Add InventoryManagementSystem app

InventoryManagementSystem/src/main/java/inventory/DiscountCalculator.java

@@ -0,0 +1,93 @@
+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;
+    }
+}