code: public void train() {
      if (flags.trainFiles != null) {
        train(flags.baseTrainDir, flags.trainFiles, defaultReaderAndWriter);
      } else if (flags.trainFileList != null) {
        String[] files = flags.trainFileList.split(",");
        train(files, defaultReaderAndWriter);
      } else {
        train(flags.trainFile, defaultReaderAndWriter);
      }
    }
docs: Train the classifier based on values in flags. It will use the first of
  these variables that is defined: trainFiles (and baseTrainDir),
  trainFileList, trainFile.