函数式编程 - Stone

理解
注意
使用

理解

lambda的本质是一个匿名方法。譬如(x,y)->{return x+y}

注意

Lambda好用但是要注意：

filter判空
对于争用资源，并行带来的压力

使用

判空filter

Optional

Optional.ofNullable(ids).orElseGet(Collections::emptyList).stream()
        .distinct()
        .map(p -> getPosition(positionParams, p, function))
        .filter(p -> p != null)
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

要对每一个元素对象（p）进行判空。

noticeSet.stream()
  .filter(p -> p != null)
  .filter(p -> gravityMap.get(String.valueOf(p)) != null)            
  .filter(p -> gravityMap.get(String.valueOf(p)) == gravity.getValue()).collect(Collectors.toSet());

排序sorted&比较Comparator

        Comparator<SimpleNotice> byBeginDate = (s1, s2) -> s2.getBeginDate().compareTo(s1.getBeginDate());//根据begin倒序

            return allSimpleNotice.stream().filter(p -> p != null).sorted(byBeginDate).collect(Collectors.toList());

新老写法比较

Collections.sort(bpmBdEvaluates, new Comparator<BpmBdEvaluate>() {
    @Override
    public int compare(BpmBdEvaluate o1, BpmBdEvaluate o2) {
        return o1.getUpdated_at().compareTo(o2.getUpdated_at());
    }
});
return bpmBdEvaluates.get(bpmBdEvaluates.size() - 1);

使用Lambda

Collections.sort(bpmBdEvaluates, (o1, o2) -> o1.getUpdated_at().compareTo(o2.getUpdated_at()));

map

oldNotice.stream().map(p -> new SimpleNotice(p.getId(), NoticeType.PERMESSAGE.getValue(),
                    p.getTitle(), p.getTitle(), p.getBeginDate(), (p.isRead() ? 1 : 0))).collect(Collectors.toSet());

groupBy

        List<Province> provinceList = provinceNameMap.entrySet().stream().map(p -> new Province(p.getKey(), p.getValue(), citysMap.get(p.getKey()))).collect(Collectors.toList());

collector

positionList.stream().collect(Collectors.toMap(DataPosition::getId, x -> x.getPosition().intValue())); positionList.stream().collect(Collectors.toMap(DataPosition::getId, x -> x.getPosition().intValue(), (key1, key2) -> {
                return key2;//如果存在重复value

This work is licensed under a CC A-S 4.0 International License.