Igor Sorokin explains how to implement DataFrameNaFunctions:
Unfortunately, C&P comes in to play, therefore, if at some point in time a default value for ‘trackLength’ is also required, you may end up changing both of these methods. Another disadvantage is that if another similar method, which requires the same default values, is added, code duplication is unavoidable.
A possible solution, which helps to reduce boilerplate, is DataFrameNaFunctions, which is intended to be used for handling missing data: replacing specific values, dropping ‘null’ and ‘NaN’, and setting default values
Read on for an example.