Java8 Utils

1、字符串分割为List

        String param = "a,ew,were,erer,wer,yht,asd,dasd,daasd,asd";
        Optional.ofNullable(param)
                 .filter(StringUtils::isNotBlank)
                 .map(Splitter.on(",").trimResults().omitEmptyStrings()::splitToList)
                 .orElse(Lists.newArrayListWithExpectedSize(0));

2、List转字符串，以特殊符号分割

        Optional.of(Lists.newArrayList())
                .filter(CollectionUtils::isNotEmpty)
                .map(Joiner.on(",")::join)
                .orElse(null);

3、图片转Base64

        try {
            // url 图片地址
            byte[] data = FileManage.downLoadFromUrl(url);
            String encode = Base64.getEncoder().encodeToString(data);
            // 根据需要是否添加base64前缀
            return "data:image/png;base64," + encode;
        } catch (IOException e) {
            e.printStackTrace();
        }

4、List 转 Map  key:对象中某一个属性，value ：可以是属性值、也可以是对象

        List<CouponModel> listModel = Lists.newArrayList();
        Map<Integer, CouponModel> integerCouponModelMap = Optional.of(listModel)
                .filter(CollectionUtils::isNotEmpty)
                .map(list -> list.stream().collect(Collectors.toMap(CouponModel::getCouponId, Function.identity())))
                .orElse(Maps.newHashMap());

Java 8允许在接口中加入具体方法。接口中的具体方法有两种，default方法和static方法，identity()就是Function接口的一个静态方法。Function.identity()返回一个输出跟输入一样的Lambda表达式对象，等价于t -> t形式的Lambda表达式

4、List 以对象中某一属性分组

List<Model> list= Lists.newArrayList();
Map<Integer, List<Model>> map = list.stream().collect(Collectors.groupingBy(Model::getId));

5、list 转 list<Object, List<Object>>

Map<Integer, List<Integer>> sourceIdToPromotionIdsMap = Optional.of(promotionDtoList)
                            .filter(dtoList -> !dtoList.isEmpty())
                            .map(dtoList -> dtoList.stream().collect(Collectors.groupingBy(PromotionDto::getSourceId,
                                    Collectors.mapping(PromotionDto::getPromotionId, Collectors.toList()))))
                            .orElse(Maps.newHashMapWithExpectedSize(0));

6、分组求和

Map<Integer, Integer> skuToConfirmQuantityMap = Optional.of(ecBpInOutDocDto)
                    .map(EcBpInOutDocDto::getBpDocNo2)
                    .map(e -> ecBpInOutDocService.queryConfirmItemByBpDocNo2AndStatus(e, EcBizDocHandleStatus.S9.getValue()))
                    .filter(CollectionUtils::isNotEmpty)
                    .map(list -> list.stream().collect(Collectors.groupingBy(EcBpInOutDocItemDto::getProductSkuId, Collectors.summingInt(EcBpInOutDocItemDto::getQuantity))))
                    .orElse(Maps.newHashMapWithExpectedSize(0));