Ruby(2008-02-17)

###############################   Class   ##############################
class Song
 def initialize(name, artist, duration)
  @name = name
  @artist = artist
  @duration = duration
 end
end
#initialize is a special method in Ruby programs.When you call Song.new to create
#a new Song object, Ruby allocates some memory to hold an uninitialized object and
#then calls that object’s initialize method, passing in any parameters that were passed to new.
#In Ruby, an instance variable is simply a name preceded by an “at” sign (@).

song = Song.new("Bicylops", "Fleck", 260)
puts song.inspect

song1 = Song.new "Bicylops", "Fleck", 260
puts song1.to_s

#In Ruby, classes are never closed: you can always add methods to an existing class.
#We said that Ruby supports to_s for all objects
class Song
 def to_s
  "Song: #@name--#@artist (#@duration)"
 end
end

song = Song.new("Bicylops", "Fleck", 260)
puts song.to_s

##################################   Inheritance and Messages   #######################
class KaraokeSong < Song
 def initialize(name, artist, duration, lyrics)
  super(name, artist, duration)
  @lyrics = lyrics
 end
end

song = KaraokeSong.new("my Way", "Sinatra", 225, "And now, the...")
puts song.to_s

#when Ruby comes across the method invocation song.to_s, it doesn’t actually know
#where to find the method to_s. Instead, it defers the decision until the program is run.
#At that time, it looks at the class of song. If that class implements a method with the
#same name as the message, that method is run. Otherwise, Ruby looks for a method in
#the parent class, and then in the grandparent, and so on up the ancestor chain. If it runs
#out of ancestors without finding the appropriate method, it takes a special action that
#normally results in an error being raised.

class KaraokeSong < Song
 def to_s
  super + " [#@lyrics]"
 end
end

song = KaraokeSong.new("my Way", "Sinatra", 225, "And now, the...")
puts song.to_s

#################################   Objects and Attributes   ####################
#Ruby offers an interesting and powerful compromise, giving you
#the simplicity of single inheritance and the power of multiple inheritance.
#A Ruby class has only one direct parent, so Ruby is a singleinheritance
#language. However, Ruby classes can include the functionality
#of any number of mixins (a mixin is like a partial class definition).
#This provides a controlled multiple-inheritance-like capability
#with none of the drawbacks.

class Song
 def name
  @name
 end
 def artist
  @artist
 end
 def duration
  @duration
 end
end

song = Song.new("Bicylops", "Fleck", 260)
puts song.artist
puts song.name
puts song.duration

#Because this is such a common idiom, Ruby provides a convenient shortcut:
#attr_reader creates these accessor methods for you.
#In this example, we named the accessor methods name, artist, and
#duration. The corresponding instance variables, @name, @artist, and @duration,
#will be created automatically.
class Song
 attr_reader :name, :artist, :duration
end

song = Song.new("Bicylops", "Fleck", 260)
puts song.artist
puts song.name
puts song.duration

###################################   Writable Attributes   ############################
class Song
 def duration=(new_duration)
  @duration = new_duration
 end
end
song = Song.new("Bicylops", "Fleck", 260)
puts song.duration
song.duration = 257
puts song.duration

#Ruby provides a shortcut for creating these simple attribute-setting methods.
class Song
 attr_writer :duration
end

song = Song.new("Bicylops", "Fleck", 260)
song.duration = 257

############################   Virtual Attributes   ###########################
class Song
 def duration_in_minutes
  @duration / 60.0
 end
 def duration_in_minutes=(new_duration)
  @duration = (new_duration * 60)
 end
end
song = Song.new("Bicylops", "Fleck", 260)
puts song.duration_in_minutes
song.duration_in_minutes = 4.2
puts song.duration

#############################   Class Variables   #############################
#A class variable is shared among all objects of a class.
#class variables must be initialized before they are used.

class Song
 @@plays = 0
 
 def initialize(name, artist, duration)
  @name = name
  @artist =  artist
  @duration = duration
  @plays = 0
 end
 
 def play
  @plays += 1
  @@plays += 1
 end
end

s1 = Song.new("Song1", "Artist1", 234)
s2 = Song.new("Song2", "Artist2", 345)
s1.play
s2.play
s1.play
s2.play

#Class variables are private to a class and its instances. If you want to make them accessible
#to the outside world, you’ll need to write an accessor method. This method could
#be either an instance method

#########################################   Class Methods   #####################
class Example
 #instance method
 def instance_method
 end
 
 #class method
 def Example.class_method
 end
end

class SongList
 MAX_TIME = 5 * 60
 
 def SongList.is_too_long(song)
  return song.duration > MAX_TIME
 end
end

song1 = Song.new("Bicylops", "Fleck", 260)
SongList.is_too_long(song1)
song2 = Song.new("The Calling", "Santana", 468)
SongList.is_too_long(song2)