Grouping by List of Map in Java 8

Question

I have a List like this:

List<Map<String, Long>>

Is there a way, using lambda, to convert this list to:

Map<String, List<Long>>

Example:

Map<String, Long> m1 = new HashMap<>();
m1.put("A", 1);
m1.put("B", 100);

Map<String, Long> m2 = new HashMap<>();
m2.put("A", 10);
m2.put("B", 20);
m2.put("C", 100);

List<Map<String, Long>> beforeFormatting = new ArrayList<>();
beforeFormatting.add(m1);
beforeFormatting.add(m2);

After formatting:

Map<String, List<Long>> afterFormatting;

which would look like:

A -> [1, 10]
B -> [100, 20]
C -> [100]

Answer 1

You need to flatMap the entry set of each Map to create a Stream<Map.Entry<String, Long>>. Then, this Stream can be collected with the groupingBy(classifier, downstream) collector: the classifier returns the key of the entry and the downstream collector maps the entry to its value and collects that into a List.

Map<String, List<Long>> map = 
     list.stream()
         .flatMap(m -> m.entrySet().stream())
         .collect(groupingBy(Map.Entry::getKey, mapping(Map.Entry::getValue, toList())));

This code needs the following static imports:

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.toList;

With your full example:

public static void main(String[] args) {
    Map<String, Long> m1 = new HashMap<>();
    m1.put("A", 1l);
    m1.put("B", 100l);

    Map<String, Long> m2 = new HashMap<>();
    m2.put("A", 10l);
    m2.put("B", 20l);
    m2.put("C", 100l);

    List<Map<String, Long>> beforeFormatting = new ArrayList<>();
    beforeFormatting.add(m1);
    beforeFormatting.add(m2);

    Map<String, List<Long>> afterFormatting =
        beforeFormatting.stream()
                        .flatMap(m -> m.entrySet().stream())
                        .collect(groupingBy(Map.Entry::getKey, mapping(Map.Entry::getValue, toList())));

    System.out.println(afterFormatting); // prints {A=[1, 10], B=[100, 20], C=[100]}
}