Hey Phil,
I notice your moving much of the extended functionality out of the Univariates and into a private field instead. I'm just curious what your logic is behind it? I had considered wrapping vs extending initially when I was building the implementation. Now that I look back, my reasons for going with extending were more to take advantage of the already preexisting "protected" fields in the implementation of the underlying Univariate. Now that I see your changes, I think what your wrapping is more kosher in terms of encapsulation underlying implementation of the wrapped statistic.
I think we are agreeing here. The ones that I changed were StandardDeviation (wrapping, rather than extending Variance) and GeometricMean (wrapping, rather than extending SumOfLogs). The reason for this, as you point out, is that extending (in these cases) binds the implementations unecessarily. In the two cases that I changed, what is really going on is just delegation, so composition is better (IMHO).
While the difference between these approaches is minimal in the front end Univariates like Mean, Var, Median, StandardDev and Geomean, I believe the extended hierarchy of FirstMoment -> SecondMoment -> ThirdMoment -> FourthMoment takes more advantage of this extended field reuse. I'm not so sure if these should be approached as wrappers too? They do make significant reuse of the internal state of the underlying moment. What do you think? Would these be problematic to turn into wrapped implementations too?
I thought about those and they could be changed to nest instead of extend one another; but as you point out there is significant use of common fields in these classes and it would be a fair amount of work to pull them apart. In addition, some efficiency might be lost by pulling them apart. This is beyond my JVM knowledge; but it seems that it might be more expensive to create the resulting "Chinese Box" of nested moment instances for, e.g. FourthMoment, and dereference all of the fields attached to the inner moments when doing computations, rather than just accessing the protected fields directly. This is especially true of the lower order moment values themselves, which are now stored as doubles and would have to be accessed via getResult() calls to the nested moments were we to change to composition.
Phil
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