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Unzip and open the project SaleBad. Create some Item objects, a Sale object, and some SaleLines objects (via the addItem method in SaleBad). Determine how the total from the SaleBad object is calculated and explain why the class needs to be redesigned. In one or two paragraphs, explain why the current design will not suffice. For the moment, do not worry about how the redesign will happen. It occurs to you that the code needs to be more loosely coupled. Changes in tax policy on various items or on the tax rate itself should affect as few classes and methods as possible.

Similarly, because the store has been charging sales tax on food items that should be tax-free in some states, it seems appropriate to implement sales tax within the Item class, considering that tax varies by item and state. You sketch out a new UML diagram for a project called SaleBetter, reflecting this change. Despite this improvement, there is still concern about the design. You understand that tax calculations should be handled by a dedicated Tax class rather than within Item or other classes, adhering to principles of good object-oriented design such as separation of concerns and single responsibility. This leads you to create a UML diagram with a separate Tax class, which can be further specialized into PercentageTax and NoTax subclasses.

Explore the SaleEvenBetter project to see how the design now handles tax calculations. This design improves upon previous versions by decoupling tax logic from the Item class, making it easier to update tax policies without affecting core sales logic and minimizing the impact of policy changes. The Tax class hierarchy allows flexibility—PercentageTax can calculate taxes based on a percentage rate, while NoTax clearly indicates items that are tax-exempt, simplifying the management of taxed and untaxed items within the sales process.

Implement the UML design in the SaleEvenBetter project, creating the classes as specified. Verify that the tax method within PercentageTax returns the same value as the getPriceWithTax method in the current version of SaleEvenBetter for taxed items, and that the tax method in NoTax returns zero for untaxed items. Submit your revised SaleEvenBetter project once testing confirms correctness.

Paper For Above instruction

The initial sales system, SaleBad, exhibits several significant design flaws that hinder its flexibility and maintainability, particularly in handling varied tax policies. One primary issue is that the system tightly couples sales and tax calculations within the Item and Sale classes, which complicates modifications related to tax rates or exemptions. When changes are required—such as applying tax-free status to certain items or updating tax percentages—the current design necessitates extensive modifications across multiple classes. This violates the principle of encapsulation and makes the system fragile, prone to bugs, and difficult to extend or adapt to new tax regulations.

Furthermore, embedding tax computation directly within Item or Sale classes reduces the modularity of the system. Tax logic should be isolated to facilitate clarity and easier updates, especially given the dynamic nature of tax policies. For instance, tax rules vary by state and item category, requiring a flexible approach that can accommodate different tax rates and exemptions without invasive code changes. The current design also hampers testing and debugging, because tax calculations are entangled with total sale computations.

To address these issues, the redesign introduces a dedicated Tax class hierarchy, decoupled from core Item and Sale classes. Instead of embedding tax logic directly in Items, each Item now references a Tax object that determines its taxability and calculates applicable tax based on current policies. This approach adheres to the principle of single responsibility, where each class has a clear, distinct purpose—Items encapsulate product details, Tax classes handle tax rules, and Sales coordinate the overall process. By separating concerns, modifications to tax rates or exemptions only require updates within specific Tax subclasses, greatly simplifying maintenance and enhancement efforts.

Implementing a hierarchy within the Tax class, with subclasses such as PercentageTax and NoTax, further improves system flexibility. PercentageTax can apply a percentage rate, adjustable as policies change, while NoTax clearly indicates items exempt from taxation. This polymorphic approach allows the sales system to handle taxed and untaxed items seamlessly, reducing complex conditional logic and improving readability. Additionally, the system gains the ability to support future tax policies, such as tiered taxes or special exemptions, through straightforward class extensions.

The benefits of this redesigned approach extend beyond flexibility. It enhances testability by isolating tax calculation logic, promotes code reuse, and reduces the risk of errors during policy updates. The system becomes more aligned with object-oriented design principles, especially encapsulation, modularity, and open/closed principle. Overall, this redesign fosters a more robust, adaptable, and maintainable sales system capable of handling the complex and evolving landscape of sales tax regulation.

References

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