Merge 2 maps with keys as values of second map

Question

I have 2 maps defined as Map<String, Set<String>> :

map1 : {
   Honda=[Accord, Civic, CRV], 
   Toyota=[Corola, RAV4, Yaris]
}
map2 : {
   Accord=[5w30, 10w40],
   Civic=[0w30, 5w30, 5w40],
   CRV=[5w30, 5w40, 10w40],
   Corola=[0w30, 10w40], 
   RAV4=[0w20, 0w30, 5w30]
}

I need merge this 2 maps so the result map key will be brand name (e.g. Honda, Toyota) and values will be all of oils. Some like this:

mergeMap: {
   Honda=[0w30, 5w30, 5w40, 10w40],
   Toyota=[0w20, 0w30, 5w30, 10w40]
}

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I wrote sam old school method which use forEach blocks, but I think that it can be better using streams and so on. Is it possible? If yes, how?

Answer 1

Since the resulting map will have the same keys, you can use mapValues.

In the lambda, flatMap each of the strings in the values set to the corresponding value in the second map, if it exists.

map1.mapValues { (_, v) -> v.flatMap { map2[it] ?: emptyList() }.toSortedSet() }

This gives you a Map<String, SortedSet<String>>.

Note that this will sort the “0w30” strings lexicographically, as they are strings, so you won’t get the exact order you want. I don’t know what these strings represent, but you should probably represent them with a data class implementing Comparable instead. That’d give you the correct order.