How do you implement the open/closed principle in your projects in practice?

The Open/Closed Principle means that software entities should be open for extension but closed for modification.

In practice, I implement this principle through:

• Using abstractions (interfaces and abstract classes) to add new implementations without changing existing code.
• Applying design patterns such as Strategy, Decorator, Factory, which allow extending functionality through new classes.
• Avoiding changing already tested and working code; instead, I add new classes or methods.

For example, if I have a payment processing system, I create a PaymentProcessor interface and implement different classes for various payment methods. When adding a new payment method, I simply add a new class without changing existing code:

public interface PaymentProcessor {
    void processPayment(double amount);
}

public class CreditCardProcessor implements PaymentProcessor {
    public void processPayment(double amount) {
        // implementation for credit card
    }
}

public class PayPalProcessor implements PaymentProcessor {
    public void processPayment(double amount) {
        // implementation for PayPal
    }
}

Thus, the system is extended without modifying existing classes.