How to add values in hashmap with the same key and when value is stored as another hashmap

Question

I have a list of maps and I want to add values if the keys are equal. Values also stored as map. As output I want a single map as in example below.

I expect the output of

[
    {user1={service1=10.24, service2=0.11, service3=18.76}}, 
    {user1={service1=200.25, service2=0.05, service4=0.00}}
    {user2={service1=50.25, service4=0.05, service5=0.00}}
]

to be

{
    user1={service1=210.49, service2=0.16, service3=18.76, service4=0.00},
    user2={service1=50.25, service4=0.05, service6=0.00}
}

What does the declaration of your variable look like? List<Map<String, Map<String, Double>>>? — wilmol
– wilmol, Commented Aug 15, 2019 at 8:12

Adrian · Accepted Answer · 2019-08-15 09:51:03Z

2

it's a combination of different collectors:

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

Example:

List<Map<String, Map<String, Double>>> list = List.of(
         Map.of("user1", Map.of("service1", 10.24, "service2", 0.11, "service3", 18.76)),
         Map.of("user1", Map.of("service1", 200.25, "service2", 0.05, "service3", 0.0)),
         Map.of("user2", Map.of("service1", 50.25, "service2", 0.05, "service3", 0.0))
);

will produce

{
 user1={service2=0.16, service1=210.49, service3=18.76}, 
 user2={service2=0.05, service1=50.25, service3=0.0}
}

edited Aug 15, 2019 at 9:51

answered Aug 15, 2019 at 9:40

Adrian

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