convert list of map to map using flatMap

Question

How I can merge List> to Map using flatMap?

Here\'s what I\'ve tried:

Answer 1

A very simple way would be to just use putAll:

Map result = new HashMap<>();
response.forEach(result::putAll);

If you particularly want to do this in a single stream operation then use a reduction:

response.stream().reduce(HashMap::new, Map::putAll);

Or if you really want to use flatMap:

response.stream().map(Map::entrySet).flatMap(Set::stream)
    .collect(toMap(Map.Entry::getKey, Map.Entry::getValue, Map::putAll));

Note the merging function in the final alternative.