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

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+package com.example;
+import java.util.HashMap;
+import java.util.Map;
+
+public class Category {
+    private String name;
+
+    // Constructor
+    private Category(String name) {
+        this.name = name;
+    }
+
+    // Create 1 static cache map when initialized, then points to the HashMap reference.
+    private static final Map<String, Category> categories = new HashMap<>();
+
+    // Factory
+    public static Category of(String nameOf){
+        if (nameOf == null){
+            throw new IllegalArgumentException("Category name can't be null");
+        }
+
+        if (nameOf.isBlank()){
+            throw new IllegalArgumentException("Category name can't be blank");
+        }
+
+        // Normalize to capital letter
+        String normalized = normalize(nameOf);
+
+        // Check the cache, create new or return cached instance
+        if (categories.containsKey(normalized)){
+            return categories.get(normalized);
+        }
+        else {
+            Category category = new Category(normalized);
+            categories.put(normalized, category);
+            return category;
+        }
+    }
+
+    // Method to normalize to capital letter
+    private static String normalize(String input) {
+        input = input.trim().toLowerCase();
+        if (input.isEmpty()) throw new IllegalArgumentException("Category name must not be blank");
+        input = input.substring(0,1).toUpperCase() + input.substring(1);
+        return input;
+    }
+
+    public String getName(){
+        return this.name;
+    }
+}

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

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+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; // kg
+
+    public ElectronicsProduct(UUID uuid, String nameInput, Category categoryInput, BigDecimal priceInput, int warrantyMonths, BigDecimal weightInput) {
+        super(uuid, nameInput, categoryInput, ensurePositive(priceInput, "Price"));
+
+        this.warrantyMonths = ensurePositiveInt(warrantyMonths);
+        this.weight = ensurePositive(weightInput, "Weight");
+    }
+
+    // Make sure price, weight is positive
+    private static BigDecimal ensurePositive(BigDecimal value, String valueType) {
+        // Uses built in constant for 0
+        if (value.compareTo(BigDecimal.ZERO) < 0) {
+            throw new IllegalArgumentException(valueType + " cannot be negative.");
+        }
+        return value;
+    }
+
+    // Make sure warranty is positive
+    private static int ensurePositiveInt(int value) {
+        if (value < 0) {
+            throw new IllegalArgumentException("Warranty months cannot be negative.");
+        }
+        return value;
+    }
+
+    @Override
+    public String productDetails() {
+        return "Electronics: " + name() + ", Warranty: " + warrantyMonths + " months";
+    }
+
+    @Override
+    public BigDecimal calculateShippingCost() {
+        BigDecimal baseShipping = BigDecimal.valueOf(79.0);
+        BigDecimal heavyShipping = baseShipping.add(BigDecimal.valueOf(49.0));
+        BigDecimal maxWeight = BigDecimal.valueOf(5.0);
+        // See if weight is larger than 0
+        if (weight.compareTo(maxWeight) > 0) {
+            return heavyShipping;
+        }
+        return baseShipping;
+    }
+
+    @Override
+    public Double weight() {
+        return Double.parseDouble(String.valueOf(weight));
+    }
+}

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.UUID;
+
+public class FoodProduct extends Product implements Perishable, Shippable {
+    private final LocalDate expirationDate;
+    private final BigDecimal weight; // kg
+
+    public FoodProduct(UUID uuid, String nameInput, Category categoryInput, BigDecimal priceInput, LocalDate expirationDateInput, BigDecimal weightInput) {
+        super(uuid, nameInput, categoryInput, ensurePositive(priceInput, "Price"));
+
+        this.expirationDate = expirationDateInput;
+        this.weight = ensurePositive(weightInput, "Weight");
+    }
+
+    // Make sure price, weight is positive
+    private static BigDecimal ensurePositive(BigDecimal value, String valueType) {
+        // Uses built in constant for 0
+        if (value.compareTo(BigDecimal.ZERO) < 0) {
+            throw new IllegalArgumentException(valueType + " cannot be negative.");
+        }
+        return value;
+    }
+
+    @Override
+    public String productDetails() {
+        return "Food: " + name() + ", Expires: " + expirationDate;
+    }
+
+    @Override
+    public BigDecimal calculateShippingCost() {
+        // Multiply weight with 50
+        return weight.multiply(BigDecimal.valueOf(50));
+    }
+
+    @Override
+    public Double weight() {
+        return Double.parseDouble(String.valueOf(weight));
+    }
+
+    @Override
+    public LocalDate expirationDate() {
+        return expirationDate;
+    }
+}

