What is an example of the Liskov Substitution Principle

Question

I have heard that the Liskov Substitution  Principle  LSP  is a fundamental principle of object oriented design  What is it and what are some examples of its use

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This formulation of the LSP is way too strong:

If for each object o1 of type S there is an object o2 of type T such that for all programs P de?ned in terms of T, the behavior of P is unchanged when o1 is substituted for o2, then S is a subtype of T.

Which basically means that S is another, completely encapsulated implementation of the exact same thing as T. And I could be bold and decide that performance is part of the behavior of P...

So, basically, any use of late-binding violates the LSP. It's the whole point of OO to to obtain a different behavior when we substitute an object of one kind for one of another kind!

The formulation cited by wikipedia is better since the property depends on the context and does not necessarily include the whole behavior of the program.

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The Liskov Substitution Principle  LSP  lsp  is a concept in Object Oriented Programming that states      Functions that use pointers or   references to base classes must be   able to use objects of derived classes   without knowing it    At its heart LSP is about interfaces and contracts as well as how to decide when to extend a class vs  use another strategy such as composition to achieve your goal   The most effective way I have seen to illustrate this point was in Head First OOA amp D  They present a scenario where you are a developer on a project to build a framework for strategy games   They present a class that represents a board that looks like this     All of the methods take X and Y coordinates as parameters to locate the tile position in the two-dimensional array of Tiles  This will allow a game developer to manage units in the board during the course of the game   The book goes on to change the requirements to say that the game frame work must also support 3D game boards to accommodate games that have flight  So a ThreeDBoard class is introduced that extends Board   At first glance this seems like a good decision  Board provides both the Height and Width properties and ThreeDBoard provides the Z axis   Where it breaks down is when you look at all the other members inherited from Board  The methods for AddUnit  GetTile  GetUnits and so on  all take both X and Y parameters in the Board class but the ThreeDBoard needs a Z parameter as well   So you must implement those methods again with a Z parameter  The Z parameter has no context to the Board class and the inherited methods from the Board class lose their meaning  A unit of code attempting to use the ThreeDBoard class as its base class Board would be very out of luck   Maybe we should find another approach  Instead of extending Board  ThreeDBoard should be composed of Board objects  One Board object per unit of the Z axis   This allows us to use good object oriented principles like encapsulation and reuse and doesn   t violate LSP

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Let me try  consider an interface    interface Planet      This is implemented by class   class Earth implements Planet       public  radius      public function construct  radius             this- gt radius    radius            You will use Earth as    planet   new Earth 6371    calc   new SurfaceAreaCalculator  planet    calc- gt output      Now consider one more class which extends Earth   class LiveablePlanet extends Earth     public function color             Now according to LSP  you should be able to use LiveablePlanet in place of Earth and it should not break your system  Like    planet   new LiveablePlanet 6371       Earlier we were using Earth here  calc   new SurfaceAreaCalculator  planet    calc- gt output      Examples taken from here

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Long story short  let s leave rectangles rectangles and squares squares  practical example when extending a parent class  you have to either PRESERVE the exact parent API or to EXTEND IT   Let s say you have a base ItemsRepository     class ItemsRepository                  return int Returns number of deleted rows            public function delete                    perform a delete query          numberOfDeletedRows   10           return  numberOfDeletedRows            And a sub class extending it   class BadlyExtendedItemsRepository extends ItemsRepository                   return void Was suppose to return an INT like parent  but did not  breaks LSP             public function delete                    perform a delete query          numberOfDeletedRows   10              we broke the behaviour of the parent class         return            Then you could have a Client working with the Base ItemsRepository API and relying on it          Class ItemsService is a client for public ItemsRepository  API   the public delete method         Technically  I am able to pass into a constructor a sub-class of the ItemsRepository    but if the sub-class won t abide the base class API  the client will get broken      class ItemsService                   var ItemsRepository             private  itemsRepository                   param ItemsRepository  itemsRepository             public function   construct ItemsRepository  itemsRepository                 this- gt itemsRepository    itemsRepository                            Notice how this is suppose to return an int  My clients expect it based on the        ItemsRepository API in the constructor                    return int             public function delete                 return  this- gt itemsRepository- gt delete               The LSP is broken when substituting parent class with a sub class breaks the API s contract   class ItemsController                  Valid delete action when using the base class              public function validDeleteAction                  itemsService   new ItemsService new ItemsRepository              numberOfDeletedItems    itemsService- gt delete                 numberOfDeletedItems is an INT                          Invalid delete action when using a subclass              public function brokenDeleteAction                  itemsService   new ItemsService new BadlyExtendedItemsRepository              numberOfDeletedItems    itemsService- gt delete                 numberOfDeletedItems is a NULL              You can learn more about writing maintainable software in my course  https   www udemy com enterprise-php

