我可能会这样写:
List<Long> deskIds = service.getAllNodesDepthFirst().stream() .flatMap(p -> p.getDesks().stream()) .map(Desk::getId) .collect(toList());

我可能会这样写:
List<Long> deskIds = service.getAllNodesDepthFirst().stream() .flatMap(p -> p.getDesks().stream()) .map(Desk::getId) .collect(toList());