Решение Задачи на стримы от Java Gym Rat

src/main/java/com/algos/gym_rat/Main.java

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+package com.algos.gym_rat;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Main {
+
+    public static void main(String[] args) {
+
+        Product product1 = new Product("Молоко", 10, 2.0);
+        Product product2 = new Product("Масло", 1, 10.0);
+        Product product3 = new Product("Сыр", 5, 2.0);
+        Product product4 = new Product("Хлеб", 3, 1.0);
+        Product product5 = new Product("Лаваш", 4, 5.0);
+
+        Order order1 = new Order("1", "Вася");
+        order1.setProducts(new ArrayList<>(List.of(product1, product2, product3, product4)));
+
+        System.out.println(OrderAnalysis.calculateOrderTotal(order1));
+
+        Order order2 = new Order("2", "Коля");
+        order2.setProducts(new ArrayList<>(List.of(product2, product4, product5)));
+
+        List<Order> orders = new ArrayList<>(List.of(order1, order2));
+        System.out.println(OrderAnalysis.getUniqueProducts(orders));
+
+        System.out.println(OrderAnalysis.findMostPopularProduct(orders, 3));
+        System.out.println(OrderAnalysis.findMostPopularProductNice(orders, 3));
+    }
+}

src/main/java/com/algos/gym_rat/Order.java

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+package com.algos.gym_rat;
+
+import java.util.List;
+
+public class Order {
+    String orderId;
+    String customerName;
+    List<Product> products;
+
+    public Order(String orderId, String customerName) {
+        this.orderId = orderId;
+        this.customerName = customerName;
+    }
+
+    public List<Product> getProducts() {
+        return products;
+    }
+
+    public void setProducts(List<Product> products) {
+        this.products = products;
+    }
+}

src/main/java/com/algos/gym_rat/OrderAnalysis.java

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+package com.algos.gym_rat;
+
+import java.util.*;
+import java.util.stream.Collectors;
+
+public class OrderAnalysis {
+
+    // Рассчитать полную стоимость заказа
+    public static Double calculateOrderTotal(Order order) {
+
+        return order
+                .getProducts()
+                .stream()
+                .mapToDouble(p -> p.getQuantity() * p.getPrice())
+                .sum();
+    }
+
+    //Получить список наименований товаров из заказов
+    public static Set<String> getUniqueProducts(List<Order> orders) {
+
+        return orders
+                .stream()
+                .flatMap(o -> o.getProducts().stream())
+                .map(Product::getName)
+                .collect(Collectors.toSet());
+    }
+
+    //Вывести наименования N популярных товаров в порядке уменьшения популярности
+    //Популярный товар - товар, который больше остальных покупается
+    public static List<String> findMostPopularProduct(List<Order> orders, int topN) {
+
+        Map<String, Integer> popularProducts = new HashMap<>();
+        for (Order order : orders) {
+            List<Product> products = order.getProducts();
+            for (Product product : products) {
+                popularProducts.merge(product.getName(), product.getQuantity(), Integer::sum);
+            }
+        }
+
+        return popularProducts
+                .entrySet()
+                .stream()
+                .sorted((e1, e2) -> {
+                    return e2.getValue().compareTo(e1.getValue());
+                })
+                .limit(topN)
+                .map(Map.Entry::getKey)
+                .collect(Collectors.toList());
+    }
+    public static List<String> findMostPopularProductNice(List<Order> orders, int topN) {
+
+        return orders
+                .stream()
+                .flatMap(o -> o.getProducts().stream())
+                .collect(Collectors.groupingBy(Product::getName, Collectors.summingInt(Product::getQuantity)))
+                .entrySet()
+                .stream()
+                .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
+                .limit(topN)
+                .map(Map.Entry::getKey)
+                .collect(Collectors.toList());
+    }
+}

src/main/java/com/algos/gym_rat/Product.java

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+package com.algos.gym_rat;
+
+public class Product {
+
+    String name;
+    Integer quantity;
+    Double price;
+
+    public Product(String name, Integer quantity, Double price) {
+        this.name = name;
+        this.quantity = quantity;
+        this.price = price;
+    }
+
+    public Integer getQuantity() {
+        return quantity;
+    }
+
+    public Double getPrice() {
+        return price;
+    }
+
+    public String getName() {
+        return name;
+    }
+}