Liskov Substitution Principle

Dmytro Tus

Full Stack Web developer

The Liskov Substitution Principle in practical software development. The principle defines that objects of a superclass shall be replaceable with objects of its subclasses without breaking the application. That requires the objects of your subclasses to behave in the same way as the objects of your superclass

Let's write the code which violates this principle and then code which complies the principle.

// VIOLATES the LS principle
<?php

interface Payable {
      public function pay();  // violates the LS principle
}

class Order implements Payable{

        public function pay() {
               return 'the product was payed';  // returns string
        }   
}

class Gift implements Payable{

        public function pay() {
            return [
                 'status' => 'ok',
                 'message' => 'the gift was payed'
             ]; // returns array
        }
}

The code which compiles the Liskov Substitution principle

// complies the LS principle
<?php
interface Payable {
      public function pay(): array;
}

class Order implements Payable{

    public function pay(): array {
            return [
                'status' => 'ok',
                'message' => 'the product was payed'
            ];
    }   
}

class Gift implements Payable{

    public function pay(): array {
        return [
                'status' => 'ok',
                'message' => 'the gift was payed'
            ];
    }
}

That is all. Happy coding. 🥳