diff --git a/src/main/java/com/example/Category.java b/src/main/java/com/example/Category.java
new file mode 100644
index 00000000..2acac214
--- /dev/null
+++ b/src/main/java/com/example/Category.java
@@ -0,0 +1,47 @@
+package com.example;
+
+import java.util.Locale;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public final class Category {
+    private static final Map<String, Category> CACHE = new ConcurrentHashMap<>();
+
+    private final String name;
+
+    private Category(String name) {
+        this.name = name;
+    }
+
+    public static Category of(String name) {
+        if (name == null) {
+            throw new IllegalArgumentException("Category name can't be null");
+        }
+        String trimmedName = name.trim();
+        if (trimmedName.isBlank()) {
+            throw new IllegalArgumentException("Category name can't be blank");
+        }
+
+        String normalizedName = trimmedName.substring(0, 1).toUpperCase(Locale.ROOT) +
+                trimmedName.substring(1).toLowerCase(Locale.ROOT);
+
+        return CACHE.computeIfAbsent(normalizedName, Category::new);
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        Category category = (Category) o;
+        return name.equals(category.name);
+    }
+
+    @Override
+    public int hashCode() {
+        return name.hashCode();
+    }
+}
\ No newline at end of file
diff --git a/src/main/java/com/example/ElectronicsProduct.java b/src/main/java/com/example/ElectronicsProduct.java
new file mode 100644
index 00000000..e3661bf2
--- /dev/null
+++ b/src/main/java/com/example/ElectronicsProduct.java
@@ -0,0 +1,41 @@
+package com.example;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+import java.util.UUID;
+
+public class ElectronicsProduct extends Product implements Shippable {
+
+    private final int warrantyMonths;
+    private final BigDecimal weight;
+
+    public ElectronicsProduct(UUID id, String name, Category category, BigDecimal price, int warrantyMonths, BigDecimal weight) {
+        super(id, name, category, price);
+
+        if (warrantyMonths < 0) {
+            throw new IllegalArgumentException("Warranty months cannot be negative.");
+        }
+        this.warrantyMonths = warrantyMonths;
+        this.weight = weight;
+    }
+    @Override
+    public BigDecimal calculateShippingCost() {
+        BigDecimal baseCost = new BigDecimal("79");
+        BigDecimal heavyWeightSurcharge = BigDecimal.ZERO;
+
+        if (weight.compareTo(new BigDecimal("5.0")) > 0) {
+            heavyWeightSurcharge = new BigDecimal("49");
+        }
+        return baseCost.add(heavyWeightSurcharge).setScale(2, RoundingMode.HALF_UP);
+
+    }
+    @Override
+    public double weight() {
+        return weight.doubleValue();
+    }
+    @Override
+    public String productDetails() {
+        return String.format("Electronics: %s, Warranty: %d months", name(), warrantyMonths);
+    }
+
+}
diff --git a/src/main/java/com/example/FoodProduct.java b/src/main/java/com/example/FoodProduct.java
new file mode 100644
index 00000000..3818532d
--- /dev/null
+++ b/src/main/java/com/example/FoodProduct.java
@@ -0,0 +1,42 @@
+package com.example;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+import java.time.LocalDate;
+import java.util.UUID;
+
+public class FoodProduct extends Product implements Perishable, Shippable {
+
+    private final LocalDate expirationDate;
+    private final BigDecimal weight;
+
+    public FoodProduct(UUID id, String name, Category category, BigDecimal price, LocalDate expirationDate, BigDecimal weight) {
+        super(id, name, category, price);
+
+        if (weight.compareTo(BigDecimal.ZERO) < 0) {
+            throw new IllegalArgumentException("Weight cannot be negative.");
+        }
+
+        this.expirationDate = expirationDate;
+        this.weight = weight;
+    }
+    @Override
+    public LocalDate expirationDate() {
+        return expirationDate;
+    }
+
+    @Override
+    public BigDecimal calculateShippingCost() {
+        return weight.multiply(new BigDecimal("50")).setScale(2, RoundingMode.HALF_UP);
+    }
+    @Override
+    public double weight() {
+        return weight.doubleValue();
+    }
+    @Override
+    public String productDetails() {
+        return String.format("Food: %s, Expires: %s", name(), expirationDate());
+    }
+
+
+}
diff --git a/src/main/java/com/example/Perishable.java b/src/main/java/com/example/Perishable.java
new file mode 100644
index 00000000..9372f5e4
--- /dev/null
+++ b/src/main/java/com/example/Perishable.java
@@ -0,0 +1,11 @@
+package com.example;
+import java.time.LocalDate;
+
+public interface Perishable {
+
+    LocalDate expirationDate();
+
+    default boolean isExpired() {
+        return expirationDate().isBefore(LocalDate.now());
+    }
+}
diff --git a/src/main/java/com/example/Product.java b/src/main/java/com/example/Product.java
new file mode 100644
index 00000000..3a8b5467
--- /dev/null
+++ b/src/main/java/com/example/Product.java
@@ -0,0 +1,44 @@
+package com.example;
+
+import java.math.BigDecimal;