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In a very simple sentence  we can say   The child class must not violate its base class characteristics  It must be capable with it  We can say it s same as subtyping

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SOLID  Wiki Liskov substitution principle  LSP   Preconditions cannot be strengthened in a subtype  Postconditions cannot be weakened in a subtype  Invariants of the super type must be preserved in a subtype    Precondition and postcondition are method s types About   Preconditions e g  function s parameterType and parameterValue  can be the same or weaker   Postconditions e g  function s returnedType and returnedValue  can be the same or stronger  Invariant variable About  of super type should stay invariant   Swift class C1    class C2  C1    class C3  C2     class A       func foo a  C2  - gt  C2           return C2            class B  A       override func foo a  C1  - gt  C3           return C3            Java class C1    class C2 extends C1    class C3 extends C2     class A       public C2 foo C2 a            return new C2             class B extends A         Override     public C3 foo C2 a      You are available pass only C2 as parameter         return new C3             Subtyping  Sybtype should not require from caller more then supertype Sybtype should not expose for caller less then supertype   Contravariance of argument types and covariance of the return type     Contravariance of method arguments in the subtype  Covariance of return types in the subtype  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     Variance  Covariance  Contravariance

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LISKOV SUBSTITUTION PRINCIPLE  From Mark Seemann book  states that we should be able to replace one implementation of an interface with another without breaking either client or implementation It   s this principle that enables to address requirements that         occur in the future  even if we can   t foresee them today   If we unplug the computer from the wall  Implementation   neither the wall outlet  Interface  nor the computer  Client  breaks down  in fact  if it   s a laptop computer  it can even run on its batteries for a period of time   With software  however  a client often expects a service to be available          If the service was removed  we get a NullReferenceException  To deal with this type of situation  we can create an implementation of an interface that does    nothing     This is a design pattern known as Null Object  4  and it corresponds roughly to unplugging the computer from the wall  Because we   re using loose coupling  we can replace a real implementation with something that does         nothing without causing trouble

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I encourage you to read the article  Violating Liskov Substitution Principle  LSP    You can find there an explanation what is the Liskov Substitution Principle  general clues helping you to guess if you have already violated it and an example of approach that will help you to make your class hierarchy be more safe

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Some addendum  I wonder why didn t anybody write about the Invariant   preconditions and post conditions of the base class that must be obeyed by the derived classes  For a derived class D to be completely sustitutable by the Base class B  class D must obey certain conditions    In-variants of  base class must be preserved by the derived class Pre-conditions of the base class must not be strengthened by the derived class Post-conditions of the base class must not be weakened by the derived class    So the derived must be aware of the above three conditions imposed by the base class  Hence  the rules of subtyping are pre-decided  Which means   IS A  relationship shall be obeyed only when certain rules are obeyed by the subtype  These rules  in the form of invariants  precoditions and postcondition  should be decided by a formal  design contract    Further discussions on this available at my blog  Liskov Substitution principle

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LSP concerns invariants  The classic example is given by the following pseudo-code declaration  implementations omitted   class Rectangle       int getHeight       void setHeight int value            postcondition  width didn   t change           int getWidth       void setWidth int value            postcondition  height didn   t change          class Square extends Rectangle      Now we have a problem although the interface matches  The reason is that we have violated invariants stemming from the mathematical definition of squares and rectangles  The way getters and setters work  a Rectangle should satisfy the following invariant  void invariant Rectangle r        r setHeight 200      r setWidth 100      assert r getHeight      200 and r getWidth      100     However  this invariant  as well as the explicit postconditions  must be violated by a correct implementation of Square  therefore it is not a valid substitute of Rectangle

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The clearest explanation for LSP I found so far has been  The Liskov Substitution Principle says that the object of a derived class should be able to replace an object of the base class without bringing any errors in the system or modifying the behavior of the base class  from here  The article gives code example for violating LSP and fixing it

