code: public DataSet getDataSet(String fileName) {
          if (dataSetCache.containsKey(fileName)) {
              return dataSetCache.get(fileName);
          }
          DataSet dataSet;
          try {
              dataSet = loadDataSetAsStream(fileName);
          } catch (IOException e) {
              e.printStackTrace();
              return null;
          }
          dataSetCache.put(fileName, dataSet);
          return dataSet;
      }
docs: Retrieves DataSet object (from cache or loads it) for provided file name.
  
  @param fileName a file name
  @return loaded DataSet object