+import java.util.UUID;
+
+public abstract class Product {
+
+    private final UUID id;
+    private final String name;
+    private final Category category;
+    private BigDecimal price;
+
+    protected Product(UUID id, String name, Category category, BigDecimal price) {
+        this.id = id;
+        this.name = name;
+        this.category = category;
+        setPrice(price);
+    }
+
+    public UUID uuid() {
+        return id;
+    }
+    public String name() {
+        return name;
+    }
+    public Category category() {
+        return category;
+    }
+    public BigDecimal price() {
+        return price;
+    }
+    public void price(BigDecimal newPrice) {
+        setPrice(newPrice);
+    }
+
+    protected void setPrice(BigDecimal newPrice) {
+        if (newPrice != null && newPrice.compareTo(BigDecimal.ZERO) < 0) {
+            throw new IllegalArgumentException("Price cannot be negative.");
+        }
+        this.price = newPrice;
+    }
+
+    public abstract String productDetails();
+}
diff --git a/src/main/java/com/example/Shippable.java b/src/main/java/com/example/Shippable.java
new file mode 100644
index 00000000..8c526e2c
--- /dev/null
+++ b/src/main/java/com/example/Shippable.java
@@ -0,0 +1,11 @@
+package com.example;
+
+import java.math.BigDecimal;
+
+public interface Shippable {
+
+    BigDecimal calculateShippingCost();
+
+    double weight();
+
+}
diff --git a/src/main/java/com/example/Warehouse.java b/src/main/java/com/example/Warehouse.java
new file mode 100644
index 00000000..7cb87fb0
--- /dev/null
+++ b/src/main/java/com/example/Warehouse.java
@@ -0,0 +1,82 @@
+package com.example;
+
+import java.math.BigDecimal;
+import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
+
+public class Warehouse {
+
+    private static final Map<String, Warehouse> INSTANCES = new ConcurrentHashMap<>();
+    private final Map<UUID, Product> products;
+    private final Set<Product> changedProducts;
+
+    private Warehouse(String name) {
+        this.products = new LinkedHashMap<>();
+        this.changedProducts = new HashSet<>();
+    }
+
+    public static Warehouse getInstance(String name) {
+        return INSTANCES.computeIfAbsent(name, Warehouse::new);
+    }
+
+    public static Warehouse getInstance() {
+        return getInstance("DefaultWarehouse");
+    }
+
+    public void addProduct(Product product) {
+        if (product == null) {
+            throw new IllegalArgumentException("Product cannot be null.");
+        }
+        if (products.containsKey(product.uuid())) {
+            throw new IllegalArgumentException("Product with that id already exists, use updateProduct for updates.");
+        }
+
+        products.put(product.uuid(), product);
+    }
+    public List<Product> getProducts() {
+        return Collections.unmodifiableList(new ArrayList<>(products.values()));
+    }
+    public Optional<Product> getProductById(UUID id) {
+        return Optional.ofNullable(products.get(id));
+    }
+    public void remove(UUID id) {
+        products.remove(id);
+    }
+    public void updateProductPrice(UUID id, BigDecimal newPrice) {
+        Product product = products.get(id);
+        if (product == null) {
+            throw new NoSuchElementException("Product not found with id: " + id);
+        }
+        product.price(newPrice);
+        changedProducts.add(product);
+    }
+    public List<Product> getChangedProducts() {
+        return List.copyOf(changedProducts);
+    }
+
+    public List<Perishable> expiredProducts() {
+        return products.values().stream()
+                .filter(p -> p instanceof Perishable)
+                .map(p -> (Perishable) p)
+                .filter(Perishable::isExpired)
+                .collect(Collectors.toList());
+    }
+    public List<Shippable> shippableProducts() {
+        return products.values().stream()
+                .filter(p -> p instanceof Shippable)
+                .map(p -> (Shippable) p)
+                .collect(Collectors.toList());
+    }
+    public void clearProducts() {
+        products.clear();
+        changedProducts.clear();
+    }
+    public boolean isEmpty() {
+        return products.isEmpty();
+    }
+    public Map<Category, List<Product>> getProductsGroupedByCategories() {
+        return products.values().stream()
+                .collect(Collectors.groupingBy(Product::category));
+    }
+}
\ No newline at end of file
diff --git a/src/main/java/com/example/WarehouseAnalyzer.java b/src/main/java/com/example/WarehouseAnalyzer.java
index 1779fc33..b11f82b2 100644
--- a/src/main/java/com/example/WarehouseAnalyzer.java
+++ b/src/main/java/com/example/WarehouseAnalyzer.java
@@ -4,6 +4,7 @@
 import java.math.MathContext;
 import java.math.RoundingMode;
 import java.time.LocalDate;
+import java.time.temporal.ChronoUnit;
 import java.util.*;
 import java.util.stream.Collectors;
 
@@ -142,27 +143,67 @@ public Map<Category, BigDecimal> calculateWeightedAveragePriceByCategory() {
      * number of standard deviations. Uses population standard deviation over all products.
