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Ruby Programming
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Class Methods

Sometimes a class needs to provide methods that work without being tied to any particular object.

We've already come across one such method. The new method creates a new Song object but is not itself associated with a particular song.

aSong = Song.new(....)

You'll find class methods sprinkled throughout the Ruby libraries. For example, objects of class File represent open files in the underlying file system. However, class File also provides several class methods for manipulating files that aren't open and therefore don't have a File object. If you want to delete a file, you call the class method File.delete , passing in the name.

File.delete("doomedFile")

Class methods are distinguished from instance methods by their definition. Class methods are defined by placing the class name and a period in front of the method name.

class Example

  def instMeth              # instance method   end

  def Example.classMeth     # class method   end

end

Jukeboxes charge money for each song played, not by the minute. That makes short songs more profitable than long ones. We may want to prevent songs that take too long from being available on the SongList. We could define a class method in SongList that checked to see if a particular song exceeded the limit. We'll set this limit using a class constant, which is simply a constant (remember constants? they start with an uppercase letter) that is initialized in the class body.

class SongList
  MaxTime = 5*60           #  5 minutes
  def SongList.isTooLong(aSong)
    return aSong.duration > MaxTime
  end
end
song1 = Song.new("Bicylops", "Fleck", 260)
SongList.isTooLong(song1) false
song2 = Song.new("The Calling", "Santana", 468)
SongList.isTooLong(song2) true

Ruby Programming
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