Re: LSP Was: Mixing OO and DB

Robert Martin <unclebob_at_objectmentor.com> writes:
>Maybe I'm missing something. The principle has been around for a long
>time. The old quote from the May, 1988 Sigplan Notices was:

  There, it is being attributed to a »forthcoming« work of Leavens:

      Gary T. Leavens, Verifying Object-Oriented Programs that
      use Subtypes. Massachusetts Institute of Technology,
      Laboratory for Computer Science, Technical Report TR-439,
      February 1989.

  Barabara Liskov in a later work has used simpler wording to   explain the meaning of »subtype« (without quantifiers).

  I do not apreciate the original »LSP« that much, because it   lacks explicit reference to specification (contract).

  I would prefer the following criterion (in my own wording):

      »B is a subtype of P, when it fulfill P's specification.«

  The Circle/Eclipse problem is related to the   value/storage-mismatch. I am not aware of a source expressing   this explicitly, so I would like to elaborate in my own   wording:

  In the following paragraphs »type« will refer to a »primitive   type« (a set of values), while above it referred to a   »behavior type« (a type of objects).

  When B is a subset of P, then each element b of B /is a/ P   (i.e., is also an element of P). But the inverse holds for   storage: A storage cell P* for P values /is a/ storage cell   for B values.

      B < P ==> P* < B*

  For example, B := {0,1}, P := {0,1,2,3}, b e B, p e P, then   each b is a p, but each P-storage is a B-storage (notice the   changed order).

  Each circle is an eclipse, but each storage for eclipses is   a storage for circles, while a storage for circles is not   a storage for eclipses.

  All the discussions about this only stem from lack of care to   distinguish between values and storages. The LSP is not needed   to resolve this. One only needs to make clear whether an   object of a given type is specified to represent a value or a   storage for a value (thing of »String« versus »StringBuffer«   in Java). Received on Sat Feb 23 2008 - 15:35:51 CET