Return empty element from Java 8 map operation

I don’t get why you (and all answers) make it so complicated. You have a mapping operation and a filtering operation. So the easiest way is to just apply these operation one after another. And unless your method already returns an Optional, there is no need to deal with Optional.

input.stream().map(i -> crazyFunction(i))
              .filter(out -> out!=null && !out.equals(0))
              .collect(Collectors.toList());

It may be simplified to

input.stream().map(context::crazyFunction)
              .filter(out -> out!=null && !out.equals(0))
              .collect(Collectors.toList());

But you seem to have a more theoretical question about what kind of List to generate, one with placeholders for absent values or one with a different size than the input list.

The simple answer is: don’t generate a list. A List is not an end in itself so you should consider for what kind of operation you need this list (or its contents) and apply the operation right as the terminal operation of the stream. Then you have your answer as the operation dictates whether absent values should be filtered out or represented by a special value (and what value that has to be).

It might be a different answer for different operations…