how to merge a collection of Maps using streams

Question

I have a collection of maps:

Collection<Map<String,Double>> myCol = table.values();

I would like to transform this into a Map

Map<String, Double>

such that, for a matching key, values are summed up. Using a for loop, it is rather simple:

Map<String, Double> outMap = new HashMap<>();
for (Map<String, Double> map : myCol) {
    outMap =  mergeMaps(outMap, map);
}

and mergeMaps() is defined as

mergeMaps(Map<String, Double> m1, Map<String, Double> m2){    
    Map<String, Double> outMap = new TreeMap<>(m1);
    m2.forEach((k,v) -> outMap.merge(k,v,Double::sum)); /*sum values if key exists*/
    return outMap;
}

However, I would like to use streams to get a map from collection. I have tried as follows:

Map<String, Double> outMap = new HashMap<>();    
myCol.stream().forEach(e-> outMap.putAll(mergeMaps(outMap,e)));
return outMap;

This works without a problem. However, can I still improve it? I mean, how can I use collectors in it?

Answer 1

From your input, you can fetch the stream of maps and then flatmap it to have a Stream<Map.Entry<String, Double>>. From there, you collect them into a new map, specifying that you want to sum the values mapped to the same key.

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

....

Map<String, Double> outMap = 
    myCol.stream()
         .flatMap(m -> m.entrySet().stream())
         .collect(toMap(Map.Entry::getKey, Map.Entry::getValue, Double::sum));

Alternatively, you can use groupingBy instead of toMap:

.collect(groupingBy(Map.Entry::getKey, summingDouble(Map.Entry::getValue)));