cohesion

Cohesion is how closely related the responsibilities inside a module, class, or function are — A module has high cohesion when everything inside it works toward one clear purpose.

low cohesion — bad

type Utils struct{}

func (u *Utils) RegisterUser() {}
func (u *Utils) SendEmail()    {}
func (u *Utils) GeneratePDF()  {}
func (u *Utils) CalculateTax() {}

high cohesion — good

type UserService struct{}

func (s *UserService) RegisterUser() {}
func (s *UserService) DeleteUser()   {}
func (s *UserService) GetUser()      {}

coupling

Coupling is the degree of dependency between different modules, classes, or components in a software system.

we have two types of coupling:

• low coupling — it’s good and means that components can change with a minimal change to other components

interface OrderRepository {
    void save(Order order);
}

class OrderService {
    private final OrderRepository repository;

    OrderService(OrderRepository repository) {
        this.repository = repository;
    }

    void createOrder(Order order) {
        repository.save(order);
    }
}

• high coupling — it’s bad and means changing a component forcing a lot of changes in to many other components

class OrderService {
    private MySQLDatabase db = new MySQLDatabase();

    void createOrder(Order order) {
        db.connect();
        db.executeSQL("INSERT INTO orders ...");
    }
}

so, the goal is to have: low coupling with high cohesion

The 5 class quality metrics: