{"id":3871,"date":"2017-11-22T13:30:04","date_gmt":"2017-11-22T08:00:04","guid":{"rendered":"https:\/\/www.innovationm.com\/blog\/?p=3871"},"modified":"2017-11-22T13:41:23","modified_gmt":"2017-11-22T08:11:23","slug":"solid-liskovs-substitution-principle","status":"publish","type":"post","link":"https:\/\/www.innovationm.com\/blog\/solid-liskovs-substitution-principle\/","title":{"rendered":"SOLID: Liskov&#8217;s Substitution Principle"},"content":{"rendered":"<h3>Liskov&#8217;s Substitution Principle<\/h3>\n<p>Previous Principle:\u00a0<a href=\"https:\/\/www.innovationm.com\/blog\/solid-open-close-principle\/\" target=\"_blank\" rel=\"noopener\"><strong>Open Close Principle<\/strong><\/a><\/p>\n<blockquote><p><em><code>Let q(x) be a property provable about objects of x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T.<\/code><\/em><\/p>\n<p>All it is saying that, Any method which is using a reference of base class can use any child class object without knowing it.<\/p><\/blockquote>\n<p style=\"text-align: center;\">or<\/p>\n<blockquote><p>In other words, A subclass should override the methods of parent class in such way that does not break the functionality from a client point of view.<\/p><\/blockquote>\n<p style=\"text-align: center;\">or<\/p>\n<blockquote><p>If a method is using the Base class then the reference to the Base class can be replaced with a Derived class without affecting the functionality of the method.<\/p><\/blockquote>\n<p>A classic problem of Likov&#8217;s Substitution Principle is Rectangle and Square, as we know a Square\u00a0<code>is-a<\/code>\u00a0Rectangle so we will try to establish an\u00a0<code>IS-A<\/code>\u00a0relationship using inheritance like below:<\/p>\n<pre class=\"lang:java decode:true\">public class Rectangle {\r\n    private int length;\r\n    private int breadth;\r\n    \r\n    public int getLength() {\r\n        return length;\r\n    }\r\n    \r\n    public void setLength(int length) {\r\n        this.length = length;\r\n    }\r\n    \r\n    public int getBreadth() {\r\n        return breadth;\r\n    }\r\n    \r\n    public void setBreadth(int breadth) {\r\n        this.breadth = breadth;\r\n    }\r\n    \r\n    public int getArea() {\r\n        return this.length * this.breadth;\r\n    }\r\n}\r\n\r\n\/\/ Here we're trying to have is-a relationship.\r\npublic class Square extends Rectangle {\r\n\r\n    \/\/ Breadth and Height is same in Square\r\n    \r\n    @Override\r\n    public void setBreadth(int breadth) {\r\n        super.setBreadth(breadth);\r\n        super.setLength(breadth);\r\n    }\r\n    \r\n    @Override\r\n    public void setLength(int length) {\r\n        super.setLength(length);\r\n        super.setBreadth(length);\r\n    }\r\n}<\/pre>\n<p>Now let&#8217;s assume we have a client method\u00a0<code>calculateArea<\/code>\u00a0which takes\u00a0<code>Rectangle<\/code>\u00a0as parameter and have test cases based on\u00a0<code>Rectangle<\/code>\u00a0class like following.<\/p>\n<pre class=\"lang:java decode:true\">public void calculateArea(Rectangle r) {\r\n        r.setBreadth(2);\r\n        r.setLength(3);\r\n        \r\n        \/\/ From the code, the expected behavior is that \r\n        \/\/ the area of the rectangle is equal to 6.\r\n        \r\n        assert r.getArea() == 6 : \"Error in calculating area\";\r\n}<\/pre>\n<p>What will happen if we pass a\u00a0<code>Square<\/code>\u00a0object to this method? The test case will fail as it will give us an output 9 for\u00a0<code>Square<\/code>. So here the Likov&#8217;s Substitution Principle violates.<\/p>\n<p>So here cannot be an\u00a0<code>is-a<\/code>\u00a0relationship between\u00a0<code>Square<\/code>\u00a0and\u00a0<code>Rectangle<\/code>. What is the problem with\u00a0<code>is-a\u00a0<\/code>relationship here?<\/p>\n<ol>\n<li><code>Square<\/code>\u00a0doesn&#8217;t need Height and Breadth, as the sides of Square are equal so it is a wastage of memory.<\/li>\n<li>To work appropriately the client (In this case\u00a0<code>calculateArea<\/code>\u00a0method) need to know the details of derive class\u00a0<code>Square<\/code>, for this we have to change the client code which breaks the\u00a0<code>Open Close Principle<\/code>.<\/li>\n<\/ol>\n<p>So this principle make sure that new Derive classes are extending the Base class without changing their behaviour.<\/p>\n<p><strong>Next:\u00a0<a href=\"https:\/\/www.innovationm.com\/blog\/solid-interface-segregation-principle\/\" target=\"_blank\" rel=\"noopener\">Interface Segregation Principle<\/a><\/strong><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Liskov&#8217;s Substitution Principle Previous Principle:\u00a0Open Close Principle Let q(x) be a property provable about objects of x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T. All it is saying that, Any method which is using a reference of base class can use [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":3888,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2,3,71,8],"tags":[215,165,13],"class_list":["post-3871","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-android","category-ios","category-mobile","category-mobile-architecture-and-design","tag-design-pattern","tag-mobile","tag-mobile-design"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>SOLID: Liskov&#039;s Substitution Principle - InnovationM - Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.innovationm.com\/blog\/solid-liskovs-substitution-principle\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"SOLID: Liskov&#039;s Substitution Principle - InnovationM - Blog\" \/>\n<meta property=\"og:description\" content=\"Liskov&#8217;s Substitution Principle Previous Principle:\u00a0Open Close Principle Let q(x) be a property provable about objects of x of type T. 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