List<Long> deskIds = new ArrayList<>();
for(ProvidedService memberService : service.getAllNodesDepthFirst()){
   for(Desk d : memberService.getDesks()){
     deskIds.add(d.getId());
   }
}

List<Long> deskIds = new ArrayList<>();
service.getAllNodesDepthFirst().stream().forEach(srv -> {
    deskIds.addAll(srv.getDesks().stream().map(Desk::getId).collect(Collectors.toList()));
});