Interface Segregation

Dmytro Tus

Full Stack Web developer

The interface segregation principle (ISP) states that no code should be forced to depend on methods it does not use.

Here is the code which violates the Interface segregation principle

// Violates the interface Segregation Principle
<?php
interface Payable {
      public function pay(): array;
      public function convertCurrency(): float;
}

class Order implements Payable{

    public function pay(): array {
            return [
                'status' => 'ok',
                'message' => 'the product was payed'
            ];
    }   
    
    public function convertCurrency(): float {
        return 232.3;
    }
}

class Gift implements Payable{

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

    public function convertCurrency(): float {
        return null;
    }
}

Here is the code which complies the Interface segregation principle

<?php
interface Payable {
      public function pay(): array;
}

interface Convertable {
      public function convertCurrency(): float;
}

class Order implements Payable, Convertable{

    public function pay(): array {
            return [
                'status' => 'ok',
                'message' => 'the product was payed'
            ];
    }   
    
    public function convertCurrency(): float {
        return 232.3;
    }
}

class Gift implements Payable{

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

You need to be cafeful with interfaces. It is better to have single method inside interface than have many methods inside interface which are not used by the clients of interfaces.

Thats all 🙂