MATH-870, RealVector.sparseIterator() has become useless. Its interface was confusing. Indeed, the Javadoc states
Specialized implementations may choose to not iterate over all dimensions, either because those values are unset, or are equal to defaultValue(), or are small enough to be ignored for the purposes of iteration.
The reference to defaultValue() (which does not exist) suggests that the interface offers the flexibility to chose the value which is not stored in the sparse implementation. On the other hand, "small enough to be ignored" suggests that this default value is zero.
In version 3.1, this method will be deprecated, to be removed in 4.0.