code: public <T extends RegressionEvaluation> T evaluateRegression(DataSetIterator iterator, List<String> columnNames) {
          return (T)doEvaluation(iterator, new org.deeplearning4j.eval.RegressionEvaluation(columnNames))[0];
      }
docs: Evaluate the (single output layer only) network for regression performance
  
  @param iterator    Data to evaluate on
  @param columnNames Column names for the regression evaluation. May be null.
  @return Regression evaluation