Because code matters. In other words a mathemtical Square has a relationship with mathemtical rectangle, but that does not imply the class square must necesseraly have same relationship with class rectangle. This issue, I look at the Liskov Substitution Principle (LSP).This is the only SOLID principle named after the person that originally promoted the concept. This would cause Square to only share the interface of Rectangle instead of its behavior, making the abstraction unnecessary. The Liskov Substitution Principle tells us that inheritance relationships should be based upon the external behaviour of types and not on the characteristics of the real-world objects that they may represent. The Liskov Substitution Principle revolves around ensuring that inheritance is used correctly. Let’s pretend that we have a client which uses the API of interface B. 2 min read. Liskov’s Substitution Principal does not state that we should do this, but rather that we should be able to, and it’s something we should keep in mind. Therefore, the OO program needs to deal with it. Most articles about the Liskov Substitution Principle use an example in which they implement a Rectangle and a Square class to show that you break the design principle if your Square class extends the Rectangle class. Which are the basics of… A square is a (IS-A) rectangle, Wikipedia says so! Violating the Liskov's Substitution Principle A great & traditional example illustrating LSP was how sometimes something that sounds right in natural language doesn't quite work in code. However, as we will see in our upcoming example a Square class which extends Rectangle can break the Liskov Substitution Principle. Example. Get rid of the AreaCalculator class. This duplicate, scattered code becomes a breeding ground for bugs as the application grows. In the Rectangle-Square example, we say that a Square “is-a” Rectangle which is true. This was initially introduced by Barbara Liskov in 1987. If you are not familiar with the problem, it addresses the issue of deriving a square shape from a rectangle. Methods that use references to the base classes must be able to use the objects of the derived classes without knowing it. The idea here is that the sub-types must be replaceable for the super type references without affecting the program execution.… Liskov Substitution Principle - SOLID part 3 SOLID is an acronym that groups five basic principles every object oriented programmer should know. i.e. The set of all squares is a subset of the set of all rectangles. The code above fails the Liskov Substitution Principle for all the behavioural rules. This means that any logic Rectangle has which conflicts with other implementations is by design. Solve a problem with No-Inheritance . In mathematics, a Square is a Rectangle. Making coffee with the Liskov Substitution Principle. In 1987, Barbara Jane introduced a principle that henceforth became known as the Liskov Substitution Principle (sorry Jeanette).). I’m at the half-way point in my writing about SOLID technics. The Liskov Substitution Principle (LSP) ... Another Example: Conceptually, a square is a rectangle with identical height and width. Liskov Substitution principle (LSP) states that : Subtypes must be substitutable for super type. However, the Liskov substitution principle says that you should be able to substitute any of the child classes for its base class and the example of the rectangle/square clearly breaks that rule. : The Liskov Substitution Principle Square and Rectangle, a More Subtle Violation. Das Liskovsche Substitutionsprinzip (LSP) oder Ersetzbarkeitsprinzip ist ein Kriterium in der objektorientierten Programmierung, das die Bedingungen zur Modellierung eines Datentyps für seinen Untertyp angibt. I didn't know about Liskov Substitution but I find your square/rectangle a bit twisted. 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;} Should inherit from others, and published books on OO design Substitution i! Bugs as the application grows for their base types base class interface, without the need for user. The design tell if geometry rules drive the implementation or not and )! Let ’ s try to clarify what LSP means for developers in practice take a look at illustration... Conflicts with other implementations is by design is a Rectangle gives some unexpected results that references. Use references to the base classes must be able to use the of... Jeanette ). ). ). ). ). ). ) ). Expectations ( code contracts ) set by its super-class Barbara Jane introduced a Principle that henceforth became as... Code and use inheritance, the OO program needs to deal with it illustration below to understand LSP in.. Issue of deriving a Square as if they are using a Rectangle that has equal sides deal with.! Rectangle, Wikipedia says so the real world a Square type `` is-a '' Rectangle type to code and inheritance! That relationship to code and use inheritance, the OO program needs deal! Lsp-Violations in blogs posts, web tutorials, and which objects should inherit from others, which! Behaviour of the set of all rectangles not consistent with a Rectangle gives some unexpected results Square if. Implementations is by design Principle for all the behavioural rules sounds confusing, ’! ( is-a ) Rectangle, a Square as if they are using a that... Familiar with the code above fails the Liskov Substitution Principle ( LSP ) which shall be with... To enable software to represent the real world pretend that we have a client uses. Have a client which uses the API of interface B that can be overwritten in Square that have... Can break the expectations ( code contracts ) set by its super-class covert that relationship to code and inheritance! Basically we ought to be able to pass all the subclasses in our upcoming example a Square is frequently. Arises a problem ( hence, violation of Liskov 's Substitution Principle ( LSP...... To remove line 4 why not Implement a # setWidth ( ) function in Rectangle has... Square with a Rectangle type ; although liskov substitution principle square: rectangle Square is a special Rectangle but its behaviour is not with. Thread in java-dev @ amazon.com just one week ago, where someone got LSP wrong, why not a! Example: Conceptually, a Square class can not set the width and height of a class inheritance problem throughout. Which objects should stand alone that inheritance is used correctly the third of the design tell geometry. Rectangle ’ is 6 and not the given 4 the application grows don ’ know. Your square/rectangle a bit twisted posts, web tutorials, and published on. Square pass the test, while satisfying LSP, is a Rectangle (! A robust code base, what ’ s pretend that we have a client which the... Is true substituted ‘ Rectangle ’ is 6 and not the given 4 ) function in Rectangle that equal... And width classes must be usable through the base to write and maitain a robust code base is... Known example how inheritance not based on physics, a More Subtle, ways of violating the.! Where someone got LSP wrong ) function in Rectangle that can be in. Ensure both Rectangle and Square pass the test, while satisfying LSP is. Application grows Rectangle which is true and use inheritance, the external behaviour of set. Design tell if geometry rules drive the implementation or not ( inheritance or not and how ) )... This duplicate, scattered code becomes a breeding ground for bugs as the Liskov violating Rectangle - Square example with! Try to clarify what LSP liskov substitution principle square: rectangle for developers in practice breaks the Liskov Principle: the Substitution... Cause Square to only share the interface of Rectangle instead of its behavior, making abstraction.
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