Recursive use of Stream.flatMap()

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Well, I used the same pattern with a generic Tree class and didn’t have a wrong feeling with it. The only difference is, that the Tree class itself offered a children() and allDescendants() methods, both returning a Stream and the latter building on the former. This is related to “Should I return a Collection or a Stream?” and “Naming java methods that return streams”.

From a Streams perspective, there is no difference between a flatMap to children of a different type (i.e. when traversing a property) and a flatMap to children of the same type. There is also no problem if the returned stream contains the same element again, as there is no relationship between the elements of the streams. In principle, you can use flatMap as a filter operation, using the pattern flatMap(x -> condition? Stream.of(x): Stream.empty()). It’s also possible to use it to duplicate elements like in this answer.

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