See the code below. What is the problem of the following code?

The ShoppingCart class mixes responsibilities by handling both item management and discount calculation. It violates the Single Responsibility Principle and lacks a clear separation of concerns. Additionally, it uses public fields and lacks encapsulation.

Think again, how can you apply the Open-Closed Principle to support new types of discounts?

Good Code Example:

To adhere to the Open-Closed Principle, we can introduce a discount strategy pattern, where each discount type is encapsulated in a separate class implementing a common interface. This allows for adding new discount types without modifying existing code.

1. IDiscountStrategy interface

2. PercentageDiscountStrategy class

3. FixedAmountDiscountStrategy class

4. ShoppingCart class

In the good code example, the responsibilities are properly separated. The ShoppingCart class focuses on managing items and calculating the total, while the discount calculation is delegated to the IDiscountStrategy interface and its implementations (PercentageDiscountStrategy and FixedAmountDiscountStrategy). This adheres to the Single Responsibility Principle and promotes code reusability.

Encapsulation is achieved by encapsulating the items list and discountStrategy field as private fields, ensuring proper access control. The use of an interface allows for easy extension and swapping of different discount strategies, promoting flexibility and maintainability.

The code examples showcase how the Open-Closed Principle can be applied by utilizing abstractions, interfaces, and polymorphism to allow for extensibility and the addition of new functionalities without modifying existing code.