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I guess everyone kind of covered what LSP is technically  You basically want to be able to abstract away from subtype details and use supertypes safely    So Liskov has 3 underlying rules     Signature Rule   There should be a valid implementation of every operation of the supertype in the subtype syntactically  Something a compiler will be able to check for you  There is a little rule about throwing fewer exceptions and being at least as accessible as the supertype methods   Methods Rule  The implementation of those operations is semantically sound    Weaker Preconditions   The subtype functions should take at least what the supertype took as input  if not more   Stronger Postconditions  They should produce a subset of the output the supertype methods produced       Properties Rule   This goes beyond individual function calls     Invariants   Things that are always true must remain true  Eg  a Set s size is never negative   Evolutionary Properties   Usually something to do with immutability or the kind of states the object can be in  Or maybe the object only grows and never shrinks so the subtype methods shouldn t make it      All these properties need to be preserved and the extra subtype functionality shouldn t violate supertype properties    If these three things are taken care of   you have abstracted away from the underlying stuff and you are writing loosely coupled code    Source  Program Development in Java - Barbara Liskov

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The LSP in simple terms states that objects of the same superclass should be able to be swapped with each other without breaking anything   For example  if we have a Cat and a Dog class derived from an Animal class  any functions using the Animal class should be able to use Cat or Dog and behave normally

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A square is a rectangle where the width equals the height  If the square sets two different sizes for the width and height it violates the square invariant  This is worked around by introducing side effects  But if the rectangle had a setSize height  width  with precondition 0  lt  height and 0  lt  width  The derived subtype method requires height    width  a stronger precondition  and that violates lsp   This shows that though square is a rectangle it is not a valid subtype because the precondition is strengthened  The work around  in general a bad thing  cause a side effect and this weakens the post condition  which violates lsp   setWidth on the base has post condition 0  lt  width  The derived weakens it with height    width   Therefore a resizable square is not a resizable rectangle

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Here is an excerpt from this post that clarifies things nicely        in order to comprehend some principles  it   s important to realize when it   s been violated  This is what I will do now   What does the violation of this principle mean  It implies that an object doesn   t fulfill the contract imposed by an abstraction expressed with an interface  In other words  it means that you identified your abstractions wrong   Consider the following example   interface Account                  Withdraw  money amount from this account                 param Money  money         return mixed             public function withdraw Money  money     class DefaultAccount implements Account       private  balance      public function withdraw Money  money                if    this- gt enoughMoney  money                 return                     this- gt balance- gt subtract  money             Is this a violation of LSP  Yes  This is because the account   s contract tells us that an account would be withdrawn  but this is not always the case  So  what should I do in order to fix it  I just modify the contract   interface Account                  Withdraw  money amount from this account if its balance is enough         Otherwise do nothing                 param Money  money         return mixed             public function withdraw Money  money       Voil    now the contract is satisfied   This subtle violation often imposes a client with the ability to tell the difference between concrete objects employed  For example  given the first Account   s contract  it could look like the following   class Client       public function go Account  account  Money  money                if   account instanceof DefaultAccount  amp  amp    account- gt hasEnoughMoney  money                 return                     account- gt withdraw  money             And  this automatically violates the open-closed principle  that is  for money withdrawal requirement  Because you never know what happens if an object violating the contract doesn t have enough money  Probably it just returns nothing  probably an exception will be thrown  So you have to check if it hasEnoughMoney   -- which is not part of an interface  So this forced concrete-class-dependent check is an OCP violation    This point also addresses a misconception that I encounter quite often about LSP violation  It says the    if a parent   s behavior changed in a child  then  it violates LSP     However  it doesn   t     as long as a child doesn   t violate its parent   s contract

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Substitutability is a principle in object-oriented programming stating that  in a computer program  if S is a subtype of T  then objects of type T may be replaced with objects of type S  Let s do a simple example in Java  Bad example public class Bird      public void fly       public class Duck extends Bird    The duck can fly because it is a bird  but what about this  public class Ostrich extends Bird    Ostrich is a bird  but it can t fly  Ostrich class is a subtype of class Bird  but it shouldn t be able to use the fly method  that means we are breaking the LSP principle  Good example public class Bird   public class FlyingBirds extends Bird      public void fly       public class Duck extends FlyingBirds   public class Ostrich extends Bird

