How does RMSE get calculated?

[alexkokorin2]

My understanding is that it should be get calculated on a validation set but this model forecasts values for the future (I don't provide a true 'future' values to calculated RMSE on), so how do we get that RMSE error?

[alexkokorin2]

// compute RMSE to be used in confidence interval computation
            final double rmseValidation = ArimaSolver.computeRMSEValidation(
                data, ForecastUtil.testSetPercentage, paramsXValidation);

Found this piece of code, close the issue.