You can use map
:
List<String> names =
personList.stream()
.map(Person::getName)
.collect(Collectors.toList());
EDIT :
In order to combine the Lists of friend names, you need to use flatMap
:
List<String> friendNames =
personList.stream()
.flatMap(e->e.getFriends().stream())
.collect(Collectors.toList());