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Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it    When I first read about LSP  I assumed that this was meant in a very strict sense  essentially equating it to interface implementation and type-safe casting   Which would mean that LSP is either ensured or not by the language itself   For example  in this strict sense  ThreeDBoard is certainly substitutable for Board  as far as the compiler is concerned   After reading up more on the concept though I found that LSP is generally interpreted more broadly than that   In short  what it means for client code to  know  that the object behind the pointer is of a derived type rather than the pointer type is not restricted to type-safety   Adherence to LSP is also testable through probing the objects actual behavior   That is  examining the impact of an object s state and method arguments on the results of the method calls  or the types of exceptions thrown from the object   Going back to the example again  in theory the Board methods can be made to work just fine on ThreeDBoard   In practice however  it will be very difficult to prevent differences in behavior that client may not handle properly  without hobbling the functionality that ThreeDBoard is intended to add   With this knowledge in hand  evaluating LSP adherence can be a great tool in determining when composition is the more appropriate mechanism for extending existing functionality  rather than inheritance

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This principle was introduced by Barbara Liskov in 1987 and extends the Open-Closed Principle by focusing on the behavior of a superclass and its subtypes    Its importance becomes obvious when we consider the consequences of violating it  Consider an application that uses the following class   public class Rectangle       private double width     private double height      public double Width           get               return width              set               width   value                 public double Height           get               return height              set               height   value                   Imagine that one day  the client demands the ability to manipulate squares in addition to rectangles  Since a square is a rectangle  the square class should be derived from the Rectangle class    public class Square   Rectangle        However  by doing that we will encounter two problems   A square does not need both height and width variables inherited from the rectangle and this could create a significant waste in memory if we have to create hundreds of thousands of square objects  The width and height setter properties inherited from the rectangle are inappropriate for a square since the width and height of a square are identical  In order to set both height and width to the same value  we can create two new properties as follows   public class Square   Rectangle     public double SetWidth           set               base Width   value         base Height   value                  public double SetHeight           set               base Height   value         base Width   value                   Now  when someone will set the width of a square object  its height will change accordingly and vice-versa   Square s   new Square     s SetWidth 1      Sets width and height to 1   s SetHeight 2      sets width and height to 2     Let s move forward and consider this other function   public void A Rectangle r        r SetWidth 32      calls Rectangle SetWidth       If we pass a reference to a square object into this function  we would violate the LSP because the function does not work for derivatives of its arguments  The properties width and height aren t polymorphic because they aren t declared virtual in rectangle  the square object will be corrupted because the height won t be changed    However  by declaring the setter properties to be virtual we will face another violation  the OCP  In fact  the creation of a derived class square is causing changes to the base class rectangle

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It states that if C is a subtype of E then E can be replaced with objects of type C without changing or breaking the behavior of the program  In simple words  derived classes should be substitutable for their parent classes  For example  if a Farmer   s son is Farmer then he can work in place of his father but if a Farmer   s son is a cricketer then he can   t work in place of his father  Violation Example  public class Plane     public void startEngine                      public class FighterJet extends Plane        public class PaperPlane extends Plane    In the given example FighterPlane and PaperPlane classes both extending the Plane class which contain startEngine   method  So it s clear that FighterPlane can start engine but PaperPlane can   t so it   s breaking LSP  PaperPlane class although extending Plane class and should be substitutable in place of it but is not an eligible entity that Plane   s instance could be replaced by  because a paper plane can   t start the engine as it doesn   t have one  So the good example would be  Respected Example  public class Plane      public class RealPlane     public void startEngine         public class FighterJet extends RealPlane    public class PaperPlane extends Plane

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Likov s Substitution Principle states that if a program module is using a Base class  then the reference to the Base class can be replaced with a Derived class without affecting the functionality of the program module   Intent - Derived types must be completely substitute able for their base types   Example - Co-variant return types in java

