Open/Closed Principle

Open for extension, closed for modification

Open/Closed Principle (OCP)

Software entities should be open for extension but closed for modification. You should be able to add new functionality without changing existing code. This is achieved through abstractions and polymorphism.

java

// ❌ OCP Violation
// BAD: Must modify this class every time a new payment method is added
public class PaymentProcessor {
    public void processPayment(String type, double amount) {
        if (type.equals("credit_card")) {
            // process credit card
        } else if (type.equals("paypal")) {
            // process PayPal
        } else if (type.equals("crypto")) {
            // process crypto — requires modifying existing code!
        }
    }
}

java

// ✅ OCP Applied
// GOOD: Add new payment methods without changing existing code
interface PaymentMethod {
    void processPayment(double amount);
    boolean validate();
}

class CreditCardPayment implements PaymentMethod {
    public void processPayment(double amount) {
        System.out.println("Processing credit card: $" + amount);
    }
    public boolean validate() { return true; }
}

class PayPalPayment implements PaymentMethod {
    public void processPayment(double amount) {
        System.out.println("Processing PayPal: $" + amount);
    }
    public boolean validate() { return true; }
}

// NEW payment method — NO modification needed!
class CryptoPayment implements PaymentMethod {
    public void processPayment(double amount) {
        System.out.println("Processing crypto: $" + amount);
    }
    public boolean validate() { return true; }
}

// PaymentProcessor is closed for modification
class PaymentProcessor {
    public void process(PaymentMethod method, double amount) {
        if (method.validate()) {
            method.processPayment(amount);
        }
    }
}