java - 如何使用 lambda 流迭代嵌套列表？

Question

我正在尝试将以下代码重构为带有 `stream 的 lambda 表达式，尤其是嵌套的 foreach 循环:

public static Result match (Response rsp) {
    Exception lastex = null;

    for (FirstNode firstNode : rsp.getFirstNodes()) {
        for (SndNode sndNode : firstNode.getSndNodes()) {
            try {
                if (sndNode.isValid())
                return parse(sndNode); //return the first match, retry if fails with ParseException
            } catch (ParseException e) {
                lastex = e;
            }
        }
    }

    //throw the exception if all elements failed
    if (lastex != null) {
        throw lastex;
    }

    return null;
}

我从:

rsp.getFirstNodes().forEach().?? // how to iterate the nested 2ndNodes?

Answer 1

看flatMap:

flatMap(Function<? super T,? extends Stream<? extends R>> mapper)
Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.

代码示例假设 isValid()不扔

Optional<SndNode> sndNode = rsp.getFirstNodes()
  .stream()
  .flatMap(firstNode -> firstNode.getSndNodes().stream())  //This is the key line for merging the nested streams
  .filter(sndNode -> sndNode.isValid())
  .findFirst();

if (sndNode.isPresent()) {
    try {
        parse(sndNode.get());
    } catch (ParseException e) {
        lastex = e;
    }
}