Reducing Map by using Java 8 Stream API

Question

I have a Map in the following form:

Map<Integer, Map<String,Double>> START

Let INNER be the inner map, i.e.

Map<String,Double>

For example, I'd like to reduce START map in a new one

Map<Integer, Double> END

which have the same keys, but different values. In particular, for each key, I want the new Double value be the SUM of values in the INNER map for the corresponding key.

How could I achieve this by using JAVA 8's STREAM API?

Thanks everyone.

EDIT: A sample map is

------------------------------
|      |  2016-10-02   3.45   |
| ID1  |  2016-10-03   1.23   |
|      |  2016-10-04   0.98   |
------------------------------
|      |  2016-10-02   1.00   |
| ID2  |  2016-10-03   2.00   |
|      |  2016-10-04   3.00   |
------------------------------

e I'd like a new map like the following one:

--------------------------------
|      |                       |
| ID1  |  SUM(3.45,1.23,0.98)  |
|      |                       |
--------------------------------
|      |                       |
| ID2  |  SUM(1.00,2.00,3.00)  |
|      |                       |
--------------------------------

Answer 1

This should be a good example:

public class Main {

    public static void main(final String[] args) {
        final Map<Integer, Map<String, Double>> tmp = new HashMap<>();
        tmp.put(1, new HashMap<String, Double>() {{
            put("1", 3.45);
            put("2", 1.23);
            put("3", 0.98);
        }});
        tmp.put(2, new HashMap<String, Double>() {{
            put("1", 1.00);
            put("2", 2.00);
            put("3", 3.00);
        }});

        System.out.println(tmp.entrySet().stream()
                .collect(
                    Collectors.toMap(Map.Entry::getKey, 
                    data -> 
                        data.getValue()
                            .values().stream()
                            .mapToDouble(Number::doubleValue).sum())));
    }

}

output will be {1=5.66, 2=6.0} and all this does is takes entry set of map, gets a stream of it and collects to new map sums of inner map values.

Answer 2

It will work for you

    Map<Integer, Double> collect = START.entrySet()
        .stream()
        .collect(
            Collectors.toMap(
                Map.Entry::getKey, 
                e -> e.getValue()
                      .values()
                      .stream()
                      .reduce(0d, (a, b) -> a + b)
                )
        );