Java 8 streams - merging a list of Maps

Question

I'm trying to use a stream of List<Map<String, Integer>> to merge all maps included into one. This is to aggregate multiple request responses into one.

Consider this dataset (using JSON to make it easier to read):

[
    {"field1": 1, "field2": 5},
    {"field2": 6, "field3": 10},
    {"field1": 3, "field4": 15}
]

I want it to result in the following (order does not matter):

{"field1": 4, "field2": 11, "field3": 10, "field4": 15}

I want to sum all keys and combine them into a single map. Is there a nice way to do this with the Stream interface in Java 8?

I discovered Collectors.groupingBy(o -> o, Collectors.counting()) , so perhaps I'm on the right lines, I just need to figure out how to actually merge them in advance of this without screwing up the results.

Thanks in advance.

Answer 1

counting() will just return the number of values that are grouped by the function you apply for each key. If you want to sum them, the downstream collector you are looking for is summingInt (or summingLong if you may have overflows):

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.summingInt;

...

Map<String, Integer> map =
    list.stream()
        .flatMap(m -> m.entrySet().stream())
        .collect(groupingBy(Map.Entry::getKey, summingInt(Map.Entry::getValue)));