In many cases when I work with Map Reduce my mapper or my reducer might
take a single value and map it to multiple keys -
The reducer might also take a single key and emit multiple values
I don't think that functions like flatMap and reduceByKey will work or are
there tricks I am not aware of
To emulate a Mapper, flatMap() is exactly what you want. Since it
flattens, it means you return an Iterable of values instead of 1
value. That can be a Collection containing many values, or 1, or 0.
For a reducer, to really reproduce what a Reducer does in Java, I
think you will need groupByKey()
