Class Cohesion (LCOM4)

LCOM4 answers a question you've asked in every code review: is this class one thing, or several things sharing a filename? It doesn't judge style; it counts, and the count is the number of classes hiding inside.

Count it yourself, once

Draw an (imaginary) line between two methods when they touch the same field or call each other. LCOM4 is the number of islands left when you're done:

class UserService
{
    private Database $db;
    private Mailer $mailer;

    // Island 1: these two methods share $db...
    public function find(int $id): User
    {
        return $this->db->find($id);
    }

    public function save(User $user): void
    {
        $this->db->save($user);
    }

    // Island 2: ...and these two share $mailer. No bridge between islands.
    public function sendWelcome(string $email): void
    {
        $this->mailer->send($email, 'welcome');
    }

    public function sendGoodbye(string $email): void
    {
        $this->mailer->send($email, 'goodbye');
    }
}

find and save connect through $db. sendWelcome and sendGoodbye connect through $mailer. Nothing connects the two groups: LCOM4 = 2. This "service" is a repository and a notifier that happen to share a constructor.

How to read it

LCOM4	Meaning
1	Cohesive. All methods work on the same state. This is the goal.
2+	The class contains that many independent classes.
0	No methods (interfaces, empty classes). Nothing to measure.

The beautiful property of LCOM4 is that it comes with its own refactoring plan: the islands are the new classes.

class UserRepository      // island 1, with $db
class WelcomeNotifier     // island 2, with $mailer

Split along the islands and nothing breaks, because by definition no method of one island touched the state of the other.

When to tolerate it

An LCOM4 of 2 is not automatically an emergency. Some patterns legitimately score above 1: a class of pure static helpers, or a DTO whose getters each touch their own field. Read the number together with size: a 40-line class at LCOM4 = 2 is a shrug, a 400-line god class at LCOM4 = 4 is four refactorings you already know how to do. On Monolog, the average is 1.47 with a maximum of 10: one class somewhere is really ten.

See it on your code

ast-metrics analyze .

The Coupling block of the summary reports the average and maximum LCOM4. The HTML report lists the score per class on the Classes page. To cap it, the architecture ruleset ships a max_responsibilities rule for your .ast-metrics.yaml:

ast-metrics ruleset add architecture