Right now the DescriptiveStatistics is for ResizableDoubleArray, but if the user has the double data she wishes to get DescriptiveStatistics for there is no way of doing this (besides the inefficient way of creating a ResizableDoubleArray from the double).
It would be possible to use StatUtils, but it does not contain near as much functionality as DescriptiveStatistics.
Idea: Create a DescriptiveStatistics for double also. I'm not sure which way of implementing it would be the best, but one way would be to make DescriptiveStatistics abstract with all but the apply-method implemented as now. The apply-method should be made as abstract which then has to implemented in the extensions together with a field containing the values. (addValue, windowSize etc. has to go in the ResizableDoubleArray-extension.) The double-extension should then have a constructor taking double as a parameter.