>>
>>>Len,
>>>
>>>> Christopher:
>>>>
>>>> CS:
>>>>>While I can quite understand the short-term convenience of
>>>>>it, my problem with method overriding is that it torpedoes
>>>>>the Liskov Substitution Principle: an instance of a
>>>>>subclass is no longer guaranteed to be an instance of the
>>>>>baseclass. That's more than just a bizarre abuse -- it's an
>>>>>overthrow of polymorphism as well as inheritance.
>>>>
>>>> I don't agree. Identity and behavior are orthogonal aspects
>>>> in OO.
>>>
>>>Not so. If an instance X of class A is defined as necessarily
>>>having behavior P, but a programmer now states that entity X
>>>in this situation must now have behavior Q but not P, then,
>>>for all I know, X is no longer an A.
>>
>> LSP does not relate identity to behavior. In fact it does not
>> deal with identity or behavior directly - instead it promotes
>> substitutability and correctness resulting from behavior.
>> (see
>> http://en.wikipedia.org/wiki/Liskov_substitution_principle)
>
>Thanks for the pointer. So I followed its references. In the
>first, Liskov's original 1987 paper enunciates "the following
>substitution property" (where S is a subtype of T):
>
>"If for each object o1 of type S there is an object o2 of type T
>such that for all programs P defined in terms of T, the behavior
>of P is unchanged when o1 is substituted for o2, then S is a
>subtype of T." (01)
The quote above does not contradict my point because neither identity or
behavior of o1-->o2 is involved. The behavior of P, on the other hand, can not
be directly related to identity of o1 or o2. Moreover, it can be inferred that
P defines the scope and criteria for identity of o1 and o2. (02)
>
>Even her (and Wing's) 1999 paper, the second Wikipedia
>reference, in its abstract opens with this sentence:
>
>"We present a way of defining the subtype relation that ensures
>that subtype objects preserve behavioral properties of their
>supertypes."
>
>So it would seem that the LSP does directly concern behavior.
> (03)
LSP concerns behavior of some abstract program P that implies set of operations
for which subtypes preserve invariance of some properties. Nothing here
directly relates identity of individual type to its behavior. (04)
>And behavior in such circumstances must surely preserve
>identity. Then If a program transmutes identity, neither type
>nor subtype would necessarily apply any more.
> (05)
That is correct because program is the scope where subtype holds. (06)
>>
>> I can understand your concern about method overriding done
>> incorrectly in violation of LSP, but that does not mean that
>> allowing overriding "torpedoes LSP".
>
>Not necessarily, but overriding does invite practices which can
>easily torpedo both integrity and identity and thereby the
>intent of the LSP.
> (07)
On the contrary - overriding according to LSP is the way to enforce behavioral
properties defined by the program operating with abstract types. It provides
the way to refine behavior without imposing unnecessary constraints. That is
intent of LSP and not linking identity of types to their behavior. (08)
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