code: private static void restrictFeedItemToAdGroup(
        AdWordsServicesInterface adWordsServices,
        AdWordsSession session,
        FeedItem feedItem,
        long adGroupId)
        throws RemoteException {
      // Get the FeedItemTargetingService.
      FeedItemTargetServiceInterface feedItemTargetService =
          adWordsServices.get(session, FeedItemTargetServiceInterface.class);
      FeedItemAdGroupTarget adGroupTarget = new FeedItemAdGroupTarget();
      adGroupTarget.setFeedId(feedItem.getFeedId());
      adGroupTarget.setFeedItemId(feedItem.getFeedItemId());
      adGroupTarget.setAdGroupId(adGroupId);
  
      FeedItemTargetOperation operation = new FeedItemTargetOperation();
      operation.setOperator(Operator.ADD);
      operation.setOperand(adGroupTarget);
  
      FeedItemTargetReturnValue returnValue =
          feedItemTargetService.mutate(new FeedItemTargetOperation[] {operation});
      FeedItemAdGroupTarget addedAdGroupTarget = (FeedItemAdGroupTarget) returnValue.getValue(0);
      System.out.printf(
          "Feed item target for feed ID %d and feed item ID %d "
              + "was created to restrict serving to ad group ID %d.%n",
          addedAdGroupTarget.getFeedId(),
          addedAdGroupTarget.getFeedItemId(),
          addedAdGroupTarget.getAdGroupId());
    }
docs: Restricts the feed item to an ad group.