problem1

src/main/java/yourssu/order/domain/Order.java

@@ -1,11 +1,21 @@
 package yourssu.order.domain;
 
+import java.util.ArrayList;
 import java.util.List;
 
 public class Order {
     private final List<OrderItem> items;
 
-    public Order(List<OrderItem> items) {
-        this.items = items;
+    public List<OrderItem> getItems() {
+        return items;
+    }
+
+    public Order() {
+        items = new ArrayList<>();
+    }
+
+
+    public void addItem(OrderItem item) {
+        items.add(item);
     }
 }

src/main/java/yourssu/order/domain/OrderItem.java

@@ -10,4 +10,12 @@ public OrderItem(String name, int price, int count) {
         this.price = price;
         this.count = count;
     }
+
+    public int calculateTotalPrice() {
+        return price * count;
+    }
+
+    public int getCount() {
+        return count;
+    }
 }

src/main/java/yourssu/order/service/OrderAnalyticsService.java

@@ -1,4 +1,21 @@
 package yourssu.order.service;
 
+import yourssu.order.domain.Order;
+import yourssu.order.domain.OrderItem;
+
+import java.util.List;
+
 public class OrderAnalyticsService {
+
+    public double calculateAverageCost(List<Order> orders) {
+        int totalPrice = orders.stream().flatMap(order -> order.getItems().stream())
+                .mapToInt(OrderItem::calculateTotalPrice)
+                .sum();
+
+        int count = orders.stream().flatMap(order -> order.getItems().stream())
+                .mapToInt(OrderItem::getCount)
+                .sum();
+
+        return totalPrice / (double) count;
+    }
 }

src/main/java/yourssu/user/domain/OrderUser.java

@@ -0,0 +1,22 @@
+package yourssu.user.domain;
+
+import yourssu.order.domain.Order;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class OrderUser {
+    private final List<Order> orders;
+
+    public OrderUser() {
+        orders = new ArrayList<>();
+    }
+
+    public List<Order> getOrders() {
+        return orders;
+    }
+
+    public void addOrder(Order order) {
+        orders.add(order);
+    }
+}

src/test/java/yourssu/Problem1Test.java

@@ -0,0 +1,37 @@
+package yourssu;
+
+import org.junit.jupiter.api.Test;
+import yourssu.order.domain.Order;
+import yourssu.order.domain.OrderItem;
+import yourssu.order.service.OrderAnalyticsService;
+import yourssu.user.domain.OrderUser;
+
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.byLessThan;
+
+public class Problem1Test {
+
+    @Test
+    void calculationAverage() {
+        // given
+        Order order1 = new Order();
+        order1.addItem(new OrderItem("A", 1000, 3));
+        order1.addItem(new OrderItem("B", 2000, 6));
+
+        Order order2 = new Order();
+        order2.addItem(new OrderItem("A", 1000, 3));
+        order2.addItem(new OrderItem("B", 2000, 6));
+
+        OrderUser user = new OrderUser();
+        user.addOrder(order1);
+        user.addOrder(order2);
+
+        OrderAnalyticsService service = new OrderAnalyticsService();
+        // when
+        double averageCost = service.calculateAverageCost(user.getOrders());
+
+        // then
+        assertThat(averageCost).isCloseTo(15000 / 9F, byLessThan(0.0001));
+    }
+}