Java List> sum of group by of map key

Question

I have a: List<Map<String, Long>> items = new ArrayList<>();

I would like to get a Map in which the key is grouped by, and the value is the sum.

Example: List

• Item 0
  • foo -> 1
  • bar -> 2
• Item 1
  • foo -> 4
  • bar -> 3

Result: Map

• foo -> 5
• bar -> 5

I know how to do this the "long" way, but was trying to discover a lambda/streaming/groupby approach using the java 8 features. any thoughts?

Answer 1

You can use groupingBy collector:

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

items.stream()
        .flatMap(m -> m.entrySet().stream())
        .collect(groupingBy(Map.Entry::getKey, summingLong(Map.Entry::getValue)));

Or you can use toMap:

import static java.util.stream.Collectors.toMap;

items.stream()
        .flatMap(m -> m.entrySet().stream())
        .collect(toMap(Map.Entry::getKey, Map.Entry::getValue, Long::sum));