615:, I don't think it's valid to infer anything about preconditions, postconditions, new exceptions, or "any other new behaviour" based on Liskov and Wing's original forumlation. That may be one interpretation or application of the Principle, but to claim that it is "mandated" by the Principle is an opinion that may not be shared by the bulk of practitioners in the field, and at the very least, should be accompanied by a reference. --
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LSP and substitutability are terrible names for it. Nothing is being substituted for anything! The principle is about a single object being of two different behavioral types (one of which is a behavioral subtype of the other), not about two different objects! If the supertype is an abstract class, there cannot even *be* any objects whose class is that class, even though behavioral subtyping is perfectly applicable to such cases!
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1169:"(correctness, task performed, etc.)" is moved to a separate sentence to make the first sentence more clean. Finally, it is my belief that everybody understands that "substitute" means "replace", and as "substitute" is the word normally used in computer science, and in the name of the article, I suggest simply using that. If not, " (replaced)" could be added after "substituted".
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looks to me suspiciously like a cumbersome way of saying that methods must be covariant in exceptions thrown; this could even be unified with covariance of return value into a generalised "covariance of response" - function types are covariant in the return, in any throws and indeed in any other form
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Disagree. Liscov substitution principle stands on its own. It is more abstract than its source link, if people want the abstraction, let them click. Removing fancy abstractions to their own articals makes the source-link-articals more readable for introductory readers. Artical design should encourge
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The incorrect citation is still there, at the end of the first paragraph. The correct succinct quote should be "“What is wanted here is something like the following substitution property: 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
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I believe we should all do our best to erase all traces of the terms "Liskov
Substitution Principle" and "substitutability" from the Internet, starting with this article. Behavioral subtyping is a very meaningful and important concept, and the name "behavioral subtyping" is a great name for it, but
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You are very probably more knowledgeable than me about the topic (I'm following a reduced CS curriculum - one year). I have learned this from a theoretical Dutch 2nd bachelor-level textbook on OO, given by a prof enamoured of Design by
Contract. My perspective on it is small. And mandating is a bit
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This is a common misunderstanding. An undecidable property could possibly be proven; a program with one statement -- Halt -- is trivially provable as halting, even though in general halting is undecidable. The problem is, for any particular program, failure to construct a proof might mean the
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I find the current wording rather messy and therefore hard to read, and I believe it is important that the very first sentence is clear, easy to understand, and to the point. Note that with the new wording, there is no need to name the subtype, so introducing "S" is avoided. Also, the parenthesis
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I just put up some changes - I found a little confusion in the "preconditions are weakened" vs "preconditions cannot strengthen" area: my formal logic background and some confusion in my undergrad students who referenced this article made me change it. I also added a bit about the functions using
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On Aug.25, 2009, a word 'mutable' was added to 'The
Principle' paragraph. I could not find any place in Liskov works where she insists that this "principle" somehow depends on the objects mutability. Although it may be true that the principle is trivial for immutable objects, the mutability issue
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of response mechanism a language might invent. Of course, not actually throwing a given type of exception is an acceptable implementation of covariant throwing, just as (accepting but) not requiring some input parameter is an acceptable implementation of contravariance for inputs. -- Eddy.
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DO NOT MERGE! Inheritance is about shared implementation. LSP is about contract and interface. LSP applies even with no inheritance. And as noted above, you can break LSP in a subclass -- for example, by reducing the range of values accepted in a method. This renders the subclass not a valid
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I think the desired final state is that the terms LSP and substitutability are mentioned in a subsection somewhere towards the bottom of the
Behavioral subtyping article for historical and debunking reasons, and that the LSP article is turned into a redirect to Behavioral subtyping.
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Thus, Liskov and Wing's notion of "subtype" is based on the notion of substitutability; that is, if S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program (e.g.,
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Object inherited from can be seen and/or used in two ways: 1) bottom-up: as parent class (top) or 2) top-down: primitive library one want to use and extend from. In second case "Liskov substitution principle" does not apply and contract inheritance is nuisance.
