InterfaceSegregation

Interface Segregation Principle (ISP)

Definition: A client should never be forced to implement an interface that it doesn’t use, or clients shouldn’t be forced to depend on methods they do not use.

Problem:

interface PriceInterface
{
    public function getPrice(): float;
    public function getDiscount(): float;
    public function getAdditionalPrice(): float;
}

class SimpleItem implements PriceInterface
{
    public function __construct(private float $price)
    {}

    public function getPrice(): float
    {
        return $this->price;
    }

    public function getDiscount(): float
    {
        return 0.0;
    }

    public function getAdditionalPrice(): float
    {
        return 0.0;
    }
}

class DiscountItem implements PriceInterface
{
    public function __construct(
        private float $price,
        private float $discount
    ){}

    public function getPrice(): float
    {
        return $this->price;
    }

    public function getDiscount(): float
    {
        return $this->discount;
    }

    public function getAdditionalPrice(): float
    {
        return 0.0;
    }
}

Using:

/** @var PriceInterface[] $items */
$items = [
    'Simple Item:' => new SimpleItem(33.44),
    'Discount Item:' => new DiscountItem(33.44, 0.3),
];

foreach ($items as $key => $item) {
    echo $key.PHP_EOL;
        if($item instanceof SimpleItem){
            echo sprintf('Price %01.2f'.PHP_EOL, $item->getPrice());
        }elseif($item instanceof DiscountItem){
            echo sprintf('Price %01.2f'.PHP_EOL, $item->getPrice());
            echo sprintf('Discount %01.2f'.PHP_EOL, $item->getDiscount());
        }else{
            echo sprintf('Price %01.2f'.PHP_EOL, $item->getPrice());
            echo sprintf('Discount %01.2f'.PHP_EOL, $item->getDiscount());
            echo sprintf('AdditionalPrice %01.2f'.PHP_EOL, $item->getAdditionalPrice());
        }
}

Run/Output:

> php solid_runner ISP

Simple Item:
Price 33.44
Discount Item:
Price 33.44
Discount 0.30

Main problem is a interface PriceInterface. Some classes which implement this interface have to implement methods which they will not use. For example, SimpleItem implements not used methods getDiscount() and getAdditionalPrice(). DiscountItem implements not used method getAdditionalPrice().

Solution:

interface PriceInterface
{
    public function getPrice(): float;
}

interface DiscountInterface
{
    public function getDiscount(): float;
}

interface AdditionalPriceInterface
{
    public function getAdditionalPrice(): float;
}

class SimpleItem implements PriceInterface
{
    public function __construct(private float $price)
    {}

    public function getPrice(): float
    {
        return $this->price;
    }
}

class DiscountItem implements PriceInterface, DiscountInterface
{
    public function __construct(
        private float $price,
        private float $discount
    ){}

    public function getPrice(): float
    {
        return $this->price;
    }

    public function getDiscount(): float
    {
        return $this->discount;
    }
}

Using:

$items = [
    'Simple Item:' => new SimpleItem(33.44),
    'Discount Item:' => new DiscountItem(33.44, 0.3),
];

foreach ($items as $key => $item) {
    echo $key.PHP_EOL;
    if($item instanceof PriceInterface){
        echo sprintf('Price %01.2f'.PHP_EOL, $item->getPrice());
    }
    if($item instanceof DiscountInterface){
        echo sprintf('Discount %01.2f'.PHP_EOL, $item->getDiscount());
    }
    if($item instanceof AdditionalPriceInterface){
        echo sprintf('AdditionalPrice %01.2f'.PHP_EOL, $item->getAdditionalPrice());
    }
}

Run/Output:

> php solid_runner ISP

Simple Item:
Price 33.44
Discount Item:
Price 33.44
Discount 0.30

We divided PriceInterface into several interfaces (PriceInterface, DiscountInterface, AdditionalPriceInterface). As result - we implemented only the necessary methods