如何打开List <Map>，按Map键分组并按Map值减少

Question

I have a List<Map<String, Double>> representing some calculations that are stored by 'category' in the Map key.

[{a: 1, b:2}, {a:4, b:5})

I want to calculate the average by 'category', something like: {a: 2.5 , b: 3.5} .

I tried with something like

.collector(Collectors.groupingBy..)

but the first parameter requests to provide the discrimination function, which I do not know how to fish it from the Map item.

Any idea how to do that with Java8? Thank you!

Answer 1

You can use flatMap to extract all the entries of all the maps into a single Stream . Then you can group them by the key and calculate the average of the values.

Map<String,Double> average =
    list.stream()
        .flatMap(m->m.entrySet().stream())
        .collect(Collectors.groupingBy(Map.Entry::getKey,
                                       Collectors.averagingDouble(Map.Entry::getValue)));

Answer 2

You can use Collectors.averagingDouble as downstream

return listOfMap.stream()
        .flatMap(a -> a.entrySet().stream())
        .collect(Collectors.groupingBy(Map.Entry::getKey,
                Collectors.averagingDouble(Map.Entry::getValue)));