Java 8 map collection and add a default value if the collection is empty

Question

I've got a use case where I must return a collection with at least 1 element. The incoming collection may have 0 or more elements.

so this could be done fairly easily

Set<ObjectB> setOfB = collectionOfA.isEmpty() ? 
        new HashSet<ObjectB>() {{ add(new ObjectB()); }}  : 
        collectionOfA
           .stream()
           .map(item -> new ObjectB(item))
           .collect(Collectors.toSet()); 

BUT....

I'm also trying to use this as a chance to better familiarize myself with the Java 8 tools and features, and so i'm trying to see if this can be done without the conditional test and in a more Java 8-like way.

Thoughts and suggestions are most appreciated!

Answer 1

I think you've already got it pretty much as simple as possible. Remember that Java 8 is still the same language; don't overdo trying to make everything functional.

The one improvement I would make to your code would be to use Collections.singleton(new ObjectB()) instead of the awkward and problematic double-brace initialization.

Answer 2

You'll have to have a condition since the count is only available when you call a reduction operation. However, for the single element set you can do:

Set<ObjectB> setOfB = collectionOfA.isEmpty() ? 
    Stream.of(new ObjectB()).collect(Collectors.toSet()) : 
    collectionOfA
       .stream()
       .map(item -> new ObjectB(item))
       .collect(Collectors.toSet());

which can be reduced to

Set<ObjectB> setOfB = ( collectionOfA.isEmpty() ? 
          Stream.of(new ObjectB()) : 
          collectionOfA.stream().map(item -> new ObjectB(item)) )
         .collect(Collectors.toSet())