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--- principles:liskov_substitution_principle [2013-01-15 17:06] – created christian
+++ principles:liskov_substitution_principle [2021-09-02 18:26] – old revision restored (2021-09-02 10:48) 65.21.179.175
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-====== Liskov Substitution Principle ======
+====== Liskov Substitution Principle (LSP) ======
 ===== Variants and Alternative Names =====
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 ===== Principle Statement =====
+> "Subtypes must be substitutable for their base types."((Robert C. Martin: //Agile Software Development, Principles, Patterns, and Practices//, p. 111))
 ===== Description =====
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 ===== Strategies =====
+  * Only strengthen invariants in subclasses; never weaken them
+  * Only waken preconditions when overriding methods
+  * Only strengthen postconditions when overriding methods
+  * Use Delegation instead of Inheritance
+  * Figure out better abstractions
+===== Caveats =====
+See section [[#contrary principles]].
 ===== Origin =====
+Barbara Liskov: //[[http://portal.acm.org/citation.cfm?id=62141|Data abstraction and hierarchy]]//
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 ==== Contrary Principles ====
-  * [[principles:Keep It Simple Stupid]]
+  * [[Keep It Simple Stupid]] (KISS): Not adhering to the LSP can be easier.
 ==== Complementary Principles ====
-  * [[principles:Information Hiding/Encapsulation]]
+  * [[Model Principle]] (MP): MP demands inheritance relations to resemble an "is-a" relationship. This means that an object of the subclass is also an object of the superclass. This is always true in a technical sense as this is how object-oriented programming languages handle inheritance hierarchies. However MP demands that is shall be true in the model, too. This is slightly different from LSP which rather is about a "is-substitutable-for" relationship.
-  * [[principles:Invariant Avoidance Principle]]
+  * [[Principle of Separate Understandability]] (PSU): When building inheritance hierarchies, LSP constrains how subclasses are constructed. Namely they should comply with the superclass contract. PSU on the other hand demands that the superclass shall be separately understandable, which means that knowledge of concrete subclasses and their needs should not be necessary to understand the superclass. So a superclass should not have a specific functionality, etc. just because a particular subclass needs this. In contrast to that the superclass of course may provide ''protected'' features for subclasses in general. But it should be inherently clear that subclasses in general may need this functionality without looking at a particular one.
 ==== Principle Collections ====
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 {{page>collections:OOD Principle Language#Box}}
 {{page>collections:SOLID#Box}}
 ===== Example =====
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 /*[[wiki:Incomplete]]*/
 /*[[wiki:Complete]]*/
 ===== Further Reading =====
-  * Robert C. Martin: //Agile Software Development, Principles, Patterns, and Practices//
+  * Robert C. Martin: //Agile Software Development, Principles, Patterns, and Practices//, p. 111--125
   * [[http://www.butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod|ButUncleBob: Principles of OOD]]
+  * [[wiki>LiskovSubstitutionPrinciple]]
+  * [[wp>Liskov Substitution Principle]]