From: Emiel van de Laar <emiel@...>
Date: 2004-07-27T20:22:04+09:00
Subject: Inheritance vs Encapsulation

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Greetings,

Lately I've been pondering about the trade offs one must make when
implementing classes which maintain internal collections. My question
is, when is it wise to inherit and when to encapsulate. I've attached
some sample Ruby code to demonstrate my thinking. My current issues
are with C# but I believe it to be a general OO issue.

Inheriting from collection classes, i.e. Array, Hash, is simple, fast
and takes relatively little code. However every method available is
exposed to the implementing class. Something we don't always want.

Encapsulation(by association) prevents method pollution by exposing
only what is needed, which gives us more control. For code completion
this is nice because we aren't confronted with an excessive amount of
methods; we only see what is needed. 

Another benefit of encapsulation is that you can replace your internal
collection at a later stadium with an alternative implementation,
perhaps for performance reasons. As long as the interface to the
implementing class remains the same you are free to do as you
please internally.

At times though, it can be a pain exposing all the methods we need;
sometimes we need them all. I know Ruby offers us the "method_missing"
feature which I have used a lot, however, as far as I know C# doesn't
offer me such a luxury.

Thanks in advance.

Cheers,

Emiel
-- 
Emiel van de Laar
PGP pubkey: %finger emiel@il.fontys.nl

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class Duck
	attr_accessor :name

	def initialize name
		@name = name
	end
end

#
# Inheritance
#
class SomeDucks < Array 
end

#
# Encapsulation
#
class MoreDucks 
	def initialize
		@ducks = []
	end

	def << duck
		@ducks << duck
	end

	def shift
		@ducks.shift
	end

	def each
		@ducks.each { |duck| yield duck }
	end
end

class Zoo
	def initialize
		@some_ducks = SomeDucks.new
		@more_ducks = MoreDucks.new

		%w{daffy donald hewey}.each do |name|
			@some_ducks << Duck.new(name)
			@more_ducks << Duck.new(name)
		end
	end

	def display_ducks
		@some_ducks.each { |duck| puts duck.name }
		puts "--"
		@more_ducks.each { |duck| puts duck.name }
	end
end

if __FILE__ == $0
	Zoo.new.display_ducks
end


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