coursera-ibm-java-developer-course/InventoryManagementSystem/src/main/java/inventory/DiscountCalculator.java

package inventory;

import java.util.Objects;

import static inventory.ProductFactory.BOOK_TYPE;

/**
 * Strategy pattern implementation for calculating different types of discounts.
 * Requirements:
 * - Student discount: 10% off books only
 * - Bulk discount: 15% off when buying 5+ items
 * - No discount option
 * - Return discount amount and description
 */
public class DiscountCalculator {

    /**
     * Inner class to hold discount calculation results.
     */
    public static class DiscountResult {
        private final double discountAmount;
        private final String description;

        public DiscountResult(double discountAmount, String description) {
            if (discountAmount < 0) {
                throw new IllegalArgumentException("Discount amount must be greater than 0");
            }
            this.discountAmount = discountAmount;
            this.description = Objects.requireNonNullElse(description, "");
        }

        public double getDiscountAmount() {
            return discountAmount;
        }

        public String getDescription() {
            return description;
        }
    }

    /**
     * @param product - the product being purchased
     * @param quantity - quantity being purchased
     * @param discountType - type of discount to apply (STUDENT, BULK, NONE)
     * @return DiscountResult with amount and description
     */
    public static DiscountResult calculateDiscount(Product product, int quantity, String discountType) {

        double discountAmount = 0.0;
        String description = "No discount applied";

        switch (discountType.toUpperCase()) {

            case "STUDENT":
                if (isEligibleForStudentDiscount(product)) {
                    discountAmount = product.getPrice() * quantity * 0.10;
                    description = "Student discount: 10% off books";
                } else {
                    description = "Student discount only applies to books";
                }
                break;

            case "BULK":
                if (isEligibleForBulkDiscount(quantity)) {
                    discountAmount = product.getPrice() * quantity * 0.15;
                    description = "Bulk discount: 15% off for 5+ items";
                } else {
                    description = "Bulk discount requires 5+ items";
                }
                break;

            case "EMPLOYEE":
                discountAmount = product.getPrice() * quantity * 0.20;
                description = "Employee discount: 20% off";
                break;

            case "NONE":
            default:
                // No discount
                break;
        }

        return new DiscountResult(discountAmount, description);
    }

    private static boolean isEligibleForStudentDiscount(Product product) {
        return BOOK_TYPE.equals(product.getType());
    }

    private static boolean isEligibleForBulkDiscount(int quantity) {
        return quantity >= 5;
    }
}