The templates used in the java package are very high mainteance and the if conditions are hard to track. As started in the discussion here: https://github.com/apache/arrow/pull/1012, I'd like to propose that we modify the structure of the internal value vectors and code generation dynamics.
Create new abstract base vectors:
For each of these, implement all the basic functionality of a vector without using templating.
Evaluate whether to use code generation to generate specific specializations of this functionality for each type where needed for performance purposes (probably constrained to mutator and accessor set/get methods). Giant and complex if conditions in the templates are actually worse from my perspective than a small amount of hand written duplicated code since templates are much harder to work with.