Java 8 forEach applied to only some?

Question

No "if" statements, please, unless you're explaining why it's impossible to do without one.

I'm seeing how far I can go operating on streams only. I have this nuisance:

List<Cube> revised =
    cubes.filter(p)
    .map(c -> f(c))
    .map(c -> {
        if(c.prop()) {
           c.addComment(comment);
        }
        return c;
    })
    .collect(Collectors.toList());

My best idea for how to do this without an "if" is

List<Cube> revised = 
    cubes.filter(p)
    .map(c -> f(c));

revised
    .filter(Cube::prop)
    .forEach(c -> c.addComment(comment)); // can also map still

Is there a way to do this in one chain only? A branch basically has to happen in the stream if so. A method like forSome(predicate, lambda) would work.

Do not want to "roll my own" anything. I can use an "if" but I'm trying to learn how expressive functional style can be.

Answer 1

There's no need to use map that returns the same element, when you have peek. The following code "cheats" by using a short-circuit operator:

cubes.filter(p)
    .map(c -> f(c))
    .peek(c -> c.prop() && c.addComment(comment))

I think the "modern" way using Optional is far less readable:

cubes.filter(p)
    .map(c -> f(c))
    .peek(c -> Optional.of(c).filter(Cube::prop).ifPresent(c -> c.addComment(comment)))