#vulnerable code 
                              private void validatePieceAsync(final SharedTorrent torrent, final Piece piece, String torrentHash, SharingPeer peer) {
    try {
      synchronized (piece) {

        if (piece.isValid()) return;

        piece.validate(torrent, piece);
        if (piece.isValid()) {
          piece.finish();
          // Send a HAVE message to all connected peers, which don't have the piece
          PeerMessage have = PeerMessage.HaveMessage.craft(piece.getIndex());
          for (SharingPeer remote : getConnectedPeers()) {
            if (remote.getTorrent().getHexInfoHash().equals(torrentHash) &&
                    !remote.getAvailablePieces().get(piece.getIndex()))
              remote.send(have);
          }

          final boolean isTorrentComplete;
          synchronized (torrent) {
            torrent.removeValidationFuture(piece);
            torrent.notifyPieceDownloaded(piece, peer);

            boolean isCurrentPeerSeeder = peer.getAvailablePieces().cardinality() == torrent.getPieceCount();
            //if it's seeder we will send not interested message when we download full file
            if (!isCurrentPeerSeeder) {
              if (torrent.isAllPiecesOfPeerCompletedAndValidated(peer)) {
                peer.notInteresting();
              }
            }

            isTorrentComplete = torrent.isComplete();

            if (isTorrentComplete) {
              logger.debug("Download of {} complete.", torrent.getDirectoryName());

              torrent.finish();
            }
          }

          if (isTorrentComplete) {

            LoadedTorrent announceableTorrent = torrentsStorage.getLoadedTorrent(torrentHash);

            if (announceableTorrent == null) return;

            AnnounceableInformation announceableInformation = announceableTorrent.createAnnounceableInformation();


            try {
              announce.getCurrentTrackerClient(announceableInformation)
                      .announceAllInterfaces(COMPLETED, true, announceableInformation);
            } catch (AnnounceException e) {
              logger.debug("unable to announce torrent {} on tracker {}", torrent, torrent.getAnnounce());
            }

            for (SharingPeer remote : getPeersForTorrent(torrentHash)) {
              remote.notInteresting();
            }

          }
        } else {
          torrent.markUncompleted(piece);
          logger.info("Downloaded piece #{} from {} was not valid ;-(. Trying another peer", piece.getIndex(), peer);
          peer.getPoorlyAvailablePieces().set(piece.getIndex());
        }
      }
    } catch (Throwable e) {
      torrent.markUncompleted(piece);
      logger.warn("unhandled exception in piece {} validation task", e);
    }
  } 
                            #location 51 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              private void validatePieceAsync(final SharedTorrent torrent, final Piece piece, String torrentHash, SharingPeer peer) {
    try {
      synchronized (piece) {

        if (piece.isValid()) return;

        piece.validate(torrent, piece);
        if (piece.isValid()) {
          piece.finish();
          // Send a HAVE message to all connected peers, which don't have the piece
          PeerMessage have = PeerMessage.HaveMessage.craft(piece.getIndex());
          for (SharingPeer remote : getConnectedPeers()) {
            if (remote.getTorrent().getHexInfoHash().equals(torrentHash) &&
                    !remote.getAvailablePieces().get(piece.getIndex()))
              remote.send(have);
          }

          final boolean isTorrentComplete;
          synchronized (torrent) {
            torrent.removeValidationFuture(piece);
            torrent.notifyPieceDownloaded(piece, peer);

            boolean isCurrentPeerSeeder = peer.getAvailablePieces().cardinality() == torrent.getPieceCount();
            //if it's seeder we will send not interested message when we download full file
            if (!isCurrentPeerSeeder) {
              if (torrent.isAllPiecesOfPeerCompletedAndValidated(peer)) {
                peer.notInteresting();
              }
            }

            isTorrentComplete = torrent.isComplete();

            if (isTorrentComplete) {
              logger.debug("Download of {} complete.", torrent.getDirectoryName());

              torrent.finish();
            }
          }

          if (isTorrentComplete) {

            LoadedTorrent announceableTorrent = torrentsStorage.getLoadedTorrent(torrentHash);

            if (announceableTorrent == null) return;

            AnnounceableInformation announceableInformation = announceableTorrent.createAnnounceableInformation();


            try {
              announce.getCurrentTrackerClient(announceableInformation)
                      .announceAllInterfaces(COMPLETED, true, announceableInformation);
            } catch (AnnounceException e) {
              logger.debug("unable to announce torrent {} on tracker {}", torrent, torrent.getAnnounce());
            }

            for (SharingPeer remote : getPeersForTorrent(torrentHash)) {
              remote.notInteresting();
            }

          }
        } else {
          torrent.markUncompleted(piece);
          logger.info("Downloaded piece #{} from {} was not valid ;-(. Trying another peer", piece.getIndex(), peer);
          peer.getPoorlyAvailablePieces().set(piece.getIndex());
        }
      }
    } catch (Throwable e) {
      torrent.markUncompleted(piece);
      logger.warn("unhandled exception in piece {} validation task", e);
    }
    torrent.handlePeerReady(peer);
  }