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Let   s illustrate in Java   class TrasportationDevice      String name     String getName              void setName String n              double speed     double getSpeed              void setSpeed double d              Engine engine     Engine getEngine              void setEngine Engine e              void startEngine              class Car extends TransportationDevice       Override    void startEngine               There is no problem here  right  A car is definitely a transportation device  and here we can see that it overrides the startEngine    method of its superclass   Let   s add another transportation device   class Bicycle extends TransportationDevice       Override    void startEngine     problem        Everything isn   t going as planned now  Yes  a bicycle is a transportation device  however  it does not have an engine and hence  the method startEngine   cannot be implemented      These are the kinds of problems that violation of Liskov Substitution   Principle leads to  and they can most usually be recognized by a   method that does nothing  or even can   t be implemented    The solution to these problems is a correct inheritance hierarchy  and in our case we would solve the problem by differentiating classes of transportation devices with and without engines  Even though a bicycle is a transportation device  it doesn   t have an engine  In this example our definition of transportation device is wrong  It should not have an engine   We can refactor our TransportationDevice  class as follows   class TrasportationDevice      String name     String getName              void setName String n              double speed     double getSpeed              void setSpeed double d              Now we can extend TransportationDevice  for non-motorized devices   class DevicesWithoutEngines extends TransportationDevice        void startMoving               And extend TransportationDevice  for motorized devices  Here is is more appropriate to add the Engine  object   class DevicesWithEngines extends TransportationDevice        Engine engine     Engine getEngine              void setEngine Engine e              void startEngine               Thus our Car  class becomes more specialized  while adhering to the Liskov Substitution Principle   class Car extends DevicesWithEngines       Override    void startEngine               And our Bicycle  class is also in compliance with the Liskov Substitution Principle   class Bicycle extends DevicesWithoutEngines       Override    void startMoving

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Robert Martin has an excellent paper on the Liskov Substitution Principle  It discusses subtle and not-so-subtle ways in which the principle may be violated   Some relevant parts of the paper  note that the second example is heavily condensed       A Simple Example of a Violation of LSP      One of the most glaring violations of this principle is the use of C     Run-Time Type Information  RTTI  to select a function based upon the   type of an object  i e          void DrawShape const Shape amp  s      if  typeid s     typeid Square       DrawSquare static cast lt Square amp  gt  s       else if  typeid s     typeid Circle       DrawCircle static cast lt Circle amp  gt  s            Clearly the DrawShape function is badly formed  It must know about   every possible derivative of the Shape class  and it must be changed   whenever new derivatives of Shape are created  Indeed  many view the structure of this function as anathema to Object Oriented Design       Square and Rectangle  a More Subtle Violation       However  there are other  far more subtle  ways of violating the LSP    Consider an application which uses the Rectangle class as described   below       class Rectangle     public      void SetWidth double w   itsWidth w       void SetHeight double h   itsHeight w       double GetHeight   const  return itsHeight       double GetWidth   const  return itsWidth     private      double itsWidth      double itsHeight                 Imagine that one day the users demand the ability to manipulate   squares in addition to rectangles             Clearly  a square is a rectangle for all normal intents and purposes    Since the ISA relationship holds  it is logical to model the Square   class as being derived from Rectangle             Square will inherit the SetWidth and SetHeight functions  These   functions are utterly inappropriate for a Square  since the width and   height of a square are identical  This should be a significant clue   that there is a problem with the design  However  there is a way to   sidestep the problem  We could override SetWidth and SetHeight            But consider the following function       void f Rectangle amp  r      r SetWidth 32      calls Rectangle  SetWidth         If we pass a reference to a Square object into this function  the   Square object will be corrupted because the height won   t be changed    This is a clear violation of LSP  The function does not work for   derivatives of its arguments

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Would implementing ThreeDBoard in terms of an array of Board be that useful   Perhaps you may want to treat slices of ThreeDBoard in various planes as a Board  In that case you may want to abstract out an interface  or abstract class  for Board to allow for multiple implementations   In terms of external interface  you might want to factor out a Board interface for both TwoDBoard and ThreeDBoard  although none of the above methods fit

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The LSP is a rule about the contract of the clases  if a base class satisfies a contract  then by the LSP derived classes must also satisfy that contract   In Pseudo-python  class Base     def Foo self  arg                  do stuff   class Derived Base      def Foo self  arg                 do stuff    satisfies LSP if every time you call Foo on a Derived object  it gives exactly the same results as calling Foo on a Base object  as long as arg is the same

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A great example illustrating LSP  given by Uncle Bob in a podcast I heard recently  was how sometimes something that sounds right in natural language doesn t quite work in code   In mathematics  a Square is a Rectangle  Indeed it is a specialization of a rectangle  The  is a  makes you want to model this with inheritance  However if in code you made Square derive from Rectangle  then a Square should be usable anywhere you expect a Rectangle  This makes for some strange behavior    Imagine you had SetWidth and SetHeight methods on your Rectangle base class  this seems perfectly logical  However if your Rectangle reference pointed to a Square  then SetWidth and SetHeight doesn t make sense because setting one would change the other to match it  In this case Square fails the Liskov Substitution Test with Rectangle and the abstraction of having Square inherit from Rectangle is a bad one     Y all should check out the other priceless SOLID Principles Motivational Posters

