feat: add order service

src/main/java/com/demo/order/OrderService.java

@@ -0,0 +1,24 @@
+package com.demo.order;
+
+import java.math.BigDecimal;
+
+public class OrderService {
+
+    private final OrderRepository repository;
+
+    public OrderService(OrderRepository repository) {
+        this.repository = repository;
+    }
+
+    public BigDecimal total(Order order) {
+        BigDecimal sum = BigDecimal.ZERO;
+        for (OrderLine line : order.getLines()) {
+            sum = sum.add(line.getPrice().multiply(BigDecimal.valueOf(line.getQuantity())));
+        }
+        return sum;
+    }
+
+    public void submit(Order order) {
+        repository.save(order);
+    }
+}