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

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+package com.example;
+
+import java.time.LocalDate;
+
+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;
+
+public abstract class Product {
+    private final UUID id;
+    private final String name;
+    private final Category category;
+    private BigDecimal price;
+
+    protected Product(UUID uuid, String name, Category category, BigDecimal price) {
+        this.id = uuid;
+        this.name = name;
+        this.category = category;
+        this.price = price;
+    }
+
+    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) {
+        this.price = price;
+    }
+
+    abstract public String productDetails();
+}

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

@@ -0,0 +1,11 @@
+package com.example;
+import java.math.BigDecimal;
+
+interface Shippable {
+    BigDecimal calculateShippingCost();
+
+    // Return 0.0 if no weight override
+    default Double weight(){
+        return 0.0;
+    }
+}

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

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+package com.example;
+import java.math.BigDecimal;
+import java.util.*;
+
+import static java.util.stream.Collectors.groupingBy;
+import static java.util.stream.Collectors.toList;
+
+public class Warehouse {
+    // List to save what products are in the warehouse
+    private final Set<Product> products;
+    // Set to make sure no duplicates
+    private final Set<UUID> changedProducts;
+    private final String name;
+
+    // Cache to save warehouses(singletons) by key: name
+    private static final Map<String, Warehouse> instances = new HashMap<>();
+
+    // Constructor
+    private Warehouse() {
+        // HashSet with faster add, remove, contains (I've read) and no order needed
+        this.name = "Default";
+        this.products = new HashSet<>();
+        this.changedProducts = new HashSet<>();
+    }
+
+    // Constructor
+    private Warehouse(String name) {
+        this.name = name;
+        // HashSet faster add, remove, contains (I've read) and no order needed
+        this.products = new HashSet<>();
+        this.changedProducts = new HashSet<>();
+    }
+
+
+    /******************************************
+    ***************   METHODS   ***************
+    *******************************************/
+
+    public static Warehouse getInstance() {
+        return getInstance("default");
+    }
+
+    public static Warehouse getInstance(String name) {
+        if(!instances.containsKey(name)) {
+            instances.put(name, new Warehouse(name));
+        }
+        return instances.get(name);
+    }
+
+    // Get all products
+    public List<Product> getProducts() {
+        return Collections.unmodifiableList(new ArrayList<>(products));
+    }
+
+    // Get a single product by id
+    public Optional<Product> getProductById(UUID uuid) {
+        return products.stream()
+                // Find matching ID
+                .filter(item -> item.uuid().equals(uuid))
+                // Return Optional
+                .findFirst();
+    }
+
+    public void updateProductPrice(UUID uuid, BigDecimal newPrice) {
+        Product item = products.stream()
+                .filter(p -> p.uuid().equals(uuid))
+                .findFirst()
+                .orElseThrow(() -> new NoSuchElementException("Product not found with id: " + uuid));
+
+        item.setPrice(newPrice);
+        changedProducts.add(uuid);
+    }
+
+    public Set<UUID> getChangedProducts() {
+        return Collections.unmodifiableSet(changedProducts);
+    }
+
+    public void addProduct(Product item) {
+        if (item == null) {
+            throw new IllegalArgumentException("Product cannot mvn clean install -Ube null.");
+        }
+        else if (products.stream().anyMatch(p -> p.uuid().equals(item.uuid()))) {
+            throw new IllegalArgumentException("Product with that id already exists, use updateProduct for updates.");
+        }
+        products.add(item);
+    }
+
+    public void clearProducts() {
+        products.clear();
+        changedProducts.clear();
+    }
+
+    public boolean isEmpty() {
+        return products.isEmpty();
+    }
+
+    public List<Perishable> expiredProducts() {
+        return products.stream()
+                .filter(item -> item instanceof Perishable) // Find every item implementing Perishable
+                .map(item -> (Perishable) item) // Turn into Perishable instead of Product
+                .filter(Perishable::isExpired) // Find every item that has expired
+                .collect(toList()); // Finally return the list
+    }
+
+    public List<Shippable> shippableProducts() {
+        return products.stream()
+                .filter(item -> item instanceof Shippable) // Find every item implementing Shippable
+                .map(item -> (Shippable) item) // Turn Product into Shippable
+                .collect(toList()); // Finally returns the list
+    }
+
+    public void remove(UUID uuid) {
+        products.removeIf(item -> item.uuid().equals(uuid));
+        changedProducts.remove(uuid);
+    }
+
+    public Map<Category, List<Product>> getProductsGroupedByCategories() {
+        return Collections.unmodifiableMap(products.stream()
+                .collect(groupingBy(Product::category)));
+    }
+}
+