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The Liskov Substitution Principle   The overridden method shouldn   t remain empty The overridden method shouldn   t throw an error Base class or interface behavior should not go for modification  rework  as because of derived class behaviors

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Liskov s Substitution Principle LSP       All the time we design a program module and we create some class   hierarchies  Then we extend some classes creating some derived   classes       We must make sure that the new derived classes just extend without   replacing the functionality of old classes  Otherwise  the new classes   can produce undesired effects when they are used in existing program   modules       Liskov s Substitution Principle states that if a program module is   using a Base class  then the reference to the Base class can be   replaced with a Derived class without affecting the functionality of   the program module    Example   Below is the classic example for which the Liskov s Substitution Principle is violated  In the example  2 classes are used  Rectangle and Square  Let s assume that the Rectangle object is used somewhere in the application  We extend the application and add the Square class  The square class is returned by a factory pattern  based on some conditions and we don t know the exact what type of object will be returned  But we know it s a Rectangle  We get the rectangle object  set the width to 5 and height to 10 and get the area  For a rectangle with width 5 and height 10  the area should be 50  Instead  the result will be 100         Violation of Likov s Substitution Principle class Rectangle       protected int m width      protected int m height       public void setWidth int width            m width   width             public void setHeight int height            m height   height             public int getWidth             return m width             public int getHeight             return m height             public int getArea             return m width   m height           class Square extends Rectangle       public void setWidth int width            m width   width          m height   width             public void setHeight int height            m width   height          m height   height            class LspTest       private static Rectangle getNewRectangle                it can be an object returned by some factory             return new Square               public static void main String args              Rectangle r   LspTest getNewRectangle             r setWidth 5           r setHeight 10              user knows that r it s a rectangle             It assumes that he s able to set the width and height as for the base            class          System out println r getArea                now he s surprised to see that the area is 100 instead of 50               Conclusion       This principle is just an extension of the Open Close Principle and it   means that we must make sure that new derived classes are extending   the base classes without changing their behavior    See also  Open Close Principle   Some similar concepts for better structure  Convention over configuration

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I see rectangles and squares in every answer  and how to violate the LSP   I d like to show how the LSP can be conformed to with a real-world example       lt  php  interface Database        public function selectQuery string  sql   array     class SQLiteDatabase implements Database       public function selectQuery string  sql   array                  sqlite specific code          return  result           class MySQLDatabase implements Database       public function selectQuery string  sql   array                  mysql specific code          return  result             This design conforms to the LSP because the behaviour remains unchanged regardless of the implementation we choose to use   And yes  you can violate LSP in this configuration doing one simple change like so     lt  php  interface Database        public function selectQuery string  sql   array     class SQLiteDatabase implements Database       public function selectQuery string  sql   array                  sqlite specific code          return  result           class MySQLDatabase implements Database       public function selectQuery string  sql   array                  mysql specific code          return   result    gt   result      This violates LSP             Now the subtypes cannot be used the same way since they don t produce the same result anymore

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LSP says that   Objects should be replaceable by their subtypes    On the other hand  this principle points to      Child classes should never break the parent class s type definitions    and the following example helps to have a better understanding of LSP   Without LSP   public interface CustomerLayout       public void render        public FreeCustomer implements CustomerLayout                 Override     public void render              code           public PremiumCustomer implements CustomerLayout               Override     public void render            if  hasSeenAd              return    it isn t rendered in this case           code          public void renderView CustomerLayout layout       layout render        Fixing by LSP   public interface CustomerLayout      public void render        public FreeCustomer implements CustomerLayout                 Override     public void render              code           public PremiumCustomer implements CustomerLayout               Override     public void render            if  hasSeenAd              showAd     it has a specific behavior based on its requirement           code          public void renderView CustomerLayout layout       layout render