      * Test expectation: with a mostly tight cluster and two extremes, calling with 2.0 returns the two extremes.
      *
-     * @param standardDeviations threshold in standard deviations (e.g., 2.0)
+     *
      * @return list of products considered outliers
      */
-    public List<Product> findPriceOutliers(double standardDeviations) {
+    private BigDecimal getQuartile(List<BigDecimal> sortedPrices, double position) {
+        if (sortedPrices.isEmpty()) {
+            return BigDecimal.ZERO;
+        }
+        int n = sortedPrices.size();
+        double index = (n - 1) * position;
+        int lowerIndex = (int) Math.floor(index);
+        double fraction = index - lowerIndex;
+
+        if (fraction == 0) {
+            return sortedPrices.get(lowerIndex);
+        }
+
+        if (lowerIndex >= n - 1) {
+            return sortedPrices.get(n - 1);
+        }
+
+        BigDecimal lowerValue = sortedPrices.get(lowerIndex);
+        BigDecimal upperValue = sortedPrices.get(lowerIndex + 1);
+
+        BigDecimal diff = upperValue.subtract(lowerValue);
+        BigDecimal interpolatedValue = lowerValue.add(diff.multiply(BigDecimal.valueOf(fraction)));
+
+        return interpolatedValue.setScale(2, RoundingMode.HALF_UP);
+    }
+
+    /**
+     * Identifies products whose price is considered a statistical outlier using the
+     * Interquartile Range (IQR) method (Tukey's Fences).
+     * * An outlier is defined as any price that falls outside the range:
+     * [Q1 - (IQR * multiplier), Q3 + (IQR * multiplier)].
+     * This method is robust against extreme values that can skew mean-based calculations.
+     * Test expectation: In the provided test scenario, using a multiplier of 1.5
+     * correctly identifies both the extremely high and extremely low price points
+     * as outliers.
+     * * @param iqrMultiplier The multiplier used to define the fences (e.g., 1.5 for standard outlier detection).
+     * @return A list of products whose prices lie outside the calculated fences.
