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 objs = ...;
for (OldObject oldObj : oldObjects) {
  NewObject          


        

Answer 1

You can use peek.

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

Alternatively, you can mutate the elements after forming the list.

List<NewObject> list = oldObjects.stream()
                                 .map(OldObject::toNewObject)
                                 .collect(Collectors.toList());
list.forEach(o -> o.setOrange(true));

Answer 2

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.