inanzzz | S.O.L.I.D. Single Responsibility principle

A class should have only one responsibility and shouldn't do or know more than what it meant to. A class that knows too much or does too much is called "God object".

# VIOLATION
class User
{
    private $firstname;
    private $lastname;

    public function setFirstname(string $firstname)
    {
        $this->firstname = $firstname;
    }

    public function setLastname(string $lastname)
    {
        $this->lastname = $lastname;
    }

    public function output(): string
    {
        return sprintf('Hello:%s %s', $this->firstname, $this->lastname);
    }
}

$user = new User();
$user->setFirstname('Robert');
$user->setLastname('DeNiro');
echo $user->output();

As you can see above, User class seems to be responsible for outputting as well as setting and getting user information which is what causes violation.

# REFACTORED
interface UserInterface
{
    public function setFirstname(string $firstname);
    public function getFirstname(): string;
    public function setLastname(string $lastname);
    public function getLastname(): string;
}

class User implements UserInterface
{
    private $firstname;
    private $lastname;

    public function setFirstname(string $firstname)
    {
        $this->firstname = $firstname;
    }

    public function getFirstname(): string
    {
        return $this->firstname;
    }

    public function setLastname(string $lastname)
    {
        $this->lastname = $lastname;
    }

    public function getLastname(): string
    {
        return $this->lastname;
    }
}

class UserOutputer
{
    private $user;

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

    public function plain(): string
    {
        return sprintf('Hello:%s %s', $this->user->getFirstname(), $this->user->getLastname());
    }
}

$user = new User();
$user->setFirstname('Robert');
$user->setLastname('DeNiro');

$userOutputter = new UserOutputer($user);
echo $userOutputter->plain();

As you can see above, we have introduced a new class called UserOutputer and shipped outputting responsibility from User class to it. We now have:

a User class which is responsible for setting and getting user information and

a UserOutputer class which is responsible for outputting user information.

We can even easily add JSON and XML formats as output options to UserOutputer class without even touching User class.