Coupling and cohesion — the dual metrics of good design

Direct Answer

Cohesion describes how closely responsibilities inside a module belong together; coupling describes how strongly that module relies on others. High cohesion and controlled coupling often support maintainability, but they are separate qualities: a focused class may still depend on many systems, and splitting code can sometimes increase coupling.

Code Example

// Focused responsibility: pricing an order.
final class OrderPricer {
    private final TaxCalculator tax;

    OrderPricer(TaxCalculator tax) {
        this.tax = tax;
    }

    Money total(Money subtotal, Address address) {
        return subtotal.add(tax.calculate(subtotal, address));
    }
}

OrderPricer can be cohesive because its work is about pricing. Its coupling still depends on the size and stability of TaxCalculator, Money, and Address. An interface keyword alone does not settle that second question.

Common Mistakes & Pitfalls

• Using a formula: high cohesion does not automatically create low coupling.
• Assuming every split helps both: extraction can add messages, DTOs, ordering rules, or network dependencies.
• Calling every interface loose coupling: a broad or unstable interface can remain tightly coupled.
• Splitting by line count: ten tiny classes can scatter one invariant and increase coordination.
• Ignoring operational coupling: shared databases, timing, retries, and deployment order may matter more than Java imports.