3 ways to flatten a list of lists. Is there a reason to prefer one of them?

Question

Assume we have a list as follows. CoreResult has a field of type List.

final List list = new LinkedList         


        

Answer 1

Option 3 and 2 is same, so up to you if you want to map first and then flat map or just flatmap.

With option 1 you have to open another stream to perform further operations.

Answer 2

I would go for option 2 or 3. If you want to flatten a List<List<Double>>, you would do this:

List<Double> list = doubleList.stream()
                              .flatMap(List::stream)
                              .collect(Collectors.toList());

Answer 3

I'm not sure about the performance of each one, but an important aspect of the builder pattern utilized by java streams is that it allows for readability. I personally find the option 2 to be the most readable. Option 3 is good, too. I would avoid option one because it kind of "cheats" the flattening of the collection.

Put each method on its own line and determine which is the most intuitive to read. I rather like the second one:

final List<Double> B = list.stream()
                           .map(CoreResult::getField)
                           .flatMap(Collection::stream)
                           .collect(Collectors.toList());