Collect Map<String, Integer> from stream of Map<String, Map<String, Integer>>

Question

I have a nested map with key as Employee name and values as another map with key as company name and value as years of experience like below

Map<String, Map<String, Integer>> map = new HashMap<>();
    Map<String, Integer> innerMap1 = new HashMap<>();
    innerMap1.put("INfosys", 2);
    innerMap1.put("Volvo", 2);
    innerMap1.put("MH", 3);
    innerMap1.put("Piterion", 1);

    Map<String, Integer> innerMap2 = new HashMap<>();
    innerMap2.put("Tata", 2);
    innerMap2.put("Bosch", 1);
    innerMap2.put("Amber", 1);
    innerMap2.put("E2", 1);

    map.put("Rahul", innerMap1);
    map.put("Amrita", innerMap2);

Now my function should return a Map with the employee name as key and total experience as value. How can I do that using java streams (in a single stream)

public Map<String, Integer> getEmployeesWithExp(Map<String, Map<String, Integer>> map) {
    map.entrySet().stream().
                        ...

    return null;
}

I found similar topic: stackoverflow.com/questions/44085387/… — Lareth51
– Lareth51, Commented Oct 14, 2019 at 11:58

Naman · Accepted Answer · 2019-10-14 13:15:36Z

4

There probably are multiple ways but you could collect the entries into a new map and reduce the values of the inner maps to integers, e.g. like this:

Map<String, Integer> result = 
           map.entrySet().stream()
              .collect( 
                  Collectors.toMap(e -> e.getKey(), //or Map.Entry::getKey
                                    e -> e.getValue().values().stream()
                                                      .reduce(0, Integer::sum)));

edited Oct 14, 2019 at 13:15

Naman

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answered Oct 14, 2019 at 11:57

Thomas

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Comments

Xobotun · Accepted Answer · 2019-10-14 12:02:08Z

1

This is the first time I tried to use streams with maps, it was quite a good exerxcise, thanks.

I failed to do it in only one stream, though. This solution features one main stream and internal streams.

I used org.apache.commons.lang3.tuple.Pair, by the way.

Map<String, Integer> result = map.entrySet().stream()
        .map(entry -> Pair.of(entry.getKey(), entry.getValue().values().stream().reduce(0, Integer::sum)))
        .collect(Collectors.toMap(Pair::getKey, Pair::getValue));

It answered

 "Amrita" → 5
 "Rahul" → 8

I believe it is correct. :D

answered Oct 14, 2019 at 12:02

Xobotun

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Comments

Vighnesh Mahadik · Accepted Answer · 2019-10-14 12:03:42Z

1

This is simple for loops used for solution :-

    Map<String, Integer> finalMap = new HashMap<>();
    for (Entry<String, Map<String, Integer>> entry : map.entrySet()) {
        Integer exp = 0;
        for (Entry<String, Integer> entry2 : entry.getValue().entrySet()) {
            exp += entry2.getValue();
        }
        finalMap.put(entry.getKey(), exp);
    }

Output- {Amrita=5, Rahul=8}

answered Oct 14, 2019 at 12:03

Vighnesh Mahadik

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Comments

prabhat kuber · Accepted Answer · 2020-08-02 11:24:27Z

0

Can be done in simple way:-

Function< Map<String, Integer>,Integer> sumOfValue = (x) ->   x.values().stream().mapToInt(Integer::intValue).sum(); 
stringMapMap.entrySet().stream().collect(Collectors.toMap(
            e-> e.getKey(), e-> sumOfValue.apply(e.getValue())
    ));

answered Aug 2, 2020 at 11:24

prabhat kuber

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