Cohesion¶

Cohesion measures how closely related and focused the responsibilities of a single module, class, or function are. High cohesion means that a unit has a single, well-defined purpose and all its parts work toward that purpose. Low cohesion means it handles many unrelated concerns.

High cohesion is a desirable property: it makes code easier to understand, test, maintain, and reuse. It is the C in the SOLID principle of Single Responsibility, or SRP, and is the counterpart of low coupling.

Robert C. Martin’s common forms of cohesion, from weakest to strongest:

Coincidental: parts are grouped arbitrarily, worst case
Logical: parts perform similar operations, e.g., a utility class
Temporal: parts are executed at the same time
Procedural: parts follow a sequence of steps
Communicational: parts operate on the same data
Sequential: output of one part is input to the next
Functional: all parts contribute to a single well-defined task, best case

Architectes aim for high cohesion and low coupling.

<?php

// Low cohesion: the class does too many unrelated things
class Utilities {
    public function sendEmail(string $to, string $body): void { /* ... */ }
    public function calculateTax(float $price): float { return $price * 0.2; }
    public function parseDate(string $date): DateTime { return new DateTime($date); }
}

// High cohesion: each class has one focused responsibility
class Mailer {
    public function send(string $to, string $body): void { /* ... */ }
}

class TaxCalculator {
    public function calculate(float $price): float { return $price * 0.2; }
}

?>

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Cohesion¶

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