map.rb

class Cat
  def hello
    "nya"
  end
end
class Dog
  def hello
    "wan"
  end
end

class Cage
  def initialize(animal = nil)
    @animals = []
    @animals << animal if animal
  end
  def add(animal = nil)
    return self unless animal
    @animals << animal
    self
  end
  def show
    # @animals.each do |animal|
    #   p animal.hello
    # end
    # [cat, dog] #=> ["nya", "wan]
    #p @animals.map{ |animal| animal.hello }.to_a
    p @animals.map(&:hello).to_a
    self
  end
end

Cage.new.add(Cat.new).add(Dog.new).show