# Re: Composition vs. inheritance

From:
ram@zedat.fu-berlin.de (Stefan Ram)
Newsgroups:
comp.lang.java.programmer
Date:
23 Apr 2008 16:33:58 GMT
Message-ID:
<rectangle-20080423183222@ram.dialup.fu-berlin.de>
Todd <todd.heidenthal@lmco.com> writes:

Obviously, I am missing something fundamental, so I apologize if I
seem to be going in circles (no pun intended).

When you answer to a post, you should only quote
the parts you directly refer to. Google Groups

The whole rectangle square discussion only stems
from insufficiant care to distinguish between
value and storage objects.

Let Q be the set of ?quarternary digits? {0,1,2,3},
its subset {0,1} is called B; B is ?the set of
binary digits?. The inclusion

B c Q (B is a subset of Q)

is valid.

A quartary storage q* can store a quartary digit. It
also might be used to store a binary digit. However, one
can not use any binary storage to store any quartenary digit.

So, for the set of binary storages B* and the set of
quartary storages Q*:

Q* c B* (Q* is a subset of B*)

In general, if every B value is a Q value, then every Q storage
is a B storage.

If one now does not care to distinguish between values and
memories, this would be worded as ?If every B is a Q, then
every Q is a B?, which is false.

The rectangle square problem only exists as long as one does
not make it clear whether one wants to model rectangle and
square /values/ or rectangle and square /storages/.

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