Types of Cohesion

Also Known As

TL;DR

Explanation

Constantine's cohesion spectrum from weakest to strongest: Coincidental (elements grouped arbitrarily — a 'utils' class), Logical (logically similar actions — all I/O routines), Temporal (executed at the same time — startup code), Procedural (follow a sequence), Communicational (share data), Sequential (output of one feeds into next), Functional (all elements contribute to a single well-defined task — ideal). In practice aim for functional or communicational cohesion. Coincidental and logical cohesion produce the most brittle, untestable code. High cohesion combined with low coupling are the two primary structural quality objectives for any module.

Common Misconception

Why It Matters

Common Mistakes

• Accepting coincidental cohesion (random grouping) in utility classes without questioning whether the grouping makes sense.
• Temporal cohesion (things that run at the same time) masquerading as functional cohesion in initialisation classes.
• Not recognising that most 'Utils' or 'Helpers' classes exhibit coincidental or logical cohesion — split them.
• Targeting sequential or communicational cohesion as the goal when functional cohesion (single, well-defined purpose) is achievable.

Code Examples

// Coincidental cohesion — completely unrelated methods grouped together:
class Utils {
    public function formatDate(DateTime $d): string { /* ... */ }
    public function sendEmail(string $to, string $body): void { /* ... */ }
    public function calculateTax(float $amount): float { /* ... */ }
    // Nothing in common — should be 3 separate classes
}

// Types of cohesion (high to low):

// Functional (best) — every element contributes to ONE task
class EmailSender {
    public function send(Email $email): void { /* SMTP logic only */ }
}

// Sequential — output of one element is input of next
class DataPipeline {
    public function run(array $data): array {
        return $this->validate($this->transform($this->load($data)));
    }
}

// Coincidental (worst) — elements grouped by chance
class Utils {  // anti-pattern: unrelated methods lumped together
    public function sendEmail() {}
    public function formatDate() {}
    public function calculateTax() {}
}

Tags

Related Terms

References

• https://en.wikipedia.org/wiki/Cohesion_(computer_science)