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src/main/java/com/example/Category.java

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+package com.example;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.Objects;
+
+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 trimmed = name.trim();
+        if (trimmed.isEmpty())
+            throw new IllegalArgumentException("Category name can't be blank");
+
+        String normalized = name.trim();
+        normalized = normalized.substring(0, 1).toUpperCase() + normalized.substring(1).toLowerCase();
+        return CACHE.computeIfAbsent(normalized, Category::new);
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public String name() {
+        return name;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (!(o instanceof Category)) return false;
+        Category c = (Category) o;
+        return name.equals(c.name);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(name);
+    }
+
+    @Override
+    public String toString() {
+        return name;
+    }
+}

src/main/java/com/example/ElectronicsProduct.java

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+package com.example;
+
+import java.math.BigDecimal;
+import java.util.Objects;
+import java.util.UUID;
+
+/**
+ * ElectronicsProduct: shippable product with warranty.
+ */
+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.");
+        if (weight.compareTo(BigDecimal.ZERO) < 0) throw new IllegalArgumentException("Weight months cannot be negative.");
+
+        this.warrantyMonths = warrantyMonths;
+        this.weight = Objects.requireNonNull(weight, "Weight cannot be null");
+    }
+
+    @Override
+    public String productDetails() {
+        return "Electronics: %s, Warranty: %d months".formatted(name(), warrantyMonths);
+    }
+
+    @Override
+    public Double weight() {
+        return weight.doubleValue();
+    }
+
+    @Override
+    public BigDecimal calculateShippingCost() {
+        BigDecimal base = BigDecimal.valueOf(79);
+        if (weight.doubleValue() > 5.0) base = base.add(BigDecimal.valueOf(49));
+        return base;
+    }
+
+}

src/main/java/com/example/FoodProduct.java

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+package com.example;
+
+import java.math.BigDecimal;
+import java.time.LocalDate;
+import java.util.Objects;
+import java.util.UUID;
+
+/**
+ * FoodProduct: perishable and shippable.
+ */
+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 (price.compareTo(BigDecimal.ZERO) < 0) throw new IllegalArgumentException("Price cannot be negative.");
+        if (weight.compareTo(BigDecimal.ZERO) < 0) throw new IllegalArgumentException("Weight cannot be negative.");
+
+        this.expirationDate = Objects.requireNonNull(expirationDate, "Expiration date cannot be null");
+        this.weight = Objects.requireNonNull(weight, "Weight cannot be null");
+    }
+
+    @Override
+    public String productDetails() {
+        return "Food: %s, Expires: %s".formatted(name(), expirationDate);
+    }
+
+    @Override
+    public LocalDate expirationDate() {
+        return expirationDate;
+    }
+
+    @Override
+    public boolean isExpired() {
+        return expirationDate.isBefore(LocalDate.now());
+    }
+
+    @Override
+    public BigDecimal calculateShippingCost() {
+        return weight.multiply(BigDecimal.valueOf(50)); // 50 kr/kg
+    }
+
+    @Override
+    public Double weight() {
+        return weight.doubleValue();
+    }
+}

src/main/java/com/example/Perishable.java

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+package com.example;
+
+import java.time.LocalDate;
+
+/**
+ * Interface for products with expiration dates.
+ */
+public interface Perishable {
+    LocalDate expirationDate();
+
+    default boolean isExpired() {
+        return expirationDate().isBefore(LocalDate.now());
+    }
+}

src/main/java/com/example/Product.java

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+package com.example;
+
+import java.math.BigDecimal;
+import java.util.UUID;
+
+/**
+ * Base class for all products.
+ * Holds shared attributes and ensures proper validation.
+ */
+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) {
+        if (id == null) throw new IllegalArgumentException("ID cannot be null");
+        if (name == null || name.isBlank()) throw new IllegalArgumentException("Product name cannot be blank");
+        if (category == null) throw new IllegalArgumentException("Category cannot be null");
+        if (price == null) throw new IllegalArgumentException("Price cannot be null");
+
+        this.id = id;
+        this.name = name;
+        this.category = category;
+        this.price = price;
+    }
+
+    // Getters
+    public UUID uuid() {
+        return id;
+    }
+
+    public String name() {
+        return name;
+    }
+
+    public Category category() {
+        return category;
+    }
+
+    public BigDecimal price() {
+        return price;
+    }
+
+    // Setter
+    public void price(BigDecimal newPrice) {
+        if (newPrice == null) throw new IllegalArgumentException("Price cannot be null");
+        this.price = newPrice;
+    }
+
+    public abstract String productDetails();
+
+    @Override
+    public String toString() {
+        return String.format("%s (%s) - %s kr", name, category.getName(), price);
+    }
+}

src/main/java/com/example/Shippable.java

@@ -0,0 +1,11 @@
+package com.example;
+
+import java.math.BigDecimal;
+
+/**
+ * Interface for products that can be shipped.
+ */
+public interface Shippable {
+    Double weight();
+    BigDecimal calculateShippingCost();
+}

src/main/java/com/example/Warehouse.java

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+package com.example;
+
+import java.math.BigDecimal;
+import java.util.*;
+import java.util.stream.Collectors;
+
+/**
+ *  Warehouse managing products.
+ */
+public class Warehouse {
+
+    private static final Map<String, Warehouse> INSTANCES = new HashMap<>();
+    private final String name;
+    private final List<Product> products = new ArrayList<>();
+    private final Set<UUID> changedProducts = new HashSet<>();
+
+    private Warehouse(String name) {
+        this.name = name;
+    }
+
+    public static Warehouse getInstance() {
+        return getInstance("default");
+    }
+
+    public static Warehouse getInstance(String name) {
+        return INSTANCES.computeIfAbsent(name, Warehouse::new);
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void addProduct(Product product) {
+        if (product == null) throw new IllegalArgumentException("Product cannot be null.");
+        if (getProductById(product.uuid()).isPresent())
+            throw new IllegalArgumentException("Product with that id already exists, use updateProduct for updates.");
+
+        products.add(product);
+    }
+
+    public List<Product> getProducts() {
+        return List.copyOf(products);
+    }
+
+    public Optional<Product> getProductById(UUID id) {
+        return products.stream().filter(p -> p.uuid().equals(id)).findFirst();
+    }
+
+    public void updateProductPrice(UUID id, BigDecimal newPrice) {
+        Product p = getProductById(id).orElseThrow(() -> new NoSuchElementException("Product not found with id: " + id));
+        p.price(newPrice);
+        changedProducts.add(id);
+    }
+
+    public void remove(UUID id) {
+        products.removeIf(p -> p.uuid().equals(id));
+    }
+
+    public List<Perishable> expiredProducts() {
+        return products.stream().filter(p -> p instanceof Perishable per && per.isExpired())
+                .map(p -> (Perishable) p).collect(Collectors.toList());
+    }
+
+    public List<Shippable> shippableProducts() {
+        return products.stream().filter(p -> p instanceof Shippable)
+                .map(p -> (Shippable) p).collect(Collectors.toList());
+    }
+
+    public Map<Category, List<Product>> getProductsGroupedByCategories() {
+        return products.stream().collect(Collectors.groupingBy(Product::category));
+    }
+
+    public void clearProducts() {
+        products.clear();
+        changedProducts.clear();
+    }
+
+    public boolean isEmpty() {
+        return products.isEmpty();
+    }
+}
+