How to convert a Map transformation in scala to java without manually rebuilding the map?

Question

This is the transformation when written in scala -

(Map of type [K, List[V]]).groupBy(_._1).mapValues(_.unzip._2)

This would return a map with keys of all different values of the List and values with a collection of relevant indexes for each such group.

When trying to convert this into java -

One could use Google's guava library's Multimap to replace the .groupBy(_._1) which is -

...collection that maps keys to values, similar to Map, but in which each key may be associated with multiple values.

But that would be a little complicated to accomodate that for each key referring to a number of lists.

How could I perform the same task on a java HashMap> in java without deconstructing the entire map and list manually and rebuilding it?