Guava: how to customly reduce a multimap?

Question

Is the concept of reduce implemented in Guava libs? I mean I have a multimap:

[1] -> 1, 2, 5, 8
[2] -> 3, 6, 4
[3] -> 1, 0

Then I have a multiplying function:

(a, b) -> a * b

And I want to get the following map:

[1] -> 80          // 1 * 2 * 5 * 8
[2] -> 72          // 3 * 6 * 4
[3] -> 0           // 1 * 0

How do I do it in Guava?

Answer 1

I don't think there is a reduction operation in Guava. I guess you have two options.

If you're using java-8, just stream over the entry set, and collect the entries into a new map with groupingBy and reduce.

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

...

Map<Integer, Integer> map = 
    multimap.entries()
            .stream()
            .collect(groupingBy(Map.Entry::getKey,
                                reducing(1, Map.Entry::getValue, (a, b) -> a * b)));

groupingBy as you can guess, will group the entries by their key values. Then we just reduce the values grouped (which are Entry<Integer, Integer>) by first mapping their to their values and finally by multiplying them - providing the identity value which is 1 for the multiplication.

---

If you can't use java-8, you can get back a Map<Integer, Collection<Integer>> from your Multimap with Multimaps.asMap and use Maps.transformValues:

Map<Integer, Collection<Integer>> tempMap = Multimaps.asMap(oldMap);
Map<Integer, Integer> newMap = 
    Maps.transformValues(tempMap, new Function<Collection<Integer>, Integer>() {
            @Nullable
            @Override
            public Integer apply(Collection<Integer> input) {
                //do the multiplication here
                return 0;
            }
        });