Ruby/Class/Class Variables

class Song
  include Comparable
  @@plays = 0
  attr_reader :name, :artist, :duration
  attr_writer :duration
  def initialize(name, artist, duration)
    @name     = name
    @artist   = artist
    @duration = duration
    @plays    = 0
  end
  def to_s
    "Song: #@name--#@artist (#@duration)"
  end
  def play
    @plays  += 1   
    @@plays += 1
    "This  song: #@plays plays. Total #@@plays plays."
  end
  def record
    "Recording..."
  end
  def inspect
    self.to_s
  end
  def <=>(other)
    self.duration <=> other.duration
  end
end

# create instance variables by prefixing a variable name with @. 
# create class variables by prefixing a variable"s name with @@. 
# A class variable is shared by all instances of a class
# Only one copy of a class variable exists for a given class. 
# You have to initialize (declare a value for) a class variable before you use it.
 
class Animal
  @@number_animals = 0
  def initialize(color)
    @color = color
    @@number_animals += 1
  end
  def get_color
    return @color
  end
  def get_number_animals
    return @@number_animals
  end
end
dog = Animal.new("brown")
cat = Animal.new("striped")
squirrel = Animal.new("gray")
puts "Number of animals is " + squirrel.get_number_animals.to_s

class Song
  include Comparable
  @@plays = 0
  attr_reader :name, :artist, :duration
  attr_writer :duration
  def initialize(name, artist, duration)
    @name     = name
    @artist   = artist
    @duration = duration
    @plays    = 0
  end
  def to_s
    "Song: #@name--#@artist (#@duration)"
  end
  def play
    @plays  += 1   # same as @plays = @plays + 1
    @@plays += 1
    "This  song: #@plays plays. Total #@@plays plays."
  end
end
s1 = Song.new("Song1", "Artist1", 234)  # test songs..
s2 = Song.new("Song2", "Artist2", 345)
s1.play
s2.play
s1.play
s1.play