[feat] ExpenseService, Impl 추가

src/main/java/com/bbteam/budgetbuddies/domain/expense/service/ExpenseService.java

@@ -2,6 +2,9 @@
 
 import com.bbteam.budgetbuddies.domain.expense.dto.ExpenseRequestDto;
 import com.bbteam.budgetbuddies.domain.expense.dto.ExpenseResponseDto;
+import com.bbteam.budgetbuddies.domain.category.dto.CategoryResponseDTO;
+
+import java.util.List;
 
 public interface ExpenseService {
     ExpenseResponseDto createExpense(ExpenseRequestDto expenseRequestDto);

src/main/java/com/bbteam/budgetbuddies/domain/expense/service/ExpenseServiceImpl.java

@@ -0,0 +1,36 @@
+package com.bbteam.budgetbuddies.domain.expense.service;
+
+import com.bbteam.budgetbuddies.domain.category.entity.Category;
+import com.bbteam.budgetbuddies.domain.category.repository.CategoryRepository;
+import com.bbteam.budgetbuddies.domain.expense.converter.ExpenseConverter;
+import com.bbteam.budgetbuddies.domain.expense.dto.ExpenseRequestDto;
+import com.bbteam.budgetbuddies.domain.expense.dto.ExpenseResponseDto;
+import com.bbteam.budgetbuddies.domain.expense.entity.Expense;
+import com.bbteam.budgetbuddies.domain.expense.repository.ExpenseRepository;
+import com.bbteam.budgetbuddies.domain.user.entity.User;
+import com.bbteam.budgetbuddies.domain.user.repository.UserRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+
+@Service
+@RequiredArgsConstructor
+public class ExpenseServiceImpl implements ExpenseService {
+
+    private final ExpenseRepository expenseRepository;
+    private final UserRepository userRepository;
+    private final CategoryRepository categoryRepository;
+    private final ExpenseConverter expenseConverter;
+
+    @Override
+    public ExpenseResponseDto createExpense(ExpenseRequestDto expenseRequestDto) {
+        User user = userRepository.findById(expenseRequestDto.getUserId())
+                .orElseThrow(() -> new IllegalArgumentException("Invalid user ID"));
+        Category category = categoryRepository.findById(expenseRequestDto.getCategoryId())
+                .orElseThrow(() -> new IllegalArgumentException("Invalid category ID"));
+
+        Expense expense = expenseConverter.toExpenseEntity(expenseRequestDto, user, category);
+        expenseRepository.save(expense);
+
+        return expenseConverter.toExpenseResponseDto(expense);
+    }
+}