code: @SuppressWarnings("unchecked")
    public void reduceToKeys(Set<String> keys) {
      if (size > 0) {
        int sizeCopy = size;
        String[] keyListCopy = keyList.clone();
        int[] errorNumberCopy = errorNumber.clone();
        HashMap<String, Integer>[] errorListCopy = errorList.clone();
        int[] sourceNumberListCopy = sourceNumberList.clone();
        T1[][] fullValueListCopy = fullValueList.clone();
        for (int i = 0; i < fullValueListCopy.length; i++) {
          if (fullValueListCopy[i] != null) {
            fullValueListCopy[i] = fullValueListCopy[i].clone();
          }
        }
        keyList = new String[keys.size()];
        errorNumber = new int[keys.size()];
        errorList = new HashMap[keys.size()];
        sourceNumberList = new int[keys.size()];
        fullValueList = operations.createMatrix1(keys.size());
        size = 0;
        for (int i = 0; i < sizeCopy; i++) {
          if (keys.contains(keyListCopy[i])) {
            keyList[size] = keyListCopy[i];
            errorNumber[size] = errorNumberCopy[i];
            errorList[size] = errorListCopy[i];
            sourceNumberList[size] = sourceNumberListCopy[i];
            fullValueList[size] = fullValueListCopy[i];
            size++;
          }
        }
      }
    }
docs: /*
  (non-Javadoc)
  
  @see
  mtas.codec.util.collector.MtasDataCollector#reduceToKeys(java.util.Set)