code: public Optional<SubMenuItem> getSubMenuById(int parentId) {
          return  getAllSubMenus().stream().filter(subMenu->subMenu.getId() == parentId).map(m->(SubMenuItem)m).findFirst();
      }
docs: gets a submenu by it's ID. Returns an optional that will be empty when not present
  @param parentId the parent to obtain
  @return an optional that will be populated when present with the sub menu.