Is there a way to group values and then calculate sum using RxJava - operators

Question

I have a list of objects which is in form of a Flowable.
Example -
values count
A 2
B 3
C 4
A 5
C 1
I wish to group the flowable on the basis of values and then calculate the sum. Is there a better way of doing this ?
I have already tried generting a multimap. Then defining a function in the subscriber to aggregate the results. However, I feel I am not using JavaRx efficiently to do aggregations in streams.
Flowable<Response> responseFlowable = Flowable.fromIterable(generateList());
replayResponseFlowable.toMultimap(response -> response.getValues(), response -> response.getCount()).subscribe(groups-> calculateSum(groups));
}
private static void calculateSum(Map<String,Collection<Integer>> groups)
{
//iterate over the map and calculate sum for each of the groups.
}
The expected result is :
A 7
B 3
C 5
I wish to do this computation within the stream using JavaRx instead of defining a custom method. How can I do it ?

Answer 1

You can put the calculateSum in the stream to make it look nicer.
replayResponseFlowable.toMultiMap(response -> response.getValues())
.map(groups -> calculateSum(groups))
.subscribe(result -> {
print(result);
})
Since you are only aggregating the counts, you don't need to keep track of all items. Instead you can keep only the sum of the count for each value.
This can be done using .collect operator:
replayResponseFlowable
.collectInto(new HashMap<String, Integer>(), (group, response) -> {
group.merge(response.values, response.count, Integer::sum);
})
.subscribe(result -> {
print(result);
});
This will print
emitted=[{value=A, count=7}, {value=B, count=3}, {value=C, count=5}]
If you want to get the result as the flowable emits items, use .scan:
replayResponseFlowable
.scan(new HashMap<String, Integer>(), (group, response) -> {
group.merge(response.values, response.count, Integer::sum);
return group;
})
.subscribe(result -> {
print(result);
});
This will print:
emitted=[]
emitted=[{value=A, count=2}]
emitted=[{value=A, count=2}, {value=B, count=3}]
emitted=[{value=A, count=2}, {value=B, count=3}, {value=C, count=4}]
emitted=[{value=A, count=7}, {value=B, count=3}, {value=C, count=4}]
emitted=[{value=A, count=7}, {value=B, count=3}, {value=C, count=5}]