Note that the calculations in the previous example are lazy; they are only done when requested. This also means that they're performed each and every time they're requested. This can be a considerable overhead, depending on the context in which objects of these classes are used.
It's may be sensible to transform these statistical summaries into eager calculations, as we know when elements are added and removed from a list. Although there's a hair more programming to create eager versions of these functions, it has a net impact of improving performance when there's a lot of data being accumulated.
The point of eager statistical calculations is to avoid the loops that compute sums. If we compute the sums ...
