How to iterate nested for loops referring to parent elements using Java 8 streams?

问题

I want to iterate nested lists using java8 streams, and extract some results of the lists on first match. Unfortunately I have to also get a values from the parent content if a child element matches the filter.

How could I do this?

java7

Result result = new Result();

//find first match and pupulate the result object.
for (FirstNode first : response.getFirstNodes()) {
    for (SndNode snd : first.getSndNodes()) {
        if (snd.isValid()) {
            result.setKey(first.getKey());
            result.setContent(snd.getContent());
            return;
        }
    }
}

java8

 response.getFirstNodes().stream()
        .flatMap(first -> first.getSndNodes())
        .filter(snd -> snd.isValid())
        .findFirst()
        .ifPresent(???); //cannot access snd.getContent() here

回答1:

When you need both values and want to use flatMap (as required when you want to perform a short-circuit operation like findFirst), you have to map to an object holding both values

response.getFirstNodes().stream()
  .flatMap(first->first.getSndNodes().stream()
    .map(snd->new AbstractMap.SimpleImmutableEntry<>(first, snd)))
  .filter(e->e.getValue().isValid())
  .findFirst().ifPresent(e-> {
    result.setKey(e.getKey().getKey());
    result.setContent(e.getValue().getContent());
  });

In order to use standard classes only, I use a Map.Entry as Pair type whereas a real Pair type might look more concise.

In this specific use case, you can move the filter operation to the inner stream

response.getFirstNodes().stream()
  .flatMap(first->first.getSndNodes().stream()
     .filter(snd->snd.isValid())
     .map(snd->new AbstractMap.SimpleImmutableEntry<>(first, snd)))
  .findFirst().ifPresent(e-> {
    result.setKey(e.getKey().getKey());
    result.setContent(e.getValue().getContent());
  });

which has the neat effect that only for the one matching item, a Map.Entry instance will be created (well, should as the current implementation is not as lazy as it should but even then it will still create lesser objects than with the first variant).

回答2:

It should be like this:

Edit: Thanks Holger for pointing out that the code won't stop at the first valid FirstNode

response.getFirstNodes().stream()
  .filter(it -> {it.getSndNodes().stream().filter(SndNode::isValid).findFirst(); return true;})
  .findFirst()
  .ifPresent(first -> first.getSndNodes().stream().filter(SndNode::isValid).findFirst().ifPresent(snd -> {
    result.setKey(first.getKey());
    result.setContent(snd.getContent());
  }));

A test can be found here

来源：https://stackoverflow.com/questions/29235567/how-to-iterate-nested-for-loops-referring-to-parent-elements-using-java-8-stream