code: public function getUserAlias4User(UserInterface $user)
      {
          if (!isset($this->userAliasCache[$user->getUsername()])) {
              $userAliasEntityTmp = $this->getUserAliasEntity4User($user);
              // workaround for the isset check, null would be false, so we have to set false=)
              if (!$userAliasEntityTmp) {
                  $userAliasEntityTmp = false;
              }
              $this->userAliasCache[$user->getUsername()] = $userAliasEntityTmp;
          }
  
          $userAliasEntity = $this->userAliasCache[$user->getUsername()];
  
          if ($userAliasEntity) {
              $result = $userAliasEntity->getCharName();
          } else {
              $result = $user->getUsername();
          }
  
          return $result;
      }
docs: return the alias of a user, ot the username, as default
  
  @param UserInterface $user
  @return int|string