\n\nRepost of https://dev.to/rhuzaifa/is-the-liskov-substitution-principle-really-useful-3oce
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Liskov Substitution is part of SOLID Design. SOLID?
\n\n\nSOLID are 5 software development principles or guidelines based on Object-Oriented design making it easier for you to make your projects scalable and maintainable.
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Think of them as best practices.
\nYou see the L in SOLID stands for this principle.
\nIt says
\n\n\nLet Φ(x) be a property provable about objects x of type T. Then Φ(y) should be true for objects y of type S where S is a subtype of T. Barbara Liskov
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Honestly, Too scientific.
\nIn simple terms
\n\n\nReplacing an instance of a class with its child class should not produce any negative side effects or broken codebase.
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✔️ Can use the subclass of a parent class just the same as using the parent class without breaking anything.
\n✔️ Subclasses can modify/override parent class methods.
\n❌ Subclasses can modify the parent’s method signature like arguments, return type, and exceptions.
\n❌ Subclasses can define a new function not present in the parent class.
\n❌ Parent class can be modified.
The Goal of this principle is to basically prevent our old codebase from breaking due to new code. This is also in line with the Single Responsibility and the Open Close Principle.
\nWe will use a simple example for explanation.
\nThe following example violates the rule.
\nclass Animal{\n\tfunction eat(){\n\t\t// common functionality\n\t\treturn \"Eating Now\"\t// return type string\n\t}\n\n\tfunction sleep(){\n\t\t// common functionality\n\t\treturn \"I am sleeping\"\t// return type string\n\t}\n}\n\nclass Cat extends Animal{\n\tfunction eat(){\n\t\t// ... cat specific code\n\t\treturn \"Meow, whatever human\"\t// return type string\n\t}\n\n\tfunction sleep(){\n\t\t// ... cat specific code\n\n\t\t// voilating LSP: parnet sleep() does not return boolean\n\t\treturn true \n\t}\n}\n\nclass Dog extends Animal{\n\tfunction eat(){\n\t\t// ... dog specific code\n\t\treturn \"Woof, It was tasty.\"\t// return type string\n\t}\n\n\tfunction sleep(){\n\t\t// ... dog specific code\n\t\t\n\t\t// voilating LSP: parent sleep() doesn't use Error Exception\n\t\tthrow Error('I just slept') \n\t}\n}With the Liskov Substitution Principle, we would modify our code as follow
\nclass Animal{\n\tfunction eat(){\n\t\t// common functionality\n\t\treturn \"Eating Now\"\t// return type string\n\t}\n\n\tfunction sleep(){\n\t\t// common functionality\n\t\treturn \"I am sleeping\"\t// return type string\n\t}\n}\n\nclass Cat extends Animal{\n\tfunction eat(){\n\t\t// ... cat specific code\n\t\treturn \"Meow, whatever human\"\t// return type string\n\t}\n\n\tfunction sleep(){\n\t\t// ... cat specific code\n\t\treturn \"I am already sleeping\"\t// return type string\n\t}\n}\n\nclass Dog extends Animal{\n\tfunction eat(){\n\t\t// ... dog specific code\n\t\treturn \"Woof, It was actually tasty.\"\t// return type string\n\t}\n\n\tfunction sleep(){\n\t\t// ... dog specific code\n\t\treturn \"Zzzzzzzz\"\t// return type string\n\t}\n}With this approach, we can swap parent and child classes without breaking the code.
\nIt is in most cases, but there are those cases where you might want to add some more that does not quite fit in like the Birds example below
\nclass Bird{\n function fly(){}\n}\n\nclass Duck extends Bird{}\n\nclass Ostrich extends Bird{} // Duck can fly but ostrich cant:So yeah, it really depends. If it’s getting over-complicated/over-engineered or is not making sense(like bird example) then it’s best to do your own thing.
\nIt’s easy to extend old code with new code. You just have to make a new class and extend it with parent/base class without the fear of breaking the already working code. We also get this benefit from Dependency Inversion principle.
\nSo how do you see this? Do you think it’s really useful? Be sure to tell me your opinion in the comments.
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