Java 8 lambda for each call method and addAll

Question

I would like to replace the following code to utilize Java 8 streams if possible:

final List<Long> myIds = new ArrayList<>();
List<Obj> myObjects = new ArrayList<>();
// myObject populated...

for (final Obj ob : myObjects) {
   myIds.addAll(daoClass.findItemsById(ob.getId()));
}

daoClass.findItemsById returns List<Long>

Can anybody advise the best way to do this via lambdas? Many thanks.

Answer 1

List<Long> myIds = myObjects.stream()
    .map(Obj::getId)
    .map(daoClass::findItemsById)
    .flatMap(Collection::stream)
    .collect(Collectors.toList());

Answer 2

Use flatMap which combines multiple list into a single list.

myObjects.stream()
    .flatMap(ob -> daoClass.findItemsById(ob.getId()).stream())
    .collect(Collectors.toList());