Notes on Open-Closed Principle (OCP)

Introduction

• Part of the Solid Principles series.
• Previous discussion on the Single Responsibility Principle (SRP).
• Today's focus: Open-Closed Principle (OCP).

Definition of OCP

• A class should be open for extension but closed for modification.
• New features or behaviors should be added without altering existing code.

Example: FTS Hotel Management System

• Recently inaugurated hotel offering initial discounts:
  • Flat $10 discount on every bill initially.
  • Later introduced percentage-based discounts and seasonal discounts.

Initial Implementation of Discounts

1. Flat Discount Calculation:

   • Created DiscountCalculator class with calculateFlatDiscount method.
   • Method accepts billAmount, subtracts $10 from it.
   • Initially works fine for flat discounts.

2. Introducing Percentage Discount:

   • Modified DiscountCalculator to accommodate multiple discount types:
   • Added an if condition to check discount type:
     • If flat, subtract $10.
     • If percentage, return 90% of the bill.
   • This leads to complexity and violates OCP:
     • Future changes require modifications to the method, risking bugs and maintenance issues.

Refactoring to Follow OCP

• Goals:
  • Ensure system is extensible and future-proof.
• Plan:
  1. Create a base Discount class to define a common structure.
  2. Each discount type will extend this base class.
  3. The DiscountCalculator will work with these classes without knowing specifics.

Implementation Steps

1. Create Abstract Class: Discount

   • Defines an abstract method applyDiscount(double billAmount).
   • Acts as a blueprint for all discount types.

2. Implement Flat Discount:

   • Create FlatDiscount class extending Discount.
   • Override applyDiscount: return billAmount - 10.

3. Implement Percentage Discount:

   • Create PercentageDiscount class extending Discount.
   • Override applyDiscount: return billAmount * 0.9 (10% discount).

4. Update Discount Calculator:

   • Modify to work with Discount objects directly.
   • Calls discount.applyDiscount(billAmount) without needing if conditions.*

Usage in Main Program

• Example code using the new structure:
  • Create instances of FlatDiscount and PercentageDiscount.
  • Pass these objects to DiscountCalculator to apply discounts.

Adding New Discount Types

• Example: Adding a 20% seasonal discount:
  • Create SeasonalDiscount class extending Discount.
  • Override applyDiscount: return billAmount * 0.8 (20% discount).
• New types can be added without modifying existing code.*

Conclusion

• The refactored code adheres to the OCP.
• Class is open for extension and closed for modification.
• Next topic: Liskov Substitution Principle (LSP) for class hierarchy collaboration.