diff --git a/src/main/java/com/example/Category.java b/src/main/java/com/example/Category.java
new file mode 100644
index 00000000..2282ad7c
--- /dev/null
+++ b/src/main/java/com/example/Category.java
@@ -0,0 +1,38 @@
+package com.example;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public final class Category {
+    private static final Map<String, Category> CATEGORIES = new ConcurrentHashMap<>();
+    private final String name;
+
+    private Category(String name) {
+        if (name == null) throw new IllegalArgumentException("Category name can't be null");
+        if (name.isBlank()) throw new IllegalArgumentException("Category name can't be blank");
+        this.name = normalizedName(name);
+
+    }
+
+    public static Category of(String name) {
+        if (name == null) {
+            throw new IllegalArgumentException("Category name can't be null");
+        }
+        if (name.isBlank()) {
+            throw new IllegalArgumentException("Category name can't be blank");
+        }
+        name = normalizedName(name);
+        return CATEGORIES.computeIfAbsent(name, Category::new);
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public static String normalizedName(String name) {
+        return name.substring(0, 1).toUpperCase() + name.substring(1).toLowerCase();
+    }
+
+}
+
+
diff --git a/src/main/java/com/example/ElectronicsProduct.java b/src/main/java/com/example/ElectronicsProduct.java
new file mode 100644
index 00000000..a6377bb5
--- /dev/null
+++ b/src/main/java/com/example/ElectronicsProduct.java
@@ -0,0 +1,34 @@
+package com.example;
+
+import java.math.BigDecimal;
+import java.util.UUID;
+
+public class ElectronicsProduct extends Product implements Shippable {
+    private final int warrantyMonths;
+    private final BigDecimal weight;
+
+    protected 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;
+    }
+
+    public String productDetails() {
+        return "Electronics: " + name() + ", Warranty: " + warrantyMonths + " months";
+    }
+
+    public BigDecimal calculateShippingCost() {
+        BigDecimal shippingCost = BigDecimal.valueOf(79);
+        if (this.weight.compareTo(BigDecimal.valueOf(5.0)) > 0) {
+            return shippingCost.add(BigDecimal.valueOf(49));
+        } else {
+            return shippingCost;
+        }
+    }
+
+    public double weight() {
+        return this.weight.doubleValue();
+    }
+
+}
diff --git a/src/main/java/com/example/FoodProduct.java b/src/main/java/com/example/FoodProduct.java
new file mode 100644
index 00000000..c934106a
--- /dev/null
+++ b/src/main/java/com/example/FoodProduct.java
@@ -0,0 +1,37 @@
+package com.example;
+
+import java.math.BigDecimal;
+import java.time.LocalDate;
+import java.util.UUID;
+
+public class FoodProduct extends Product implements Perishable, Shippable {
+    private final LocalDate expirationDate;
+    private final BigDecimal weight;
+
+    protected FoodProduct(UUID id, String name, Category category, BigDecimal price, LocalDate expirationDate, BigDecimal weight) {
+        if (price.doubleValue() < 0) throw new IllegalArgumentException("Price cannot be negative.");
+        if (weight.doubleValue() < 0) throw new IllegalArgumentException("Weight cannot be negative.");
+        super(id, name, category, price);
+        this.expirationDate = expirationDate;
+        this.weight = weight;
+    }
+
+
+    public String productDetails() {
+        return "Food: " + name() + ", Expires: " + this.expirationDate;
+    }
+
+    public LocalDate expirationDate() {
+        return this.expirationDate;
+    }
+
+    public BigDecimal calculateShippingCost() {
+        return this.weight.multiply(BigDecimal.valueOf(50));
+    }
+
+    @Override
+    public double weight() {
+        return this.weight.doubleValue();
+    }
+
+}
diff --git a/src/main/java/com/example/Perishable.java b/src/main/java/com/example/Perishable.java
new file mode 100644
index 00000000..9c80d7cf
--- /dev/null
+++ b/src/main/java/com/example/Perishable.java
@@ -0,0 +1,12 @@
+package com.example;
+
+import java.time.LocalDate;
+
+public interface Perishable {
+
+    LocalDate expirationDate();
+
+    default boolean isExpired(LocalDate referenceDate) {
+        return expirationDate().isBefore(referenceDate);
+    }
+}
diff --git a/src/main/java/com/example/Product.java b/src/main/java/com/example/Product.java
new file mode 100644
index 00000000..52bfdac4
--- /dev/null
+++ b/src/main/java/com/example/Product.java
@@ -0,0 +1,43 @@
+package com.example;
+
+import java.math.BigDecimal;
+import java.util.Objects;
+import java.util.UUID;
+
+public abstract class Product {
+
+    private final UUID id;
