Animal
  def initialize(name)
    @name = name
  end
  def greet
    puts "#{@name} says hello"
  end
end
class Dog < Animal
end
dog = Dog.new("Rex")
dog.greet

An override replaces a method

Dog writes greet again with its own words. The dog uses the new version, and the animal keeps the parent version.

class Animal
  def initialize(name)
    @name = name
  end
  def greet
    puts "#{@name} says hello"
  end
end
class Dog < Animal
  def greet
    puts "#{@name} says woof"
  end
end
animal = Animal.new("Milo")
dog = Dog.new("Rex")
animal.greet
dog.greet

super hands a value up

Dog initialize takes a name and a breed. super hands the name to the initialize in Animal, and Dog stores the breed.

class Animal
  def initialize(name)
    @name = name
  end
  def greet
    puts "#{@name} says hello"
  end
end
class Dog < Animal
  def initialize(name, breed)
    super(name)
    @breed = breed
  end
  def profile
    puts "#{@name} is a #{@breed}"
  end
end
dog = Dog.new("Rex", "Labrador")
dog.greet
dog.profile

Example

class Animal
  def initialize(name)
    @name = name
  end
  def greet
    puts "#{@name} says hello"
  end
end
class Dog < Animal
  def initialize(name, breed)
    super(name)
    @breed = breed
  end
  def greet
    puts "#{@name} says woof"
  end
  def profile
    puts "#{@name} is a #{@breed}"
  end
  def fetch
    puts "#{@name} fetches the ball"
  end
end
animal = Animal.new("Milo")
dog = Dog.new("Rex", "Labrador")
animal.greet
dog.greet
dog.profile
dog.fetch

Key points

  • The parent class gives its methods to the child class.
  • class Dog < Animal makes Dog a child of Animal.
  • A child object can call a parent method with no change.
  • A child can add methods that the parent does not have.
  • A method written again in the child replaces the parent version for child objects.
  • super in initialize hands its values to the initialize of the parent.

In this example, super hands the name to Animal initialize, and Dog stores its breed. Dog replaces greet with its own line. profile and fetch are extra methods that only Dog has. The animal object still uses the greet line from Animal.

Note

A child class names one parent only.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Programiz Ruby Compiler

Exercise

What does super(name) do inside the initialize of Dog?