Flattening a collection

Question

Say I have a Map>

I can get the values of the map easily enough, and iterate over it to produce a single

Answer 1

A nice solution for the subcase of a Map of Maps is to store, if possible, the data in Guava's Table.

https://github.com/google/guava/wiki/NewCollectionTypesExplained#table

So for instance a Map> is replaced by Table which is already flattend. In fact, the docs say that HashBasedTable, Table's Hash implementation, is essentially backed by a HashMap>