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By insisting that the two values *MUST* be required to vary independently, you have lost an important mathematical property of the object in question! In fact, you've forbidden the very property that lets you define a Circle or Square in the first place.
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One of my professors pointed out the concept of comparability was missing, so I tightened up the previous changes further and added a short bit on comparability. This can probably do with expansion if I have missed any cases of comparability.
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Reflecting on it, perhaps the preconditions and postconditions are warranted. It's hard to imagine an interpretation of LSP that allows preconditions to be weakened in subclasses, for example. As for the rest, I'm still pondering that...
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This is why removing the post condition eliminates the problem; contrary to the article's claims, however, removing such a false-promise was doesn't "weaken" the contract; it merely makes it possible to write a mathematically correct one.
526:, though it brushes up against the topic with mention of the type inclusion rule. I think it likely that if you scan all the LCS TRs and TM's between that point and 1987 by Liskov or her grad students, you'll find an earlier reference.
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Neither an ellipse nor a rectangle are mathematically required to have their x and y axis vary independently of each other. In fact, this is the defining property of
Circles and Squares as sub-classes of Ellipses and Rectangles.
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If it's okay with people, I'll start by editing the
Behavioral subtyping article to make it not be a redirect to this page. I'll copy over all of the content from this page that does not promote the "substitutability" error.
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Even though this is an article about programming, the word property in the LSP doesn't refer to the concept of computed object attributes. The link is missleading to the reader, it should either be removed or changed to
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In general, the principle mandates that at all times objects from a class can be swapped with objects from an inheriting class, without the user noticing any other new behaviour. It has effects on the paradigms of
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In the normal case we have type T with precondition A and subtype S with precondtion A' same or weaker than A. In the incomparable case take T with conditions A and B and S with A' and B' where A' < A and B' :
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B. In this case the individual conditions can be compared for strength but not the whole condition. Does this make sense (it may not, it is late at night)? If it does, should it be added do you think?
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Definitely disagree. Liskov and
Inheritance are two separate issues. For example, you can have an inheritance relationship in an OO program while still violating the Liskov Substitution Principle.
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The article mentions "the postconditions" for a rectangle object without identifying or describing what those postconditions would be. This text is currently ambiguous and unclear.
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It seems to me that the article was written based on the 1994 paper and not based on the 1988 paper. This means that most of the discussion within is not relevant to LSP. Thoughts?
494:, the copyright date for which is 1992. The paper normally credited with advancing the principle is Barbara Liskov's "Data Abstraction and Hierarchy", which was published in 1988:
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I think Liskov herself feels the same way. If she used substitution-like terms in preliminary work, as far as I know she did *not* use any such terms in the published literature!
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No new exceptions should be thrown by methods of the subtype, except where those exceptions are themselves subtypes of exceptions thrown by the methods of the supertype.
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The article states that LSP was introduced in 1993 by
Barbara Liskov and Jeanette Wing. However, the term was already in use by 1993, having been used by
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I suspect that wasn't the earliest either. I checked the
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learning and exploring of people at many levels of understanding. Here I am thinking like a teacher. -- - Sat Nov 8 10:02:35 EST 2008
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I don't understand the comparability thing. Clearly if predicate A is weaker than or equal to B, they are necessarily comparable. --
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strong, yes. But isn't saying "mandating transparent swapping" just a more practical approach to the previous paragraph, i.e. this:
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of T without altering the desirable properties of T. Desirable properties can for instance be correctness and task performed.
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In effect, you have now contracted not to subclass
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with any object of a subtype S) without altering any of the desirable properties of T (correctness, task performed, etc.).
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T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T." This is from the 1988 paper.
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related articles on Knowledge. If you would like to participate, please visit the project page, where you can join
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This point is unclear to me. It appears that Robert Martin did, but we need a citation for something like that.