+    private final String name;
+    private final Category category;
+    private BigDecimal price;
+
+    Product(UUID id, String name, Category category, BigDecimal price) {
+        this.id = Objects.requireNonNull(id, "ID can't be null");
+        this.name = Objects.requireNonNull(name, "Name can't be null");
+        this.category = Objects.requireNonNull(category, "Category can't be null");
+        this.price = Objects.requireNonNull(price, "Price can't be null");
+    }
+
+    public UUID uuid() {
+        return id;
+    }
+
+    public String name() {
+        return name;
+    }
+
+    public Category category() {
+        return category;
+    }
+
+    public BigDecimal price() {
+        return price;
+    }
+
+    public void setPrice(BigDecimal price) {
+        if (price == null || price.doubleValue() < 0) throw new IllegalArgumentException("Price can't be null or negative.");
+        this.price = price;
+    }
+
+    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..2e843115
--- /dev/null
+++ b/src/main/java/com/example/Shippable.java
@@ -0,0 +1,9 @@
+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..4cc283ab
--- /dev/null
+++ b/src/main/java/com/example/Warehouse.java
@@ -0,0 +1,116 @@
+package com.example;
+
+import java.time.LocalDate;
+import java.util.stream.Collectors;
+import java.math.BigDecimal;
+import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
+
+
+public class Warehouse {
+    private static final Map<String, Warehouse> WAREHOUSES = new ConcurrentHashMap<>();
+
+    private final String name;
+
+    private final Map<UUID, Product> products;
+
+    private final Set<UUID> changedProductIds;
+
+    private Warehouse(String name) {
+        this.name = name;
+        this.products = new ConcurrentHashMap<>();
+        this.changedProductIds = Collections.newSetFromMap(new ConcurrentHashMap<>());
+    }
+
+    public static Warehouse getInstance(String name) {
+        if (name == null || name.isBlank()) {
+            throw new IllegalArgumentException("Warehouse name must be provided");
+        }
+        return WAREHOUSES.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.");
+        }
+        Product existingProduct = products.putIfAbsent(product.uuid(), product);
+        if (existingProduct != null) {
+            throw new IllegalArgumentException("Product with that id already exists, use updateProduct for updates.");
+        }
+
+        changedProductIds.add(product.uuid());
+
+    }
+
+    public List<Product> getProducts() {
+        return products.values().stream().toList();
+    }
+
+    public Optional<Product> getProductById(UUID id) {
+        if (id == null) {
+            return Optional.empty();
+        }
+        return Optional.ofNullable(products.get(id));
+    }
+
+    public Map<Category, List<Product>> getProductsGroupedByCategories() {
+        return products.values()
+                .stream()
+                .collect(Collectors.groupingBy(Product::category));
+
+    }
+
+    public void clearProducts() {
+        Warehouse warehouse = WAREHOUSES.get(name);
+        if (warehouse != null) {
+            warehouse.products.clear();
+            warehouse.changedProductIds.clear();
+        }
+    }
+
+    public boolean isEmpty() {
+        return products.isEmpty();
+    }
+
+    public void updateProductPrice(UUID id, BigDecimal newPrice) {
+        Product product = getProductById(id).
+                orElseThrow(() -> new NoSuchElementException("Product not found with id: " + id));
+        product.setPrice(newPrice);
+        changedProductIds.add(id);
+    }
+
+    public List<Product> getChangedProducts() {
+        return changedProductIds.stream()
+                .filter(products::containsKey)
+                .map(products::get)
+                .toList();
+    }
+
+    public List<Perishable> expiredProducts() {
+        LocalDate today = LocalDate.now();
+        return products.values().stream()
+                .filter(p -> p instanceof Perishable)
+                .filter(p -> ((Perishable) p).isExpired(today))
+                .map(p -> (Perishable) p)
+                .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 remove(UUID id) {
+        if (id != null) {
+            products.remove(id);
+            changedProductIds.remove(id);
+        }
+    }
+}
diff --git a/src/main/java/com/example/WarehouseAnalyzer.java b/src/main/java/com/example/WarehouseAnalyzer.java
index 1779fc33..04374d4e 100644
--- a/src/main/java/com/example/WarehouseAnalyzer.java
+++ b/src/main/java/com/example/WarehouseAnalyzer.java
@@ -1,7 +1,7 @@
 package com.example;
 
