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Ruby Programming
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Objects and Attributes

The Song objects we've created so far have an internal state (such as the song title and artist). That state is private to those objects---no other object can access an object's instance variables. In general, this is a Good Thing. It means that the object is solely responsible for maintaining its own consistency.

However, an object that is totally secretive is pretty useless---you can create it, but then you can't do anything with it. You'll normally define methods that let you access and manipulate the state of an object, allowing the outside world to interact with the object. These externally visible facets of an object are called its attributes.

For our Song objects, the first thing we may need is the ability to find out the title and artist (so we can display them while the song is playing) and the duration (so we can display some kind of progress bar).

class Song
  def name
    @name
  end
  def artist
    @artist
  end
  def duration
    @duration
  end
end
aSong = Song.new("Bicylops", "Fleck", 260)
aSong.artist "Fleck"
aSong.name "Bicylops"
aSong.duration 260

Here we've defined three accessor methods to return the values of the three instance attributes. Because this is such a common idiom, Ruby provides a convenient shortcut: attr_reader creates these accessor methods for you.

class Song
  attr_reader :name, :artist, :duration
end
aSong = Song.new("Bicylops", "Fleck", 260)
aSong.artist "Fleck"
aSong.name "Bicylops"
aSong.duration 260

This example has introduced something new. The construct :artist is an expression that returns a Symbol object corresponding to artist. You can think of :artist as meaning the name of the variable artist, while plain artist is the value of the variable. In this example, we named the accessor methods name, artist, and duration. The corresponding instance variables, @name, @artist, and @duration, will be created automatically. These accessor methods are identical to the ones we wrote by hand earlier.
Ruby Programming
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