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From: John Volan <johnv@ac3i.dseg.ti.com>
Subject: Re: Mutually dependent private types
Date: 1998/05/29
Message-ID: <356F38EC.AEF725EC@ac3i.dseg.ti.com>#1/1
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 <356B6D65.BD34E3EF@ac3i.dseg.ti.com> <EtLsGH.5x@jvdsys.nextjk.stuyts.nl>
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Organization: Raytheon Systems Company, Advanced C3I Systems
Newsgroups: comp.lang.ada
Date: 1998-05-29T00:00:00+00:00
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Jerry van Dijk wrote:
> 
> So perhaps you just found a new class:
> 
>         Office_Employee_List
> 
> :-)

This was offered in jest, but let's consider it seriously:

One way to implement an cross-referencing association between two
classes like Employee and Office could be to keep both classes ignorant
of the association and just have a third class that is entirely
responsible for keeping track of all the cross-references (in some sort
of table structure).  Yes, that is a valid design alternative, but there
are tradeoffs, and it doesn't work for all applications.  In particular,
if Office and Employee have primitive/inheritable/overridable operations
that take parameters of each other's type, then there is no way that the
two classes can avoid knowing about the association.
 
Also, this kind of cross-referencing association isn't the only way two
classes might be forced into mutual dependency.  My Doctors and Patients
example in my FAQ didn't involve any cross-referencing between Doctor
and Patient objects, but it did involve mutually-dependent primitive
operations.

-- 
Signature volanSignature = 
  new Signature
  ( /*name:      */ "John G. Volan",
    /*employer:  */ "Raytheon Advanced C3I Systems, San Jose",
    /*workEmail: */ "johnv@ac3i.dseg.ti.com",
    /*homeEmail: */ "johnvolan@sprintmail.com",
    /*selfPlug:  */ "Sun Certified Java Programmer",
    /*twoCents:  */ "Java would be even cooler with Ada95's " +
                    "generics, enumerated types, function types, " +
                    "named parameter passing, etc...",
    /*disclaimer:*/ "These views not packaged in COM.ti.dseg.ac3i, " +
                    "so loading them throws DontQuoteMeError. :-)" );