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Let s say we use a rectangle in our code  r   new Rectangle           r setDimensions 1 2   r fill colors red     canvas draw r     In our geometry class we learned that a square is a special type of rectangle because its width is the same length as its height  Let s make a Square class as well based on this info   class Square extends Rectangle       setDimensions width  height           assert width    height           super setDimensions width  height              If we replace the Rectangle with Square in our first code  then it will break   r   new Square           r setDimensions 1 2      assertion width    height failed r fill colors red     canvas draw r     This is because the Square has a new precondition we did not have in the Rectangle class  width    height  According to LSP the Rectangle instances should be substitutable with Rectangle subclass instances  This is because these instances pass the type check for Rectangle instances and so they will cause unexpected errors in your code   This was an example for the  preconditions cannot be strengthened in a subtype  part in the wiki article  So to sum up  violating LSP will probably cause errors in your code at some point

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An important example of the use of LSP is in software testing   If I have a class A that is an LSP-compliant subclass of B  then I can reuse the test suite of B to test A   To fully test subclass A  I probably need to add a few more test cases  but at the minimum I can reuse all of superclass B s test cases    A way to realize is this by building what McGregor calls a  Parallel hierarchy for testing   My ATest class will inherit from BTest  Some form of injection is then needed to ensure the test case works with objects of type A rather than of type B  a simple template method pattern will do    Note that reusing the super-test suite for all subclass implementations is in fact a way to test that these subclass implementations are LSP-compliant  Thus  one can also argue that one should run the superclass test suite in the context of any subclass   See also the answer to the Stackoverflow question  Can I implement a series of reusable tests to test an interface s implementation

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LSP is necessary where some code thinks it is calling the methods of a type T  and may unknowingly call the methods of a type S  where S extends T  i e  S inherits  derives from  or is a subtype of  the supertype T    For example  this occurs where a function with an input parameter of type T  is called  i e  invoked  with an argument value of type S  Or  where an identifier of type T  is  assigned a value of type S   val id   T   new S      id thinks it s a T  but is a S   LSP requires the expectations  i e  invariants  for methods of type T  e g  Rectangle   not be violated when the methods of type S  e g  Square  are called instead   val rect   Rectangle   new Square 5     thinks it s a Rectangle  but is a Square val rect2   Rectangle   rect setWidth 10     height is 10  LSP violation   Even a type with immutable fields still has invariants  e g  the immutable Rectangle setters expect dimensions to be independently modified  but the immutable Square setters violate this expectation   class Rectangle  val width   Int  val height   Int        def setWidth  w   Int     new Rectangle w  height     def setHeight  h   Int     new Rectangle width  h     class Square  val side   Int   extends Rectangle side  side       override def setWidth  s   Int     new Square s     override def setHeight  s   Int     new Square s      LSP requires that each method of the subtype S must have contravariant input parameter s  and a covariant output   Contravariant means the variance is contrary to the direction of the inheritance  i e  the type Si  of each input parameter of each method of the subtype S  must be the same or a supertype of the type Ti of the corresponding input parameter of the corresponding method of the supertype T   Covariance means the variance is in the same direction of the inheritance  i e  the type So  of the output of each method of the subtype S  must be the same or a subtype of the type To of the corresponding output of the corresponding method of the supertype T   This is because if the caller thinks it has a type T  thinks it is calling a method of T  then it supplies argument s  of type Ti and assigns the output to the type To  When it is actually calling the corresponding method of S  then each Ti input argument is assigned to a Si input parameter  and the So output is assigned to the type To  Thus if Si were not contravariant w r t  to Ti  then a subtype Xi   which would not be a subtype of Si   could be assigned to Ti   Additionally  for languages  e g  Scala or Ceylon  which have definition-site variance annotations on type polymorphism parameters  i e  generics   the co- or contra- direction of the variance annotation for each type parameter of the type T must be opposite or same direction respectively to every input parameter or output  of every method of T  that has the type of the type parameter   Additionally  for each input parameter or output that has a function type  the variance direction required is reversed  This rule is applied recursively     Subtyping is appropriate where the invariants can be enumerated   There is much ongoing research on how to model invariants  so that they are enforced by the compiler   Typestate  see page 3  declares and enforces state invariants orthogonal to type  Alternatively  invariants can be enforced by converting assertions to types  For example  to assert that a file is open before closing it  then File open   could return an OpenFile type  which contains a close   method that is not available in File  A tic-tac-toe API can be another example of employing typing to enforce invariants at compile-time  The type system may even be Turing-complete  e g  Scala  Dependently-typed languages and theorem provers formalize the models of higher-order typing    Because of the need for semantics to abstract over extension  I expect that employing typing to model invariants  i e  unified higher-order denotational semantics  is superior to the Typestate     Extension    means the unbounded  permuted composition of uncoordinated  modular development  Because it seems to me to be the antithesis of unification and thus degrees-of-freedom  to have two mutually-dependent models  e g  types and Typestate  for expressing the shared semantics  which can t be unified with each other for extensible composition  For example  Expression Problem-like extension was unified in the subtyping  function overloading  and parametric typing domains   My theoretical position is that for knowledge to exist  see section    Centralization is blind and unfit      there will never be a general model that can enforce 100  coverage of all possible invariants in a Turing-complete computer language  For knowledge to exist  unexpected possibilities much exist  i e  disorder and entropy must always be increasing  This is the entropic force  To prove all possible computations of a potential extension  is to compute a priori all possible extension   This is why the Halting Theorem exists  i e  it is undecidable whether every possible program in a Turing-complete programming language terminates  It can be proven that some specific program terminates  one which all possibilities have been defined and computed   But it is impossible to prove that all possible extension of that program terminates  unless the possibilities for extension of that program is not Turing complete  e g  via dependent-typing   Since the fundamental requirement for Turing-completeness is unbounded recursion  it is intuitive to understand how G  del s incompleteness theorems and Russell s paradox apply to extension   An interpretation of these theorems incorporates them in a generalized conceptual understanding of the entropic force    G  del s incompleteness theorems  any formal theory  in which all arithmetic truths can be proved  is inconsistent  Russell s paradox  every membership rule for a set that can contain a set  either enumerates the specific type of each member or contains itself  Thus sets either cannot be extended or they are unbounded recursion  For example  the set of everything that is not a teapot  includes itself  which includes itself  which includes itself  etc     Thus a rule is inconsistent if it  may contain a set and  does not enumerate the specific types  i e  allows all unspecified types  and does not allow unbounded extension  This is the set of sets that are not members of themselves  This inability to be both consistent and completely enumerated over all possible extension  is G  del s incompleteness theorems  Liskov Substition Principle  generally it is an undecidable problem whether any set is the subset of another  i e  inheritance is generally undecidable  Linsky Referencing  it is undecidable what the computation of something is  when it is described or perceived  i e  perception  reality  has no absolute point of reference  Coase s theorem  there is no external reference point  thus any barrier to unbounded external possibilities will fail  Second law of thermodynamics  the entire universe  a closed system  i e  everything  trends to maximum disorder  i e  maximum independent possibilities

