Open Closed Principle

Dmytro Tus

Full Stack Web developer

The Open-Closed Principle (OCP) is the second principle in the SOLID design principles. It states that software entities (such as classes, modules, and functions) should be open for extension but closed for modification.

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

// Implementation which VIOLATES open closed principle
<?php

class MakePayment{

      public function pay(int $amount, string $paymentType) {
           
            if($paymentType === 'cash') {
               return $this->payByCash($amount);
             }
             
             if($paymentType === 'card') {
               return $this->payByCard($amount);
             }
      }

       private function payByCash(int $amount) {
            return Http::post('endpoint-for-pay-by-cash');
       }
       
       private function payByCard(int $amount) {
            return Http::post('endpoint-for-pay-by-card');
       }
}

$payment = new MakePayment();
$payment->pay(23, 'cash');
$payment->pay(150, 'card');

In the example above we need to modify the MakePayment class every time when we add new payment method.

Let's rewrite the logic according to the Open Closed Principle.

Implementation which complies Open Closed Principle.

<?php

interface PaymentMethodInterface {
       public function pay(int $amount);
}

class CashPayMethod implements PaymentMethodInterface {
         public function pay(int $amount) {
            echo 'endpoint-for-pay-by-cash ' . $amount;
            // return Http::post('endpoint-for-pay-by-cash', [$amount]);
         }
}
       
class CardPayMethod implements PaymentMethodInterface {
         public function pay(int $amount) {
            echo 'endpoint-for-pay-by-card ' . $amount;
            // return Http::post('endpoint-for-pay-by-card', [$amount]);
         }
}

class MakePayment{

    public function process(int $amount, PaymentMethodInterface $paymentMethod) {
        
            $paymentMethod->pay($amount);
    
    }
}

// service when the payment need to be executed
$payment = new MakePayment();
$payment->process(23, new CashPayMethod());
$payment->process(23, new CardPayMethod());

That's all 🥳