How can I get the residuals from a curve using Apache Commons Math

Question

I'm trying to use Commons Math to fit polynomial curves to a dataset. I have:

• created a PolynomialCurveFitter,
• got the coefficients,
• created a polynomial function with those coefficients, and
• created an array of values generated by the function.

more or less manually creating the predictions.

My question is, how can I get the residuals between the fitted values and the original observations?

I see in the commons.math code that the PolynomialCurveFitter creates a LeastSquaresProblem, the leastSquaresProblem can return an Evaluation, and the evaluation has a method to return the residuals.

However, the getProblem() method in PolynomialCurveFitter is protected so I can't use it.

Is there a way get an appropriate LeastSquaresProblem from a CurveFitter so I can get residuals?

Or should I extend the Fitter to make getProblem() accessible?

Or is there another way entirely?

Here is my code so far:

public static double[] fitPolynomial(double[] xCol, double[] yCol, int degree) {

    Collection<WeightedObservedPoint> points = new ArrayList<>();

    for (int i = 0; i < xCol.length; i++) {
        points.add(new WeightedObservedPoint(1.0, xCol[i], yCol[i]));
    }

    PolynomialCurveFitter fitter = PolynomialCurveFitter.create(degree);

    double[] coeff = fitter.fit(points);

    PolynomialFunction function = new PolynomialFunction(coeff);

    double[] fitted = new double[xCol.length];

    for (int i = 0; i < xCol.length; i++) {
        fitted[i] = function.value(i);
    }

    return fitted;
}