### Question:
ProductServiceImpl implements ProductService { @Override public Optional<ProductInfo> findById(String productId) { return repository.findById(productId); }  @Override Optional<ProductInfo> findById(String productId); @Override List<ProductInfo> findUpAll(); @Override Page<ProductInfo> findAll(Pageable pageable); @Override List<ProductInfo> findByCategoryType(Integer categoryType); @Override ProductInfo save(ProductInfo productInfo); @Override @Transactional void increaseStock(List<CartDTO> cartDTOList); @Override @Transactional void decreaseStock(List<CartDTO> cartDTOList); @Override ProductInfo onSale(String productId); @Override ProductInfo offSale(String productId); @Override void delete(String productId);  }### Answer:
@Test public void findById() { Optional<ProductInfo> productInfo = productService.findById("123456"); System.out.println(productInfo); }