How to flatten a Map by turning its values into keys

It is not possible to have multiple keys identical according to equals/hashCode within the same map because it contradicts with idea of the map data structure. Every key must be unique, you can't store multiple entries with the same key in a Map.

But you can create a list of Map.Entry objects.

For that, you need to create a stream over the entry set and then flatten each entry by creating a new entry based on car brands (elements of a flatted value) for each key:

 Map<String, List<String>> cars = Map.of("fuel", List.of("BMW", "Honda"), "electric", List.of("Tesla", "Nio")); List<Map.Entry<String, String>> entries = cars.entrySet().stream() .flatMap(entry -> entry.getValue().stream() .map(brand -> Map.entry(entry.getKey(), brand))) .collect(Collectors.toList()); System.out.println(entries); 

output

[electric=Tesla, electric=Nio, fuel=BMW, fuel=Honda] 

Or maybe your intention was to create a Map<String, String> that will allow to retrieve car type based on brand (like that: "BMW" : "fuel", "Honda" : "fuel") then it'll make sense.

The overall approach will be similar to the previous one:

• create a stream over the entry set;
• flatten each entry using flatMap() by turning an element in the value-list into a new entry;
• collect elements to a map with Collectors.toMap().

But there's a caveat: all values have to be unique or there must be a rule on how to combine/discarde car types.

I'll make an assumption that all brands in the map are unique, otherwise, the code below will fail (to deal with collisions Collectors.toMap() requires the third argument - mergeFunction).

 Map<String, String> typeByBrand = cars.entrySet().stream() .flatMap(entry -> entry.getValue().stream() .map(brand -> Map.entry(brand, entry.getKey()))) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)); System.out.println(typeByBrand); 

output

{Nio=electric, Tesla=electric, BMW=fuel, Honda=fuel}