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Open/Closed Principle (OCP)

The Open/Closed Principle (OCP) states that a software module (class, function, or component) should be open for extension but closed for modification. This means that you should be able to add new functionality to a module without changing its existing code. OCP is one of the SOLID principles in object-oriented programming.

Why Follow OCP?

  1. Avoid Breaking Existing Code: Reducing the risk of introducing bugs in tested and working code.
  2. Encourage Extensibility: Allows adding new features with minimal disruption.
  3. Improves Maintainability: Modular, extendable code is easier to maintain.

Real-Life Examples

1. Payment System

Imagine an e-commerce application that needs to process different payment methods (e.g., Credit Card, PayPal, Google Pay).

Violation of OCP:

 

public class PaymentProcessor {
    public void processPayment(String paymentType) {
        if (paymentType.equals("CreditCard")) {
            System.out.println("Processing Credit Card Payment");
        } else if (paymentType.equals("PayPal")) {
            System.out.println("Processing PayPal Payment");
        } else if (paymentType.equals("GooglePay")) {
            System.out.println("Processing Google Pay Payment");
        }
    }
}

Issues:

  • Every time a new payment method is added, the processPayment method must be modified.
  • This violates OCP because the class isn’t closed for modification.
Applying OCP:

Use an abstract class or interface to define a common contract for payment methods, allowing new payment methods to be added without modifying existing code.

 

java
// Define a common interface
public interface PaymentMethod {
    void processPayment();
}

// Implement specific payment methods
public class CreditCardPayment implements PaymentMethod {
    public void processPayment() {
        System.out.println("Processing Credit Card Payment");
    }
}

public class PayPalPayment implements PaymentMethod {
    public void processPayment() {
        System.out.println("Processing PayPal Payment");
    }
}

public class GooglePayPayment implements PaymentMethod {
    public void processPayment() {
        System.out.println("Processing Google Pay Payment");
    }
}

// Payment processor uses abstraction
public class PaymentProcessor {
    public void processPayment(PaymentMethod paymentMethod) {
        paymentMethod.processPayment();
    }
}

Adding a new payment method: To add a new payment method (e.g., Apple Pay), simply create a new class implementing the PaymentMethod interface:

java
public class ApplePayPayment implements PaymentMethod {
    public void processPayment() {
        System.out.println("Processing Apple Pay Payment");
    }
}

Let’s see another example.

Imagine a notification system that supports email and SMS notifications.

Violation of OCP:

 

java
public class NotificationService {
    public void sendNotification(String type, String message) {
        if (type.equals("Email")) {
            System.out.println("Sending Email: " + message);
        } else if (type.equals("SMS")) {
            System.out.println("Sending SMS: " + message);
        }
    }
}

Issues:

  • Adding a new notification type (e.g., Push Notifications) requires modifying the sendNotification method.
  • This violates OCP.

Let’s give a try and fix it.

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