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Since last July, actually since never, there has been no discussion on the subject of mergeto|Subsitutability.
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04. Jan. 2011: The external link: The Liskov Substitution Principle, T. S. Norvell, 2003 is not available.
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I found a keynote speech with the same title from 1987 and changed the article intro to point to that. --
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substitute for the base class. It's bad practice -- but it's bad practice because it violates LSP.
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postconditions for methods in the subclass should be more strict than those in the superclass
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This page should be merged with . I've only seen it discussed with regards to inheritance.
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Actually, rather than the 1979 CLU reference manual, Wing's PhD thesis (
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should probably not be inserted without a proper comment, if ever.
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Disagree. This is a subtyping issue, not an inheritance issue. --
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I removed the mergeto|Substitutability for the following reasons:
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http://publications.csail.mit.edu/lcs/pubs/pdf/MIT-LCS-TR-299.pdf
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http://publications.csail.mit.edu/lcs/pubs/pdf/MIT-LCS-TR-225.pdf
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Feel free to put in perspective and change whatever you want:)--
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The "post conditions" listed for the rectangle object are wrong
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Find pictures for the biographies of computer scientists (see
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840:|Substitutability is what I'm actually going to argue for --
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I suggest changing the first sentence of the introduction
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with objects of type S (i.e., an object of type T may be
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It's worse than I thought. REDIRECT is more like it. --
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no new exceptions should be introduced in the subclass
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Who coined it as "The Liskov substitution principle"?
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1110:
1109:
1105:
1085:
1073:
1054:
1036:
1032:
1005:— Preceding
1003:
999:
996:
992:
988:
972:159.54.131.7
969:
941:— Preceding
936:
917:
883:
861:
847:
837:
830:
821:
788:66.0.235.201
731:
665:
610:
582:
491:
485:
452:
429:— Preceding
426:
373:
372:
356:Unreferenced
354:
353:
335:
334:
309:
308:
290:
289:
271:
270:
252:
251:
233:
232:
209:
208:
190:
189:
148:
108:
68:WikiProjects
35:
1135:substituted
894:—Preceding
782:—Preceding
734:ThomasOwens
488:Jim Coplien
457:—Preceding
58:Start-class
1232:Categories
842:over there
829:There's a
1129:T may be
1076:Principle
804:Bob Kerns
546:Bob Kerns
528:Bob Kerns
423:Dead link
244:Computing
1219:contribs
1207:unsigned
1131:replaced
1057:ADKolsky
1019:contribs
1007:unsigned
955:contribs
943:unsigned
896:unsigned
784:unsigned
715:Jgorman2
672:Jgorman2
563:ADKolsky
471:contribs
459:unsigned
431:unsigned
292:Maintain
235:Copyedit
28:deletion
1174:Jhertel
1162:subtype
1123:subtype
1074:In the
749:Doradus
696:Doradus
653:Doradus
641:Phlebas
617:Doradus
510:Doradus
273:Infobox
211:Cleanup
151:on the
765:Trumpi
500:Kevlin
254:Expand
64:scale.
836:merge
831:newer
728:Merge
337:Stubs
311:Photo
168:with:
1215:talk
1178:talk
1158:type
1127:type
1093:talk
1061:talk
1042:talk
1015:talk
976:talk
951:talk
924:ping
920:Pcap
904:talk
868:talk
854:talk
838:from
808:talk
792:talk
769:talk
753:talk
738:talk
567:talk
550:talk
532:talk
514:talk
467:talk
439:talk
36:keep
34:was
712:-->
498:--
490:in
143:Mid
1234::
1221:)
1217:•
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1017:•
978:)
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906:)
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848:--
810:)
794:)
771:)
755:)
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651:--
596::
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552:)
534:)
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473:)
469:•
441:)
393:}}
387:{{
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70::
38:.
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