Java lambda expression -- mapping and then modifying a list?

Question

Using a Java 8 lambda expression, I'm trying to do something like this.

List<NewObject> objs = ...;
for (OldObject oldObj : oldObjects) {
  NewObject obj = oldObj.toNewObject();
  obj.setOrange(true);
  objs.add(obj);
}

I wrote this code.

oldObjects.stream()
  .map(old -> old.toNewObject())
  .forEach({new.setOrange("true")})
  .collect(Collectors.toList()); 

This is invalid code because I'm then trying to do .collect() on what's returned by .forEach(), but forEach is void and does not return a list.

How should this be structured?

Answer 1

You can use Stream's peek method, which returns the Stream because it's an intermediate operation. It normally isn't supposed to have a side effect (it's supposed to be "non-interfering"), but in this case, I think the side effect (setOrange(true)) is intended and is fine.

List<NewObject> newObjects =
    oldObjects.stream()
        .map(OldObject::toNewObject)
        .peek( n -> n.setOrange(true))
        .collect(Collectors.toList());

It's about as verbose as your non-streams code, so you can choose which technique to use.