Coupling and Cohesion

1// ❌ LOW Cohesion: Class does unrelated things
2class Utils {
3    public void sendEmail(String to, String body) { }
4    public int calculateTax(double amount) { }
5    public void compressFile(String path) { }
6    public User parseUserJson(String json) { }
7    public void resizeImage(String path, int width) { }
8}
9
10// ✅ HIGH Cohesion: Each class is focused
11class EmailService {
12    public void send(String to, String body) { }
13    public void sendBulk(List<String> recipients, String body) { }
14    public void sendWithAttachment(String to, String body, File attachment) { }
15}
16
17class TaxCalculator {
18    public double calculate(double amount) { }
19    public double calculateWithDeductions(double amount, List<Deduction> deductions) { }
20}
21
22// ❌ HIGH Coupling: Direct dependencies everywhere
23class OrderService {
24    public void process(Order order) {
25        MySQLDatabase db = new MySQLDatabase();  // Direct dependency
26        StripePayment payment = new StripePayment();  // Direct dependency
27        SendGridEmail email = new SendGridEmail();  // Direct dependency
28        
29        db.save(order);
30        payment.charge(order.getTotal());
31        email.send(order.getCustomerEmail(), "Order confirmed");
32    }
33}
34
35// ✅ LOW Coupling: Depend on abstractions
36class OrderService {
37    private final Database database;
38    private final PaymentGateway payment;
39    private final EmailService email;
40    
41    // Dependencies injected - can be any implementation
42    public OrderService(Database db, PaymentGateway pay, EmailService mail) {
43        this.database = db;
44        this.payment = pay;
45        this.email = mail;
46    }
47    
48    public void process(Order order) {
49        database.save(order);
50        payment.charge(order.getTotal());
51        email.send(order.getCustomerEmail(), "Order confirmed");
52    }
53}