Classes

Composition over inheritance

Prefer composition over inheritance

As stated famously in Design Patterns by the Gang of Four, you should prefer composition over inheritance where you can. There are lots of good reasons to use inheritance and lots of good reasons to use composition. The main point for this maxim is that if your mind instinctively goes for inheritance, try to think if composition could model your problem better. In some cases it can.

You might be wondering then, "when should I use inheritance?" It depends on your problem at hand, but this is a decent list of when inheritance makes more sense than composition:

  • Your inheritance represents an "is-a" relationship and not a "has-a" relationship (Human->Animal vs. User->UserDetails).
  • You can reuse code from the base classes (Humans can move like all animals).
  • You want to make global changes to derived classes by changing a base class. (Change the caloric expenditure of all animals when they move).

Read more: Favoring Composition Over Inheritance

Bad:

PHP
class Employee
{
    private $name;
    private $email;

    public function __construct(string $name, string $email)
    {
        $this->name = $name;
        $this->email = $email;
    }

    // ...
}

// Bad because Employees "have" tax data.
// EmployeeTaxData is not a type of Employee

class EmployeeTaxData extends Employee
{
    private $ssn;
    private $salary;

    public function __construct(string $name, string $email, string $ssn, string $salary)
    {
        parent::__construct($name, $email);

        $this->ssn = $ssn;
        $this->salary = $salary;
    }

    // ...
}

Good:

PHP
class EmployeeTaxData
{
    private $ssn;
    private $salary;

    public function __construct(string $ssn, string $salary)
    {
        $this->ssn = $ssn;
        $this->salary = $salary;
    }

    // ...
}

class Employee
{
    private $name;
    private $email;
    private $taxData;

    public function __construct(string $name, string $email)
    {
        $this->name = $name;
        $this->email = $email;
    }

    public function setTaxData(EmployeeTaxData $taxData)
    {
        $this->taxData = $taxData;
    }

    // ...
}

Avoid fluent interfaces

A Fluent interface is an object oriented API that aims to improve the readability of the source code by using Method chaining.

While there can be some contexts, frequently builder objects, where this pattern reduces the verbosity of the code (for example the PHPUnit Mock Builder or the Doctrine Query Builder), more often it comes at some costs:

For more information you can read the full blog post on this topic written by Marco Pivetta.

Bad:

PHP
class Car
{
    private $make = 'Honda';
    private $model = 'Accord';
    private $color = 'white';

    public function setMake(string $make): self
    {
        $this->make = $make;

        // NOTE: Returning this for chaining
        return $this;
    }

    public function setModel(string $model): self
    {
        $this->model = $model;

        // NOTE: Returning this for chaining
        return $this;
    }

    public function setColor(string $color): self
    {
        $this->color = $color;

        // NOTE: Returning this for chaining
        return $this;
    }

    public function dump(): void
    {
        var_dump($this->make, $this->model, $this->color);
    }
}

$car = (new Car())
    ->setColor('pink')
    ->setMake('Ford')
    ->setModel('F-150')
    ->dump();

Good:

PHP
class Car
{
    private $make = 'Honda';
    private $model = 'Accord';
    private $color = 'white';

    public function setMake(string $make): void
    {
        $this->make = $make;
    }

    public function setModel(string $model): void
    {
        $this->model = $model;
    }

    public function setColor(string $color): void
    {
        $this->color = $color;
    }

    public function dump(): void
    {
        var_dump($this->make, $this->model, $this->color);
    }
}

$car = new Car();
$car->setColor('pink');
$car->setMake('Ford');
$car->setModel('F-150');
$car->dump();

Prefer final classes

The final should be used whenever possible:

  • It prevents uncontrolled inheritance chain.
  • It encourages composition.
  • It encourages the Single Responsibility Pattern.
  • It encourages developers to use your public methods instead of extending the class to get access on protected ones.
  • It allows you to change your code without any break of applications that use your class.
  • The only condition is that your class should implement an interface and no other public methods are defined.

For more information you can read the blog post on this topic written by Marco Pivetta (Ocramius).

Bad:

PHP
final class Car
{
    private $color;

    public function __construct($color)
    {
        $this->color = $color;
    }

    /**
     * @return string The color of the vehicle
     */
    public function getColor()
    {
        return $this->color;
    }
}

Good:

PHP
interface Vehicle
{
    /**
     * @return string The color of the vehicle
     */
    public function getColor();
}

final class Car implements Vehicle
{
    private $color;

    public function __construct($color)
    {
        $this->color = $color;
    }

    /**
     * {@inheritdoc}
     */
    public function getColor()
    {
        return $this->color;
    }
}
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