From: Roseanne Zhang <roseanne@...>
Date: 2007-07-09T02:32:13+09:00
Subject: Re: Polymorphic Code

To show polymorphism and duck-typing are 2 different animals :), I add 
more code and comment in the above example. Pay attention to the Radio.
#=================
class Animal
  attr_reader :name
  def initialize(name)
    @name= name
  end
  def says
    @name + " says some strange grunty sound"
  end
end

class Dog < Animal
  def says
    @name + " says Woof!"
  end
end

class Cat < Animal
  attr_reader :name
  def says
    @name + " says News"
  end
end

# Attention: Radio is not an Animal
class Radio
  attr_reader :name
  def initialize(name)
    @name= name
  end
  def says
    @name + " says News"
  end
end

animal = Dog.new("Fido")
puts animal.says
animal = Cat.new("Socks")
puts animal.says
animal = Animal.new("Suzi")
puts animal.says

# here animal is not an Animal any more!!!
# It will not compile in C++/Java
# It is fine, since ruby duck/dynamic typing
animal = Radio.new("BBC")
puts animal.says
#=================

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