Calculate the sum of a field contained in a collection that is contained in an other collection using Java Stream

Question

I want to calculate the sum of a field contained in a collection that is contained in another collection using java stream(). i only know how to do it if the calculated field is directly in the first collection, like this :

return collectionOfObjectA.stream() 
             .mapToDouble(c1 -> c1.getSalary()).sum();

but now the "collectionOfObjectA" contain another "collectionOfObjectB", and this last one is the one containing "salary" field.

ObjectA:

{
 id:1,
Collection<ObjectB>
}

and ObjectB:

{
 id:1,
 salary: 4000
}

the way i'm calculating it now is like follow:

     Double result=0.0;
        for (ObjectA obj: collectionOfObjectA())
        {
            for(ObjectB objB : objA.getcollectionOfObjectB()){
                result+= objB.getSalary();
            }
        }

        return result;

Answer 1

Before mapToDouble you have to use flatMap like this:

return collection1.stream()
        .flatMap(cA -> cA.getCollectionB().stream())
        .mapToDouble(ObjectB::getSalary)
        .sum();

Or you can use map before the flatMap:

return collection1.stream()
        .map(ObjectA::getCollectionB)
        .flatMap(List::stream)
        .mapToDouble(ObjectB::getSalary)
        .sum();

Of you can use flatMapToDouble directly as @Naman mentioned:

return collection1.stream()
        .flatMapToDouble(
                cA -> cA.getCollectionB().stream().mapToDouble(ObjectB::getSalary)
        ).sum();