code: public static synchronized FileTransferNegotiator getInstanceFor(
              final XMPPConnection connection) {
          FileTransferNegotiator fileTransferNegotiator = INSTANCES.get(connection);
          if (fileTransferNegotiator == null) {
              fileTransferNegotiator = new FileTransferNegotiator(connection);
              INSTANCES.put(connection, fileTransferNegotiator);
          }
          return fileTransferNegotiator;
      }
docs: Returns the file transfer negotiator related to a particular connection.
  When this class is requested on a particular connection the file transfer
  service is automatically enabled.
  
  @param connection The connection for which the transfer manager is desired
  @return The FileTransferNegotiator