How to Convert a Map<String, List<String>> to Map<String, String> in java 8 functional APIs

Question

I have a map like bellow,

 [key = "car", value = ["bmw", "toyota"]]
 [key = "bike", value = ["honda", "kawasaki"]]

I want to convert it to another map using java 8 functional apis like bellow,

 [key = "bmw", value = "car"]
 [key = "toyota", value = "car"]
 [key = "honda", value = "bike"]
 [key = "kawasaki", value = "bike"]

Answer 1

Flatten the map values to entries then collect them:

Map<String, String> m2 = map
    .entrySet()
    .stream()
    .flatMap(e -> e.getValue().stream().map(v -> new AbstractMap.SimpleEntry<>(v, e.getKey())))
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

This can be shortened by importing AbstractMap.SimpleEntry and Map.Entry .

Answer 2

A solution that doesn't need constructing temporary Map.Entry instances, is:

Map<String, String> result = source.entrySet().stream()
  .collect(HashMap::new, (m,e)->e.getValue().forEach(k->m.put(k,e.getKey())), Map::putAll);

You might notice the similarity to the non-stream solution

Map<String, String> result = new HashMap<>();
source.forEach((key, value) -> value.forEach(k -> result.put(k, key)));

or the pre-Java 8 solution

Map<String, String> result = new HashMap<>();
for(Map.Entry<String,List<String>> e: source.entrySet())
    for(String key: e.getValue()) result.put(key, e.getKey());