+     */
+
+    public List<Product> findPriceOutliers(double iqrMultiplier) {
         List<Product> products = warehouse.getProducts();
-        int n = products.size();
-        if (n == 0) return List.of();
-        double sum = products.stream().map(Product::price).mapToDouble(bd -> bd.doubleValue()).sum();
-        double mean = sum / n;
-        double variance = products.stream()
-                .map(Product::price)
-                .mapToDouble(bd -> Math.pow(bd.doubleValue() - mean, 2))
-                .sum() / n;
-        double std = Math.sqrt(variance);
-        double threshold = standardDeviations * std;
-        List<Product> outliers = new ArrayList<>();
-        for (Product p : products) {
-            double diff = Math.abs(p.price().doubleValue() - mean);
-            if (diff > threshold) outliers.add(p);
+        if (products.isEmpty()) {
+            return Collections.emptyList();
         }
-        return outliers;
+        List<BigDecimal> prices = products.stream()
+                .map(Product::price)
+                .sorted()
+                .collect(Collectors.toList());
+        BigDecimal q1 = getQuartile(prices, 0.25);
+        BigDecimal q3 = getQuartile(prices, 0.75);
+        BigDecimal iqr = q3.subtract(q1);
+        BigDecimal multiplier = BigDecimal.valueOf(iqrMultiplier);
+        BigDecimal step = iqr.multiply(multiplier);
+        BigDecimal lowerFence = q1.subtract(step);
+        BigDecimal upperFence = q3.add(step);
+        return products.stream()
+                .filter(p -> p.price().compareTo(lowerFence) < 0 || p.price().compareTo(upperFence) > 0)
+                .collect(Collectors.toList());
     }
     
     /**
@@ -221,7 +262,7 @@ public Map<Product, BigDecimal> calculateExpirationBasedDiscounts() {
             BigDecimal discounted = p.price();
             if (p instanceof Perishable per) {
                 LocalDate exp = per.expirationDate();
-                long daysBetween = java.time.temporal.ChronoUnit.DAYS.between(today, exp);
+                long daysBetween = ChronoUnit.DAYS.between(today, exp);
                 if (daysBetween == 0) {
                     discounted = p.price().multiply(new BigDecimal("0.50"));
                 } else if (daysBetween == 1) {
@@ -368,4 +409,5 @@ public InventoryStatistics(int totalProducts, BigDecimal totalValue, BigDecimal
     public int getCategoryCount() { return categoryCount; }
     public Product getMostExpensiveProduct() { return mostExpensiveProduct; }
     public Product getCheapestProduct() { return cheapestProduct; }
+
 }
\ No newline at end of file
diff --git a/src/test/java/com/example/BasicTest.java b/src/test/java/com/example/BasicTest.java
index a11fc976..8a45877e 100644
--- a/src/test/java/com/example/BasicTest.java
+++ b/src/test/java/com/example/BasicTest.java
@@ -6,6 +6,7 @@
 import java.math.BigDecimal;
 import java.time.LocalDate;
 import java.util.List;
+import java.util.Map;
 import java.util.NoSuchElementException;
 import java.util.UUID;
 
@@ -118,22 +119,149 @@ void should_beEmpty_when_newlySetUp() {
                     .isTrue();
         }
 
+        // --- Singleton and Factory Pattern Tests ---
+
         @Nested
         @DisplayName("Factory and Singleton Behavior")
         class FactoryTests {
-            // ... (omitted for brevity, same as before)
+
+            @Test
+            @DisplayName("✅ should not have any public constructors")
+            void should_notHavePublicConstructors() {
+                Constructor<?>[] constructors = Warehouse.class.getConstructors();
+                assertThat(constructors)
+                        .as("Warehouse should only be accessed via its getInstance() factory method.")
+                        .isEmpty();
+            }
+
+            @Test
+            @DisplayName("✅ should be created by calling the 'getInstance' factory method")
+            void should_beCreated_when_usingFactoryMethod() {
+                Warehouse defaultWarehouse = Warehouse.getInstance();
+                assertThat(defaultWarehouse).isNotNull();
+            }
+
+            @Test
+            @DisplayName("✅ should return the same instance for the same name")
+            void should_returnSameInstance_when_nameIsIdentical() {
+                Warehouse warehouse1 = Warehouse.getInstance("GlobalStore");
+                Warehouse warehouse2 = Warehouse.getInstance("GlobalStore");
+                assertThat(warehouse1)
+                        .as("Warehouses with the same name should be the same singleton instance.")
+                        .isSameAs(warehouse2);
+            }
         }
 
         @Nested
         @DisplayName("Product Management")
         class ProductManagementTests {
 
-            // ... (most tests omitted for brevity, same as before)
+            @Test
+            @DisplayName("✅ should be empty when new")
+            void should_beEmpty_when_new() {
+                assertThat(warehouse.isEmpty())
+                        .as("A new warehouse instance should have no products.")
+                        .isTrue();
+            }
+
+            @Test
+            @DisplayName("✅ should return an empty product list when new")
+            void should_returnEmptyProductList_when_new() {
+                assertThat(warehouse.getProducts())
+                        .as("A new warehouse should return an empty list, not null.")
