Editing Double dispatch (section)

===Ruby===
A common use case is displaying an object on a display port which can be a screen or a printer, or something else entirely that doesn't exist yet. This is a naive implementation of how to deal with those different media.

<syntaxhighlight lang="ruby">
class Rectangle
  def display_on(port)
    # selects the right code based on the object class
    case port
      when DisplayPort
        # code for displaying on DisplayPort
      when PrinterPort
        # code for displaying on PrinterPort
      when RemotePort
        # code for displaying on RemotePort
    end
  end
end
</syntaxhighlight>

The same would need to be written for Oval, Triangle and any other object that wants to display itself on a medium, and all would need to be rewritten if a new type of port were to be created. The problem is that more than one degree of polymorphism exist: one for dispatching the display_on method to an object and another for selecting the right code (or method) for displaying.

A much cleaner and more maintainable solution is then to do a second dispatch, this time for selecting the right method for displaying the object on the medium:

<syntaxhighlight lang="ruby">
class Rectangle
  def display_on(port)
    # second dispatch
    port.display_rectangle(self)
  end
end

class Oval
  def display_on(port)
    # second dispatch
    port.display_oval(self)
  end
end

class DisplayPort
  def display_rectangle(object)
    # code for displaying a rectangle on a DisplayPort
  end
  def display_oval(object)
    # code for displaying an oval on a DisplayPort
  end
  # ...
end

class PrinterPort
  def display_rectangle(object)
    # code for displaying a rectangle on a PrinterPort
  end
  def display_oval(object)
    # code for displaying an oval on a PrinterPort
  end
  # ...
end

</syntaxhighlight>