JDK1.8新属性lambda

List<ProductInfo> productInfoList = productService.findUpAll();

//精简方法(java8, lambda)
List<Integer> categoryTypeList = productInfoList.stream()
        .map(e -> e.getCategoryType())
        .collect(Collectors.toList());
List<ProductCategory> productCategoryList = categoryService.findByCategoryTypeIn(categoryTypeList);

        //传统方法
//        for (ProductInfo productInfo : productInfoList) {
//            categoryTypeList.add(productInfo.getCategoryType());
//        }

还可以这么写

        List<CartDTO> cartDTOList = orderDTO.getOrderDetailList().stream().map(e ->
                new CartDTO(e.getProductId(), e.getProductQuantity())
        ).collect(Collectors.toList());