11/13 stream

build.gradle

@@ -34,6 +34,7 @@ dependencies {
 	testCompileOnly 'org.projectlombok:lombok'
 	testAnnotationProcessor 'org.projectlombok:lombok'
 	testRuntimeOnly 'org.junit.platform:junit-platform-launcher'
+    testImplementation 'org.assertj:assertj-core:3.6.1'
 }
 
 tasks.named('test') {

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

@@ -8,4 +8,8 @@ public class Order {
     public Order(List<OrderItem> items) {
         this.items = items;
     }
+
+    public List<OrderItem> getItems() {
+        return items;
+    }
 }

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

@@ -10,4 +10,17 @@ public OrderItem(String name, int price, int count) {
         this.price = price;
         this.count = count;
     }
+
+
+    public int getPrice() {
+        return price;
+    }
+
+    public int getCount() {
+        return count;
+    }
+
+    public String getName() {
+        return name;
+    }
 }

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

@@ -0,0 +1,15 @@
+package yourssu.order.domain;
+
+import java.util.List;
+
+public class OrderUser {
+    private final List<Order> orders;
+
+    public OrderUser(List<Order> orders) {
+        this.orders = orders;
+    }
+
+    public List<Order> getOrders() {
+        return orders;
+    }
+}

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

@@ -1,4 +1,17 @@
 package yourssu.order.service;
 
+import yourssu.order.domain.OrderUser;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
 public class OrderAnalyticsService {
+    public static double getAverageOrderAmount(List<OrderUser> user) {
+        return user.stream()
+                .flatMap(orderUser -> orderUser.getOrders().stream())
+                .flatMap(order -> order.getItems().stream())
+                .collect(Collectors.averagingDouble(
+                        orderItem -> orderItem.getCount() * orderItem.getPrice()
+                ));
+    }
 }

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

@@ -1,6 +1,8 @@
 package yourssu.user.domain;
 
 import java.util.List;
+
+import yourssu.order.domain.Order;
 import yourssu.post.domain.Post;
 
 public class User {

src/test/java/yourssu/JavaPlaygroundApplicationTests.java

@@ -2,10 +2,63 @@
 
 import org.junit.jupiter.api.Test;
 import org.springframework.boot.test.context.SpringBootTest;
+import yourssu.order.domain.Order;
+import yourssu.order.domain.OrderItem;
+import yourssu.order.domain.OrderUser;
+import yourssu.order.service.OrderAnalyticsService;
+import yourssu.user.domain.User;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.OptionalDouble;
+
+import static org.assertj.core.api.Java6Assertions.assertThat;
+import static org.assertj.core.api.Java6Assertions.offset;
 
 @SpringBootTest
 class JavaPlaygroundApplicationTests {
+
+    private List<OrderUser> prepareData() {
+        OrderItem item1 = new OrderItem("item1", 2000, 2); // 4000
+        OrderItem item2 = new OrderItem("item2", 500, 3);  // 1500
+        Order order1 = new Order(Arrays.asList(item1, item2));
+
+        OrderItem item3 = new OrderItem("item3", 1000, 1); // 1000
+        OrderItem item4 = new OrderItem("item4", 300, 5); // 1500
+        Order order2 = new Order(Arrays.asList(item3, item4));
+
+        return Arrays.asList(new OrderUser(List.of(order1)), new OrderUser(List.of(order2)));
+    }
+
 	@Test
-	void contextLoads() {
+	void 모든_주문상품의_개별_총금액을_계산하고_하나의_리스트로_펼친다() {
+        List<OrderUser> orderUsers = prepareData();
+
+        List<Integer> expectedItemTotals = List.of(4000, 1500, 1000, 1500);
+
+        assertThat(expectedItemTotals)
+                .as("총 4개의 주문 상품 금액이 계산되어야 함")
+                .hasSize(4);
+
+        assertThat(expectedItemTotals)
+                .as("각 주문 상품의 (price * count) 결과는 [4000, 1500, 1000, 1500] 순으로 일치해야 함")
+                .containsExactlyElementsOf(expectedItemTotals);
+
+        assertThat(expectedItemTotals.stream().mapToInt(Integer::intValue).sum())
+                .as("계산된 모든 금액의 합은 8000이어야 함")
+                .isEqualTo(8000);
+
 	}
+
+    @Test
+    void 모든_주문상품의_최종_평균_금액을_정확히_계산_한다() {
+        List<OrderUser> orderUsers = prepareData();
+        double expectedAverage = 2000.0;
+
+        double averagePrice = OrderAnalyticsService.getAverageOrderAmount(orderUsers);
+
+        assertThat(averagePrice)
+                .as("모든 주문 상품의 평균 금액은 %.2f 이어야 함", expectedAverage)
+                .isEqualTo(expectedAverage, offset(0.001)); // 0.001 오차 범위 허용
+    }
 }