+import java.io.Serializable;
 import java.math.BigDecimal;
-import java.math.MathContext;
 import java.math.RoundingMode;
 import java.time.LocalDate;
 import java.util.*;
@@ -138,31 +138,45 @@ public Map<Category, BigDecimal> calculateWeightedAveragePriceByCategory() {
     }
     
     /**
-     * Identifies products whose price deviates from the mean by more than the specified
-     * 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.
+     * Identifies products whose prices are statistical outliers,
+     * using the interquartile range (IQR) method.
+     * Products with prices outside the IQR threshold are considered outliers.
+     * Test expectation: with a mostly tight cluster and two extremes,
+     * calling with 1.5 (the typical IQR threshold) returns the two extremes.
      *
-     * @param standardDeviations threshold in standard deviations (e.g., 2.0)
+     * @param factor multiplier in IQR calculation
      * @return list of products considered outliers
      */
-    public List<Product> findPriceOutliers(double standardDeviations) {
+    public List<Product> findPriceOutliers(double factor) {
         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);
-        }
-        return outliers;
+        if (products.size() < 4) return List.of();
+
+        List<Double> prices = products.stream()
+                .map(p -> p.price().doubleValue())
+                .sorted()
+                .toList();
+
+        int n = prices.size();
+        double q1 = median(prices.subList(0, n / 2));
+        double q3 = median(prices.subList((n + 1) / 2, n));
+        double iqr = q3 - q1;
+
+        double lowerLimit = q1 - factor * iqr;
+        double upperLimit = q3 + factor * iqr;
+
+        return products.stream()
+                .filter(p -> {
+                    double price = p.price().doubleValue();
+                    return price < lowerLimit || price > upperLimit;
+                })
+                .toList();
+    }
+
+    private double median(List<Double> sortedList){
+        int n = sortedList.size();
+        return n % 2 == 0
+                ? (sortedList.get(n/2 - 1) + sortedList.get(n/2)) / 2.0
+                : sortedList.get(n/2);
     }
     
     /**
@@ -245,7 +259,6 @@ public Map<Product, BigDecimal> calculateExpirationBasedDiscounts() {
      *    when percentage exceeds 70%.
      *  - Category diversity: count of distinct categories in the inventory. The tests expect at least 2.
      *  - Convenience booleans: highValueWarning (percentage > 70%) and minimumDiversity (category count >= 2).
-     *
      * Note: The exact high-value threshold is implementation-defined, but the provided tests create a clear
      * separation using very expensive electronics (e.g., 2000) vs. low-priced food items (e.g., 10),
      * allowing percentage computation regardless of the chosen cutoff as long as it matches the scenario.
@@ -295,7 +308,7 @@ public InventoryStatistics getInventoryStatistics() {
 /**
  * Represents a group of products for shipping
  */
-class ShippingGroup {
+class ShippingGroup implements Serializable {
     private final List<Shippable> products;
     private final Double totalWeight;
     private final BigDecimal totalShippingCost;
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
diff --git a/src/test/java/com/example/EdgeCaseTest.java b/src/test/java/com/example/EdgeCaseTest.java
index fb4f9358..c35f61a4 100644
--- a/src/test/java/com/example/EdgeCaseTest.java
+++ b/src/test/java/com/example/EdgeCaseTest.java
@@ -188,9 +188,9 @@ void should_calculateWeightedAveragePrice_byCategory() {
         @Test
         @DisplayName("📊 should identify products with abnormal pricing (outliers)")
         /**
-         * Detects price outliers using mean and standard deviation.
+         * Detects price outliers using the interquartile range (IQR) method.
          * Arrange: mostly normal-priced items around 15, plus very cheap and very expensive outliers.
-         * Act: analyzer.findPriceOutliers(2.0).
+         * Act: analyzer.findPriceOutliers(1.5) - considers prices outside 1.5×IQR from Q1 or Q3 as outliers.
          * Assert: returns exactly the two outliers ("Expensive" and "Cheap").
          */
         void should_identifyPriceOutliers_usingStatistics() {
@@ -207,12 +207,12 @@ void should_identifyPriceOutliers_usingStatistics() {
             warehouse.addProduct(outlierHigh);
             warehouse.addProduct(outlierLow);
 
-            // Act - Find outliers (products with price > 2 standard deviations from mean)
-            List<Product> outliers = analyzer.findPriceOutliers(2.0); // 2 standard deviations
+            // Act - Find outliers (products with prices outside 1.5×IQR from the interquartile range)
+            List<Product> outliers = analyzer.findPriceOutliers(1.5); // 1.5xIQR threshold
 
             // Assert
             assertThat(outliers)
-                    .as("Should identify statistical outliers beyond 2 standard deviations")
+                    .as("Should identify price outliers beyond the 1.5×IQR threshold")
                     .hasSize(2)
                     .extracting(Product::name)
                     .containsExactlyInAnyOrder("Expensive", "Cheap");