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There is a checklist to determine whether or not you are violating Liskov   If you violate one of the following items - gt  you violate Liskov  If you don t violate any - gt  can t conclude anything   Check list   No new exceptions should be thrown in derived class  If your base class threw ArgumentNullException then your sub classes were only allowed to throw exceptions of type ArgumentNullException or any exceptions derived from ArgumentNullException  Throwing IndexOutOfRangeException is a violation of Liskov   Pre-conditions cannot be strengthened  Assume your base class works with a member int  Now your sub-type requires that int to be positive  This is strengthened pre-conditions  and now any code that worked perfectly fine before with negative ints is broken   Post-conditions cannot be weakened  Assume your base class required all connections to the database should be closed before the method returned  In your sub-class you overrode that method and left the connection open for further reuse  You have weakened the post-conditions of that method   Invariants must be preserved  The most difficult and painful constraint to fulfill  Invariants are sometimes hidden in the base class and the only way to reveal them is to read the code of the base class  Basically you have to be sure when you override a method anything unchangeable must remain unchanged after your overridden method is executed  The best thing I can think of is to enforce these invariant constraints in the base class but that would not be easy   History Constraint  When overriding a method you are not allowed to modify an unmodifiable property in the base class  Take a look at these code and you can see Name is defined to be unmodifiable  private set  but SubType introduces new method that allows modifying it  through reflection    public class SuperType         public string Name   get  private set         public SuperType string name  int age                  Name   name           Age   age             public class SubType   SuperType         public void ChangeName string newName                  var propertyType   base GetType   GetProperty  quot Name quot   SetValue this  newName                There are 2 others items  Contravariance of method arguments and Covariance of return types  But it is not possible in C   I m a C  developer  so I don t care about them

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