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copied!<p>The Liskov Substitution Principle (LSP, <a href="/questions/tagged/lsp" class="post-tag" title="show questions tagged 'lsp'" rel="tag">lsp</a>) is a concept in Object Oriented Programming that states:</p> <blockquote> <p>Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it.</p> </blockquote> <p>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.</p> <p>The most effective way I have seen to illustrate this point was in <a href="http://oreilly.com/catalog/9780596008673/index.html" rel="noreferrer">Head First OOA&D</a>. They present a scenario where you are a developer on a project to build a framework for strategy games.</p> <p>They present a class that represents a board that looks like this:</p> <p><img src="https://oncodebynotmyself.files.wordpress.com/2011/03/board_thumb.png" alt="Class Diagram"></p> <p>All of the methods take X and Y coordinates as parameters to locate the tile position in the two-dimensional array of <code>Tiles</code>. This will allow a game developer to manage units in the board during the course of the game.</p> <p>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 <code>ThreeDBoard</code> class is introduced that extends <code>Board</code>.</p> <p>At first glance this seems like a good decision. <code>Board</code> provides both the <code>Height</code> and <code>Width</code> properties and <code>ThreeDBoard</code> provides the Z axis.</p> <p>Where it breaks down is when you look at all the other members inherited from <code>Board</code>. The methods for <code>AddUnit</code>, <code>GetTile</code>, <code>GetUnits</code> and so on, all take both X and Y parameters in the <code>Board</code> class but the <code>ThreeDBoard</code> needs a Z parameter as well.</p> <p>So you must implement those methods again with a Z parameter. The Z parameter has no context to the <code>Board</code> class and the inherited methods from the <code>Board</code> class lose their meaning. A unit of code attempting to use the <code>ThreeDBoard</code> class as its base class <code>Board</code> would be very out of luck.</p> <p>Maybe we should find another approach. Instead of extending <code>Board</code>, <code>ThreeDBoard</code> should be composed of <code>Board</code> objects. One <code>Board</code> object per unit of the Z axis.</p> <p>This allows us to use good object oriented principles like encapsulation and reuse and doesn’t violate LSP.</p>

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