lambda collecting within a ifPresent

Question

How can I use the collect method to collect within a nested ifPresent method?

So far this is what I have:

List result = list.stream()
                  .findFirst()
                  .ifPresent(info -> {
                      info.getMap()
                          .stream()
                          .filter( entry -> entry.getKey().equals("test"))
                          .map(entry::getValue)
                          .collect(Collectors.toList())                          
                   });

Answer 1

You might wanted to do this:

List result = list.stream()
    .limit(1)
    .flatMap(info -> // <-- removed brace {
        info.getMap().stream()
            .filter( entry -> entry.getKey().equals("test"))
            .map(entry::getValue)
    ) // <-- removed brace here too
    .collect(Collectors.toList());

Let me explain:

.limit(1): limits the stream only to the first element (or returns an empty stream if initial stream is empty)

.flatMap(): maps the stream into a new stream. in this case a new stream consisting of values from Entry#getValue() is returned

.collect(Collectors.toList()): at last the stream of values is collected into a list.

As Olivier Grégoire said in the comments, the following does work too:

List result = list.stream()
    .limit(1)
    .flatMap(info -> info.getMap().stream())
    .filter( entry -> entry.getKey().equals("test"))
    .map(entry::getValue)
    .collect(Collectors.toList());

Which in my opinion is more readable and clearer indicates what the intents were.

Answer 2

Assuming info.getMap returns a java.util.Map, I don't see why you need the inner stream pipeline at all.

Just use get("test") to obtain the value of the "test" key:

List result = 
    list.stream()
        .map(info -> info.getMap().get("test"))
        .filter(Objects::nonNull)
        .limit(1)
        .collect(Collectors.toList()); 

P.S., don't use a raw type. You should change the type of the returned list to List<ValueType> where ValueType is the type of values in the info.getMap() Map.