Java 8 stream API orElse usage

Question

What I'm trying to do is to filter the list, then map it and use orElse if null and then collect it back to the list. Now I can achieve it this way:

return users.stream()
    .filter(user -> id.equals(user.getId()))
    .map(
        user -> {
            if(user.getData() != null) {
                return user.getData();
            }
            return Collections.emptyMap();
        }
    )
    .collect(Collectors.toList());

But the question is: how can I make this structure better and why cannot I use orElse in this case?

Answer 1

It might be more readable with ternary conditional operator:

return users.stream()
    .filter(user -> id.equals(user.getId()))
    .map(
        user -> (user.getData() != null) 
        ? user.getData() 
        : emptyMap()
    )
    .collect(Collectors.toList())
;

In order to use orElse you'll have to create an Optional that wraps user.getData(). I'm not sure that's a good idea.

If you insist on using orElse (or even better, orElseGet, to avoid evaluating emptyMap() when it's not required), it can look like this:

return users.stream()
    .filter(user -> id.equals(user.getId()))
    .map(
        user -> Optional.ofNullable(
            user.getData()
        ).orElseGet(
            () -> emptyMap()
        )
    )
    .collect(Collectors.toList())
;