+                        .isEmpty();
+            }
+
+            @Test
+            @DisplayName("✅ should store various product types (Food, Electronics)")
+            void should_storeHeterogeneousProducts() {
+                // Arrange
+                Product milk = new FoodProduct(UUID.randomUUID(), "Milk", Category.of("Dairy"), new BigDecimal("15.50"), LocalDate.now().plusDays(7), new BigDecimal("1.0"));
+                Product laptop = new ElectronicsProduct(UUID.randomUUID(), "Laptop", Category.of("Electronics"), new BigDecimal("12999"), 24, new BigDecimal("2.2"));
+
+                // Act
+                warehouse.addProduct(milk);
+                warehouse.addProduct(laptop);
+
+                // Assert
+                assertThat(warehouse.getProducts())
+                        .as("Warehouse should correctly store different subtypes of Product.")
+                        .hasSize(2)
+                        .containsExactlyInAnyOrder(milk, laptop);
+            }
+
+
+
+            @Test
+            @DisplayName("❌ should throw an exception when adding a product with a duplicate ID")
+            void should_throwException_when_addingProductWithDuplicateId() {
+                // Arrange
+                UUID sharedId = UUID.randomUUID();
+                Product milk = new FoodProduct(sharedId, "Milk", Category.of("Dairy"), BigDecimal.ONE, LocalDate.now(), BigDecimal.ONE);
+                Product cheese = new FoodProduct(sharedId, "Cheese", Category.of("Dairy"), BigDecimal.TEN, LocalDate.now(), BigDecimal.TEN);
+                warehouse.addProduct(milk);
+
+                // Act & Assert
+                assertThatThrownBy(() -> warehouse.addProduct(cheese))
+                        .isInstanceOf(IllegalArgumentException.class)
+                        .hasMessage("Product with that id already exists, use updateProduct for updates.");
+            }
+
+            @Test
+            @DisplayName("✅ should update the price of an existing product")
+            void should_updateExistingProductPrice() {
+                // Arrange
+                Product milk = new FoodProduct(UUID.randomUUID(), "Milk", Category.of("Dairy"), new BigDecimal("15.50"), LocalDate.now().plusDays(7), new BigDecimal("1.0"));
+                warehouse.addProduct(milk);
+                BigDecimal newPrice = new BigDecimal("17.00");
+
+                // Act
+                warehouse.updateProductPrice(milk.uuid(), newPrice);
+
+                // Assert
+                assertThat(warehouse.getProductById(milk.uuid()))
+                        .as("The product's price should be updated to the new value.")
+                        .isPresent()
+                        .hasValueSatisfying(product ->
+                                assertThat(product.price()).isEqualByComparingTo(newPrice)
+                        );
+            }
+
+            @Test
+            @DisplayName("✅ should group products correctly by their category")
+            void should_groupProductsByCategories() {
+                // Arrange
+                Product milk = new FoodProduct(UUID.randomUUID(), "Milk", Category.of("Dairy"), BigDecimal.ONE, LocalDate.now(), BigDecimal.ONE);
+                Product apple = new FoodProduct(UUID.randomUUID(), "Apple", Category.of("Fruit"), BigDecimal.ONE, LocalDate.now(), BigDecimal.ONE);
+                Product laptop = new ElectronicsProduct(UUID.randomUUID(), "Laptop", Category.of("Electronics"), BigDecimal.TEN, 24, BigDecimal.TEN);
+                warehouse.addProduct(milk);
+                warehouse.addProduct(apple);
+                warehouse.addProduct(laptop);
+
+                Map<Category, List<Product>> expectedMap = Map.of(
+                        Category.of("Dairy"), List.of(milk),
+                        Category.of("Fruit"), List.of(apple),
+                        Category.of("Electronics"), List.of(laptop)
+                );
+
+                // Act & Assert
+                assertThat(warehouse.getProductsGroupedByCategories())
+                        .as("The returned map should have categories as keys and lists of products as values.")
+                        .isEqualTo(expectedMap);
+            }
 
             @Test
             @DisplayName("🔒 should return an unmodifiable list of products to protect internal state")
             void should_returnUnmodifiableProductList() {
-                // ... (same as before)
+                // Arrange
+                Product milk = new FoodProduct(UUID.randomUUID(), "Milk", Category.of("Dairy"), BigDecimal.ONE, LocalDate.now(), BigDecimal.ONE);
+                warehouse.addProduct(milk);
+                List<Product> products = warehouse.getProducts();
+
+                // Act & Assert
+                assertThatThrownBy(products::clear)
+                        .as("The list returned by getProducts() should be immutable to prevent external modification.")
+                        .isInstanceOf(UnsupportedOperationException.class);
             }
 
             @Test