From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 1108a1,7f8fc37d854731d6 X-Google-Attributes: gid1108a1,public X-Google-Thread: 109fba,7f8fc37d854731d6 X-Google-Attributes: gid109fba,public X-Google-Thread: 10461e,7f8fc37d854731d6 X-Google-Attributes: gid10461e,public X-Google-Thread: 103376,7f8fc37d854731d6 X-Google-Attributes: gid103376,public X-Google-Thread: 114809,7f8fc37d854731d6 X-Google-Attributes: gid114809,public From: Alan Lovejoy Subject: Re: Interesting but sensitive topic to discuss (HELP: - OOP and CASE tools) Date: 1996/11/08 Message-ID: <32838D59.6462@concentric.net>#1/1 X-Deja-AN: 195362845 references: <32813322.41C6@kyebek3.kjist.ac.kr> <55pqr5$136a@grimsel.zurich.ibm.com> <328109CD.6685@concentric.net> <55v177$ufo@grimsel.zurich.ibm.com> <55vfg3$513@news3.digex.net> content-type: text/plain; charset=us-ascii organization: Modulation mime-version: 1.0 newsgroups: comp.object,comp.lang.c++,comp.lang.ada,comp.lang.smalltalk,comp.ai x-mailer: Mozilla 2.01Gold (Win95; U) Date: 1996-11-08T00:00:00+00:00 List-Id: Ell wrote: > > Paul_Gover@uk.ibm.com wrote: > : In <328109CD.6685@concentric.net>, Alan Lovejoy writes: > : >Class inheritance is an abstraction mechanism for code sharing. It has nothing much to do > : >with architecture or design of a program. Proof: any program using class inheritance can > : >easily be converted into a completely equivalent program where all the leaf classes are root > : >classes: one simply duplicates all the inherited methods in each leaf class. > : > ... > > : I think from your statement about converting to equivalent programs indicates you > : consider implementation to be outside the scope of the design (since you describe two > : different implementations for the same design); in that case, do you think that > : questions of inheritance and so forth are not part of the development method? > : I think this is dangerous; in your example, the coverted program has large amounts of > : duplicated code, so making changes is harder because the programmer has to find all > : occurrences of the duplicated code, and we have lost one of the benefits of OO. > > I agree with you that inheritance should be more than a means of code > sharing. But it seems he is saying we can take all of the methods > declared and defined along the branch of an inheritance hierarchy and > aggregate them in root classes. Perhaps he will clarify. I am saying that all classes could be root classes--that is, one can have classes and objects without inheritance, and construct objects and programs that have exactly the same functionality as those that use inheritance. The only difference is the amount of code that must be written, and the effort required to make certain changes in the program. The point is that whether or not one uses inheritance does not change the **design** of the program at the architectural level at which a design methodology should concern itself (although using or not using inheritance can be said to be a difference in design, it is a physical implementation difference, not an architectural difference). Using inheritance versus duplicating code does not in itself change what an object or program does. Consider an argument from the opposite direction: One can construct many very different programs using the very same class library. Let's say the class library has 26 leaf classes, named A through Z. One program can be started by instantiating an instance of class M, and then sending it the message "startUpProgram1." Another program can be started by instantiating an instance of class C, and sending it the message "doSomeStuff." Either program may instantiate objects of any class (A through Z), and send whatever messages such instances will understand. The set of possible programs in this situations is a function of what messages can be understood by which objects, and what each method does. The inheritance hierarchy used to povide each object with a set of messages it can understand makes no difference. If the same class hierarchy can be used to create many different programs (without defining any new classes or methods), then it should be obvious that the class inheritance hierarchy and program architecture and design are not at all the same things! -- Alan L. Lovejoy |==============================================| Smalltalk Consultant | Beware of Geeks bearing GIFs! | alovejoy@concentric.net |==============================================|