How to iterate nested lists with lambda streams?

Question

I\'m trying to refactor the following code to lambda expressions with `stream, especially the nested foreach loops:

public static Result match (Response rsp) {
          


        

Answer 1

Look at flatMap:

flatMap(Function> 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.

Code sample assuming isValid() doesn't throw

Optional 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;
    }
}